Cosmic Horizons

The Alcubierre Warp Drive: On the Matter of Matter

2012-03-01T21:00:00.000+11:00

I think I am over the yearly battering with Australian Research Council Discovery Grants with the grant now submitted. The release is matched with the acceptance of a paper from left-field, and so, to quote Monte Python, "And now for something completely different".

Last year, Brendan McMonigal was an honours student with me, and we took a look at the Alcubierre warp drive, a method to travel globally faster than the speed of light, while being quite happy with Einstein's General Theory of Relativity. This stuff often freaks people out, because they get special relativity beaten into them first, without realizing what this actually means in terms of general relativity.

Whatever other people think, I love General Relativity. What you can do with the universe is actually pretty cool.

So, the warp drive, in hand waving terms, travels at arbitrary speed by messing about with space-time. But what Brendan looked at it the question of what happens to all those particles and photons that get caught up in the warp bubble.

What he found is that as the warp drive sweeps through the universe and collects up all the particles, the ions and lone electrons, and microwave background photons, and whatever else is lying around.

And then when you slow down, they all get released in a burst, which will fry all of the people waiting to meet you. This is not good (not if Nanna has come all the way to meet you at the space port).

You can read more at universetoday, but if you are interested, check out the paper. Well done Brendan!

The Alcubierre Warp Drive: On the matter of matter

Brendan McMonigal, Geraint F. Lewis, Philip O'Byrne

(Submitted on 26 Feb 2012)

The Alcubierre warp drive allows a spaceship to travel at an arbitrarilylarge global velocity by deforming the spacetime in a bubble around thespaceship. Little is known about the interactions between massive particles andthe Alcubierre warp drive, or the effects of an accelerating or deceleratingwarp bubble. We examine geodesics representative of the paths of null andmassive particles with a range of initial velocities from -c to c interactingwith an Alcubierre warp bubble travelling at a range of globally subluminal andsuperluminal velocities on both constant and variable velocity paths. The keyresults for null particles match what would be expected of massive testparticles as they approach +/- c. The increase in energy for massive and nullparticles is calculated in terms of v_s, the global ship velocity, and v_p, theinitial velocity of the particle with respect to the rest frame of theorigin/destination of the ship. Particles with positive v_p obtain extremelyhigh energy and velocity and become "time locked" for the duration of theirtime in the bubble, experiencing very little proper time between entering andeventually leaving the bubble. When interacting with an accelerating bubble,any particles within the bubble at the time receive a velocity boost thatincreases or decreases the magnitude of their velocity if the particle ismoving towards the front or rear of the bubble respectively. If the bubble isdecelerating, the opposite effect is observed. Thus Eulerian matter isunaffected by bubble accelerations/decelerations. The magnitude of the velocityboosts scales with the magnitude of the bubble acceleration/deceleration.

The structure of star clusters in the outer halo of M31

2012-02-29T09:02:00.002+11:00

I am slowly recovering from the recent round of grant writing. The internal deadline for our Australian Research Council (ARC) Discovery Projects (DPs) was on Monday, and, as ever, it was late nights and early mornings to put together the 115 pages of science case, budgets, budget justifications, publication records, career histories etc etc etc. Now all I have to do is sit back until July (when the referees' reports come in) and then to November (when the results are announced). If I get my grant, I'll write and say how great the systems is. However, if I am the three in four who will not get funded, I'll have a rant.

Anyway, even though the days have been lost to grant writing, science has advanced, and we recently had a paper accepted for publication in MNRAS, using the Hubble Space Telescope to look at clusters around our nearest big galactic neighbour, Andromeda.

Many objects are globular clusters, just like the ones we orbiting our own Milky Way. With Hubble Space Telescope images, we can easily see the individual stars in the globular and work out how the stars are distributed. Here's an example -

In the top left, we have our Hubble picture of the sky, and the bottom left is the colour-magnitude diagram. The problem is that we have not only our globular's stars in there, but also contamination from galactic stars. The upper right are the stars identified from the colour-magnitude diagram, and the bottom right is the surface brightness distribution.

All well and good. Except, we have these in the halo of Andromeda as well

These are extended clusters (otherwise known as "faint fuzzies"). These things are weird. As you can see from the bottom right, the profile of these objects are a lot flatter, and more extended than standard globulars. Now, here's a cool plot from an earlier paper;

So, this shows the size of objects in the verses their brightness. This plot used to be quite simple with objects on the left, the globulars

and on the right are dwarf galaxies

Basically, the ones on the left are not dominated by dark matter, and have simple star formation histories, whereas those on the right have lots of dark matter and complicated histories.

But what has happened over the recent years is that the gap in between has been filled in. In the Milky Way there are ultra-faint dwarfs, thought by some (not by all) to be dominated by dark matter. Whereas, from the left, the faint fuzzies are seen not to be dark matter dominated.

It's actually all a little confuddling. Perhaps we should expect the gap to be filled, but not necessarily by two different kinds of things. And the very bizarre thing, there does seem to be any faint fuzzies orbiting the Milky Way. Why? I don't know, but we are finding the universe to be more complex than thought. Well done Nial :)

The structure of star clusters in the outer halo of M31

N. R. Tanvir, A. D. Mackey, A. M. N. Ferguson, A. P. Huxor, J. I. Read, G. F. Lewis, M. J. Irwin, S. Chapman, R. Ibata, M. I. Wilkinson, A. W. McConnachie, N. F. Martin, M. B. Davies, T. J. Bridges

(Submitted on 9 Feb 2012 (v1), last revised 10 Feb 2012 (this version, v2))

We present a structural analysis of halo star clusters in M31 based on deepHubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. Theclusters in our sample span a range in galactocentric projected distance from13 to 100 kpc and thus reside in rather remote environments. Ten of theclusters are classical globulars, while four are from the Huxor et al. (2005,2008) population of extended, old clusters. For most clusters, contamination byM31 halo stars is slight, and so the profiles can be mapped reliably to largeradial distances from their centres. We find that the extended clusters arewell fit by analytic King (1962) profiles with ~20 parsec core radii and ~100parsec photometric tidal radii, or by Sersic profiles of index ~1 (i.e.approximately exponential). Most of the classical globulars also have largephotometric tidal radii in the range 50-100 parsec, however the King profile isa less good fit in some cases, particularly at small radii. We find 60% of theclassical globular clusters exhibit cuspy cores which are reasonably welldescribed by Sersic profiles of index ~2-6. Our analysis also reinforces thefinding that luminous classical globulars, with half-light radii <10 parsec,are present out to radii of at least 100 kpc in M31, which is in contrast tothe situation in the Milky Way where such clusters (other than the unusualobject NGC 2419) are absent beyond 40 kpc.

Citations to Australian Astronomy: 5 and 10 Year Benchmarks

2012-02-23T08:48:00.000+11:00

Not everything that can be counted counts, and not everything that counts can be counted.
Albert Einstein, (attributed)

I don't know how widely it is known outside academic circles, but researchers these days are surveyed and counted continuously, in an effort to show that tax-payers money is being spent on research excellence and research impact. A number of countries have held such exercises, and here we are into the second round of the Excellence in Research for Australia.

Such exercises have real impact, as some at The University of Sydney just found out; those not producing enough "research outputs" are in the firing-line for redundancies.

With the growth of online databases of papers and citations, it's now easy to get an assessment of someones research output, and with things like Google Scholar it's all nicely displayed; here's mine. People's careers get wrapped into a few metric, and the principle one is the h-index.

The idea is simple. Each paper you write is cited by others, and when you rank your papers in order of the number of citations it gets, your h-index is the number of the paper with the same number of citations. As an example, my h-index is 50, and so I have 50 papers with more than 50 citations.

This is taken to be a measure research impact. The more active you are, the more papers you produce. But you need people to read and cite them (this is the impact part). The bigger the h-index, the "better" the researcher (for some vague definition of better).

Of course, many people hate this kind of ranking system as many factors can affect your h-index, such as the field you work in, being ahead of your time etc. But it is common for people to look at h-indicies of similar people at similar level of their careers, at least as a starting point of assessing research impact.

Such ranking exercises have become a subfield, and a recent paper appeared which looked at the impact of Australian astronomers (in terms of their h-index and citations). Here's the abstract:

Citations to Australian Astronomy: 5 and 10 Year Benchmarks

Katherine H. Kenyon, Arjun Paramasivam, Jiachin Tu, Albert Zhang, Alister W. Graham

(Submitted on 18 Feb 2012 (v1), last revised 21 Feb 2012 (this version, v2))

Expanding upon Pimbblet's informative 2011 analysis of career h-indices formembers of the Astronomical Society of Australia, we provide additionalcitation metrics which are geared to a) quantifying the current performance ofb) all professional astronomers in Australia. We have trawled the staffweb-pages of Australian Universities, Observatories and Research Organisationshosting professional astronomers, and identified 383 PhD-qualified,research-active, astronomers in the nation - 131 of these are not members ofthe Astronomical Society of Australia. Using the SAO/NASA Astrophysics DataSystem, we provide the three following common metrics based on publications inthe first decade of the 21st century (2001-2010): h-index, author-normalisedcitation count and lead-author citation count. We additionally present asomewhat more inclusive analysis, applicable for many early-career researchers,that is based on publications from 2006-2010. Histograms and percentiles, plustop-performer lists, are presented for each category. Finally, building onHirsch's empirical equation, we find that the (10-year) h-index and (10-year)total citation count T can be approximated by the relation h =(0.5+sqrt{T})/sqrt{5} for h > 5.

One of the things they do is produce ranked lists of the "top" astronomers in Australia. Here it is in terms of h-index.

I'm not going to comment too much about this, but it's nice to be on the list, and I'm in excellent company also.

Right, back to grant writing!

First Galaxies and Faint Dwarfs

2012-02-20T09:08:00.000+11:00

Back in Oz after a week in the US at the University of California at Santa Barbara (UCSB). I was at a conference called First Galaxies and Faint Dwarfs at the Kavli Institute of Theoretical Physics.

The week started with us being told that this was going to be the best conference we would ever attend, and I must agree that it was. Instead of the usual barrage of 15min talks, we had 40mins of review talks, with discussion sessions, and it was great.

What was the point of the meeting? Well, that was summarized by the opening slide by Leon Koopmans;

Essentially, the goal was to bring together two communities, those that study the high redshift universe and the first forming galaxies, and those that look at the tiny dwarf galaxies in our local universe. Why? Because understanding galaxy formation and the links between the nearby and faraway will reveal the inner workings of the universe, especially the nature of dark matter.

I'm not going to go through the conference in detail (as I have to get to work), so here's few slides. All the talks are public, and there are podcasts so you can hear the discussions, and I recommend that you take a looksie.

Notice what there is not. There is not wailing and gnashing of teeth with screams of "oh what is dark matter, we don't know what it is, oh this is a disaster, ohhhh, woe is me". No. That is not where we are in the subject. The point is that we know dark matter (or, with a much lower prior, modified gravity) is out there, and we want to know what the consequences are.

So, what are the big questions. Well, they may seem quite technical, but it is where the front lines of cosmological science currently are.

Where are the dark matter subhalos? Basically, our picture of the large scale structure of the Universe, one driven by dark matter and dark energy, works really well in predicting where galaxies should be. But on small scales, it predicts that a galaxy like the Milky Way should be surrounded by a host (thousands of..) dwarf galaxies. But we only see a few 10s of these little things out there. So where are they? If you believe Carlos Frenk, the problem is solved - the first burst of star formation in the universe blew the gas out of the halos, and so they could not form stars. The basic result of this would be that we surrounded by dark halos, loads of them. But how can you see if their there? Well, we can use gravitational lensing, and we're working on it. Just as an example, here is a simulated gravitational lens, followed by an actual lens, the Horseshoe. By studying the details, we'll be able to measure the the amount of subclumps of mass out there. Cool eh?

Are dark matter halos cored or cusped? This is quite technical, but our cosmological models make some firm predictions on the distribution of mass in galaxies, predicting that the mass density should increase rapidly towards the centre. This peaky mass distribution is called cusped and so if we can measure it, we can directly test whether our models are correct. If the mass does not rise so quickly (a cored distribution) then this would be a bit of a problem. Except, things are not so simple, as the existence of baryons (to you and me, basically means gas and atoms and stars and stuff) can apparently change cusps into cores. But gas physics is hard, especially including them in the complex computer simulations of structure formation in the Universe. If halos are cored, rather than cusped, then it might means that dark matter is not cold, which brings me to the next question.
Is dark matter hot, warm or cold? Is dark matter cold (i.e. composed of things moving slowly) or hot (things moving relativistically), or somewhere in between (i.e. warm)? This is related to the above two questions, as warm dark matter doesn't form the little halos we expect around the Milky Way, and is likely to influence the core/cusp question. Currently, we don't know.
Are dwarf galaxies we see today like the dwarf galaxies that formed the Milky Way? Again, this is a tricky one. The Milky Way was built of little galaxies that fell in. But are the ones falling in now like the ones that fell in long ago? The answer is "we don't really know", but there are some clues, namely coming from the mass-metallicity relationship, which means that smaller galaxies have less metals (which, to an astronomer, is all elements heavier than helium. But stellar chemistry is quite complex, and the observations challenging, but there is some really good ground being made in this area.

There are more, of course, but these are the key questions. At the meeting we had the usually argy-bargy about theorists and observers not believing or trusting each other, but it's all part of the fun.

On a closing note, I've now visited three Kavli Institutes around the world, the one in Santa Barbara, one in Bejing and another in Cambridge. These are wonderful research environments, with great programs bringing people together. But similar to the Perimeter Institute in Canada, and the Keck Telescope, these are funded by private endowments, rather than being solely funded by government. Any Australian philanthropists out there, it would be nice to have a similar institute here :)

Mapping Growth and Gravity with Robust Redshift Space Distortions

2012-02-10T18:53:00.000+11:00

A quick post this evening, as I have been at a workshop for the SAMI instrument, and am off to Santa Barbara for the First Galaxies and Faint Dwarfs: First Galaxies and Faint Dwarfs conference next week, but a couple of things to post. The first is a paper by my ex-phd student, Juliana Kwan, who is now a postdoc in the US at the Argonne National Laboratory.

The paper is quite complex, and focuses on redshift space distortions. This can be difficult to understand, but here goes. We've mentioned a couple of times that matter in the Universe is arranged on a cosmic web, with clusters, clumps, filaments and voids. In fact, it looks something like this:

Our Milky Way galaxy is just a little dot in there. But the detail of the way the mass is distributed is a probe of our Universe, as its present structure carries the imprint of the forces that created it, including the make up of the Universe, the cosmic evolution, and even the nature of gravity itself.

What do we see when we look out into the Universe? Well, we can measure the position to a galaxy on the sky very accurately, but distance is not. But we can easily measure the redshift, or the amount features in the spectrum are moving to longer wavelength, and use our cosmology to turn this into a distance using the famous Hubble law.

However, there is a problem. The redshift we see is a mixture of two parts, one due to the cosmic expansion (the Hubble law bit) and one due to the `peculiar velocity, or how much the galaxy is whizzing about. By comparing to the Microwave Background, we know that our Milky Way is moving with a speed of about 500 km/s.

As we measure redshifts, not distances, these peculiar velocities distort the distances we calculate via the Hubble law. So, this happens

The blue on the right is the actual positions of galaxies in the cosmic web (in a simulation of the Universe). The green on the left show the effects of peculiar velocity, and things are stretched and squished from the space position.

In fact, clusters of galaxies, where velocities are typically several thousands of km/s, get stretched out into what are known as Fingers of God - although what they have to do with the Higgs boson, I don't know (and no, that's not a serious statement). Here's a real set of observations;

Now, the details of these Redshift Space Distortions allow us learn even more information about the Universe, but it is very hard to untangle. What Juliana's paper does is to look at the possible ways that can be used to extract science, and shows what needs to be done if you want to get "robust" measures. I'll write more on on what robust means later, but for now, I'll finish by saying "Well done Juliana!"

Mapping Growth and Gravity with Robust Redshift Space Distortions

Juliana Kwan, Geraint F. Lewis, Eric V. Linder

(Submitted on 6 May 2011 (v1), last revised 3 Feb 2012 (this version, v2))

Redshift space distortions caused by galaxy peculiar velocities provide awindow onto the growth rate of large scale structure and a method for testinggeneral relativity. We investigate through a comparison of N-body simulationsto various extensions of perturbation theory beyond the linear regime, therobustness of cosmological parameter extraction, including the gravitationalgrowth index, \gamma. We find that the Kaiser formula and some perturbationtheory approaches bias the growth rate by 1-sigma or more relative to thefiducial at scales as large as k > 0.07 h/Mpc. This bias propagates toestimates of the gravitational growth index as well as \Omega_m and theequation of state parameter and presents a significant challenge to modellingredshift space distortions. We also determine an accurate fitting function fora combination of line of sight damping and higher order angular dependence thatallows robust modelling of the redshift space power spectrum to substantiallyhigher k.

Brydon and the Royal Australian Observatory

2012-02-05T11:44:00.000+11:00

Now, I love a good comedy (and a few bad ones) as much as the next person, but it's grant writing time, and so I am going to be grumpy here. One thing that gets me hot under the collar is terrible portrayals of science, especially astronomy and astrophysics, in the media. Which brings me to exhibit number 1. The TV comedy, Supernova, starring welsh comedian, Rob Brydon.

The story is simple. Brydon plays Dr Paul Hamilton, who moves to work at the fictional Royal Australian Observatory, and the laughs come thick and fast in the usual "fish out of water" situations. Well, they don't really, the series was a flop, and lacks a lot of laughs, but, amazingly, a second season was made.

I don't know how I missed this when it was originally shown in 2005/2006, but its portrayal of astronomy and astronomers is, well, rubbish.

I would write pages and pages on this, but let's start with some basic things.

The Observatory: You can tell by the image above, the Royal Astronomical Observatory is sitting there, out in the Outback, in the middle of nowhere (well, close to Broken Hill, where Mad Max 2 was filmed). I'm sure this was to get an Australian "feel" to the show, and make the fish-out-of-water more fish-out-of-watery. Now, you might want to build a radio telescope out there (and that's what they have done at Narrabri, although not so deserty). There is a real optical observatory in Australia, at Siding Spring, not that far from Broken Hill. What does that look like?

It's up a mountain, where you expect to find an optical observatory. Clearly, it's a bit too green for a fish-out-of-water welsh astronomer to be in. But it is what an Australian optical observatory looks like. And there are aboriginals living there too (but, of course, people want to see aboriginals living like wild warriors out in the desert rather than think about the real challenges facing these people).

The Telescope: This is the telescope at the Royal Australian Observatory (in the TV show).

The telescope is puny and many amateurs would have better. This is the mighty Anglo-Australian Telescope

Notice the scale on this image. I remember the first time I went into the dome, I thought I was in a James Bond set. It's huge, it's impressive, it's a real telescope! The show makes another cardinal sin with regards to astronomy. Astronomers don't sit around in the dome with lights blazing, computers humming etc when they are observing. The telescope is in total darkness, away from sources of light which would cause the air to shake and blur the images. The astronomers sit in control rooms, away from the telescope, often with music thumping to keep them awake. Given the nature of the site of the Royal Australian Observatory, a crappy telescope with lights on would seem to fit.

Doing science: Now, this is the bit that really drove me crazy. The show is filled with sciencey buzzwords, and Rob Brydon's character, an expert on gas, spouts all kind of stuff. But their crappy telescope can so magic, it can see round corners and to see patches of gas at the birth of the Universe. The notion of writing a paper, the staple of research, is all very bizarre, as is giving presentations etc. I don't know what more I can say about this, it just is nothing like real science.

OK, OK - I know it's a comedy show. I know I should not be so critical. But seeing that they were filming in Sydney, which has a high density of astronomers, perhaps they could have asked for some advice to look slightly like a real astronomer. Heck, they could have even filmed up at the AAT, and had a realistic telescope dome, one more impressive than the one they showed. It may not have helped the laughs, but it would have at least looked slightly more "scientific".

As an aside, I am a fan of Rob Brydon. In fact, he was raised in Baglan, next to Neath, the town I was born in. And as a link to science fiction, Baglan is part of Port Talbot, home to the Port Talbot Steel Works.

which was apparently the inspiration for Ridley Scott's vision of a future city in Blade Runner. I've see it at night, it's a startling sight!

The cosmic history of the spin of dark matter haloes within the large scale structure

2012-02-05T08:52:00.000+11:00

What's the goal of astronomy and astrophysics? Pretty Hubble pictures? Whacky statements about multi-verses and black hole entropy? Nope, not really. The goal is to understand the Universe in terms of the laws of physics.

Like "real" science, there is a strong link between theory and observation and experimentation, each feeding off each other, and as I noted a little while ago, the development of new instrumentation on telescopes often opens a new window on the Universe. Instruments like SAMI will let us do something new, namely measure the spins and chemistry of a large number of objects.

But now the key question - If you were going to measure the spins of a whole load of galaxies, what would you expect to see? Would they be randomly orientated? Would patches of galaxies spin together in unison? Would nearby galaxies spin in opposite directions? And how would you even begin to answer this question?

There are paper-and-pen approaches to answering this question, basically known as tidal torquing theory, which basically says that as the mass that forms galaxies starts to collapse, nearby masses tug unevenly on each other, causing them to start to spin.

The problem is that we know that galaxies don't simply form from essentially isolated collapsing masses. It is a lot messier, with galaxies crashing together, and small systems being cannibalized by larger galaxies, and what we end up with at the end of the day is a cosmic web!

We see clusters (bright yellow), dark voids and filaments connecting it all. Just how are galaxy spins aligned in this mess?

Well, you can just measure it (and the just in there does some serious disservice to how tricky this really is to do). You can find the collapsed masses, and look what they are doing and where they live, and measure what their spins are.

Minor interlude: Here is a fantastic little intro from Andrew Pontzen on what we actually do when we mean galaxy here.

PhD student, Holly Trowland, Joss Bland-Hawthorn and myself have just submitted a paper to The Astrophysical Journal doing such spin measurements on cosmological simulations, finding that there are a whole range of correlations of the spins with environment, with mass and over cosmic history. In fact, it shows that if you simply take the paper-and-pen tidal torque theory, it just fails when you get into the mess of the cosmic web.

One issue is that our simulations have been looking at only dark matter, the dominant mass in the Universe. But galaxies are made of atoms and we need to know how these spin within the spinning dark matter halos. This is trickier than it sounds, but Holly is working on it, and we should have some results later in the year. But for now, well done Holly!

The cosmic history of the spin of dark matter haloes within the large scale structure

Holly E. Trowland, Geraint F. Lewis, Joss Bland-Hawthorn

(Submitted on 30 Jan 2012)

We use N-body simulations to investigate the evolution of the orientation andmagnitude of dark matter halo angular momentum within the large scale structuresince z=3. We look at the evolution of the alignment of halo spins withfilaments and with each other, as well as the spin parameter, which is ameasure of the magnitude of angular momentum. It was found that the angularmomentum vectors of dark matter haloes at high redshift have a weak tendency tobe orthogonal to filaments and high mass haloes have a stronger orthogonalalignment than low mass haloes. Since z=1, the spins of low mass haloes havebecome weakly aligned parallel to filaments, whereas high mass haloes keeptheir orthogonal alignment. This recent parallel alignment of low mass haloescasts doubt on tidal torque theory as the sole mechanism for the build up ofangular momentum. We find a significant alignment of neighboring dark matterhaloes only at very small separations, r<0.3Mpc/h, which is driven bysubstructure. A correlation of the spin parameter with halo mass is confirmedat high redshift.

My Gyroscope won't fall down - I

2012-01-23T08:07:00.000+11:00

I love this video

and used to do this very demo when teaching classical mechanics. But here's a question for you - why doesn't the wheel fall over?

If you trawl the text books, even the wonderful Feynman Lectures on Physics (a must read for any serious student of physics), the answer given is that the wheel doesn't fall down because of the conservation of angular momentum.

Alas, I think this answer is a bit of a cop out, and doesn't answer the question. Why? Let's consider the collision between two cars. We know from Newtonian mechanics that momentum is conserved, so the momentum before the collision is exactly the same as after the collision (let's ignore external forces for now, imagine the collision is on a frictionless sheet of ice).

The conservation of momentum is a consequence of Newton's third law, and in the collision all of the forces acting have equal and opposite reaction forces, with the total momentum unchanged. Basically, considering the conservation of momentum lets you ignore all of the forces going on in the collision.

But if you are one of the car drivers, you care implicitly about the forces acting, as you would very much prefer a gentle force acting on you over a long period (as provided by an air bag) as opposed to a larger force over a short period (as provided when your head hits the dashboard).

The situation with the wheel is similar, as the action of the internal forces (well, torques) act so that the total angular moment is conserved. But really, to understand what's going on here, the question you should be asking is "what force is holding the wheel up?".

I know the answer, but would like to demonstrate it with a simplified model. Alas, the simplified model is not that simple, and it's going to take a few posts to get through, but basically I'm going to make a computer model of a wheel, spin it, let it go and look at where the forces are.

But firstly, a truth about the universe, namely that it is made from particles and springs

(taken from the excellent webpage of Paul Bourke, a place with excellent graphics advice). Now, this might sound weird, but you can represent physical material, and how they move etc, as a system of masses connected by springs. Check this out

and read the tutorial here. I wish I had realised this when I did my course on vibrations and waves as an undergraduate :)

So, my simplified version of a wheel will be four masses connected to an axle, and to each other, by springs. The forces in the springs will effectively represent tensions in the wheel. I'll add a force due to gravity (pointing downwards) and the force on each spring will be represented by Hooke's law. This simplified model already has 24 variables! Three position and three velocities (in 3-d) for each mass.

You can derive the equations of motion either using standard Newtonian forces, or a little more neatly using a Lagrangian approach, but I won't write the equations here, but will save them for another post.

So here's my basic wheel. All I've done here is stretch the springs and let the thing oscillate a little Don't forget that gravity is acting downwards, which is why it is asymmetric.

OK, we can remove the stretch. But how is this a wheel. Well, let's give one of the masses a tangential push. Let's take the black mass and push it upwards.

The net effect is that the entire distribution of masses starts to move, and the wheel is rotating. Of course, it looks a little springy and bouncy, but it's how a real wheel works; all the internal masses of the wheel are talking to one another through internal forces. If we tighten up the springs a little, we can get it to be less bouncy.

Excellent. Well, at the start! But then things go pear-shaped! What's happening? Integration errors, that's what! Basically, I am using a Michael-Mouse integration scheme for these initial tests (and Euler scheme for those in the know) and small errors build up rapidly. What we end up with is energy not being conserved and madness ensuing.

But we can fix this up with a better integration scheme. I'm going to leave that to next time :)

Anglo-Australian Bowie

2012-01-13T09:37:00.001+11:00

For a long time, I have been a fan of David Bowie, and have seen him in concert a couple of times. In the good old days (early 1980s), when MTV showed nothing but music videos, I would happily while away the hours on Bowie (and Queen, and just about everything else).

But as everyone knows, people grow-up and have to go to university, and MTV went the way of pathetic shows rather than showing videos, and so I didn't really watch music videos any more.

While holidaying last week, we happened to be chilling, and decided to watch a bit of Rage as they were showing a series of Bowie videos, and while watching Let's Dance, I had a "ehh - that's familiar" kind of moment. It was driven by this image

a sight what will be familiar to astronomers the world over. No, not the people in the foreground, or the nuclear explosion in the background, but the mountains!

Why? Because they are the mighty Warrumbungles, mountains in Northern(ish) New South Wales, near the town of Coonabarabran. Why would these mountains be familiar to astronomers? Well, Siding Spring Observatory, home of the Anglo-Australian Telescope (the largest optical telescope in Australia, a world-beater in surveys of the heavens, and where I used to work), overlooks the park.

The thing that caught my eye is not just that these are the Warrumbungles, but the mountains look virtually the same as seen from the catwalk of the telescope, or the look-out point at the edge of the observatory.

Here's a view from the base of the AAT

(the red thing is the case that the 3.9m mirror was carried to the observatory in), and here's another view

So, I got a little excited! What if David Bowie had visited Siding Springs (and the AAT); I know it's not really a lot to get excited about, but at least it was something to think about.

My hopes were a little dashed after a little detective work. The clip in the video was not filmed on Siding Spring mountain, but actually White Gum Lookout in the National Park.

But why are the views so similar. This map explains it all

The look out basically sits on the line joining Siding Spring Observatory and the lovely peaks of the Warrumbungles. Apparently Bowie visiting Coonabarabran in the early 1980s was big news at the time (but the person who told me this kept well away). Ah well, even if he didn't visit the AAT, David Bowie must have driven close by :)

Weightlessness

2012-01-11T00:14:00.000+11:00

Sorry for the delay, but I've been off on a family holiday, touring and camping in New South Wales (which doesn't look a lot like the old version), coupled with a trip over the border into alien territory, namely Queensland.

I was on the Gold Coast, a tourist mecca and home to a number of theme parks. As the father of little-cusps, we headed to DreamWorld to be thrown about a little. When I was riding the Cyclone....

I remember a post a long time ago by my ex-Honours and MSc student, Luke Barnes (who has just returned to an Australia as a Super Science Fellow), namely what does it feel like to be weightless.

Of course, what I mean by weightless is something like this,

namely some astronauts cavorting about in orbit, floating about and just having a good old time of it.

So, what does weightlessness *feel* like? Many have seen astronauts training, in their gear, in water tanks. They can bob about and look something like this

Doesn't that look nice and peaceful? I know they have a job to do, but bobbing about in water is relaxing. Look, lots of people do it!

Gee, astronauts have it easy.

But is that is what weightlessness really feels like? Well, no.

When you are bobbing about in a pool, even in an astronaut spacesuit, you *feel* gravity, and you know what direction it points in. Gravity pulls you towards the centre of the Earth. When you are floating "head-up", your guts are pulled down towards your legs, as is your blood and other fluids. There are force gradients across your muscles, and basically you feel the direction of gravity.

If you closed your eyes and someone gently rotate you onto your side, you would sense it. Your guts would slosh over and put different pressures on your body, as would your blood flow and the other processes in your body. You would know that your body's orientation with regards to the gravitational field has changed.

However, a spaceship orbiting the Earth is in free fall, continuously accelerating towards the centre of the planet, but getting no closer.

What is this free fall like? Well, those who have been on a roller coaster, or up-and-down road, know what free fall feels like. But let's look at what Einstein told us. Basically, he encapsulated this in the equivalence principle. Essentially, what this says is that for someone free falling in a gravitational field, the effects of gravity disappear (we'll ignore the subtleties of tidal forces).

What this means is that, without a gravitational field, your body has no clues on what way is up. Your guts hang there, not pressing down or up, or side ways, but just hang there. Your gravitationally induced stresses in your muscles vanish, your blood pressure gradients change. Basically, your body has no clue to up.

What it feels like is going over the top of the roller coaster with that unpleasant feeling of your stomach in you mouth. Or more like this ride

but instead of the feeling of falling for seconds, the sensation goes on for minutes, hours, weeks and months.

No wonder some astronauts throw-up when they get into space!! Floating it is not!

Ah Bottomium!

2011-12-26T15:00:00.001+11:00

When I was an undergraduate student, I had thought about becoming a particle physicist, but a summer school first at the Rutherford Labs (working on proton-anti-proton scattering) and then CERN (search for charged Higgs particles) beat that out of me :)

There is a lot of chatter about the Higgs out there at the moment, but unfortunately I think that a lot of it illustrated the poor understanding of statistics by journalists (and even scientists). p-values make me weep.

But I thought I would talk about something else, the the discovery of bottomium at the LHC. Funnily enough, it's not actually a new particle, and that's something I thought I would try and explain.

Let's start with a picture - what's this?

Of course, it's an atom. Well, except we know this is just a schematic picture of an atom, a nucleus with some electrons whizzing around. A real atom is more complex than this, being described by the laws of quantum mechanics. Electrons are not little particles on defined orbits, but are quantum wave functions that (if you want to try and visualize them) look something like this -

Chemists call these things orbitals.

One of the other things you remember from high school science is that each of these orbitals has a different energy level (some have the same, but let's not worry about this at the moment), and that when atoms absorb energy, the it changes energy levels, and this energy can lost through the emission of photons. For lithium, the energy levels look like this

These discrete transitions give us discrete lines in the spectrum of the elements.

Of course, spectroscopy is vital in many fields - in astronomy it allows us to calculate the chemical composition of distant stars and galaxies.

Now, it might seem we are going a little off topic, but let's keep going. What about the nucleus of an atom? We know that the nucleus is made of protons and neutrons, and that protons carry positive charge, while neutrons are, well, neutral. This leads to a bit of a conundrum, as that positively charged protons will repel each other - how does the nucleus hold itself together?

This is where the strong force comes in. It provides an attraction which overcome the repulsion of the electromagnetic force between the protons. But the nucleus, like the atom, is governed by the rules of quantum mechanics, and so has quantized energy levels. So we get a picture like this

Because the forces are larger, the differences between the energy levels are larger than those you see in atoms, but the nucleus can undergo transitions in the same way that an atom can. Here's an example

We start with Cobalt-60, which is radioactive, and emits an electron from it's nucleus. In doing this, it changes a neutron to a proton, and so becomes Nickel instead. On the right, we can see that it can emit an electron at either one of two energies, 0.31 MeV or 1.48 MeV, and this leaves the Nickel in one of two energy states. These states decay down into the ground state through the emission of γ-ray photons. So, γ radiation is nothing but nuclei undergoing transitions in the same way as atoms!

Now, one thing that some people don't know is that if we have a Nickel atom in its ground state, and one in a higher energy level, the one at the higher energy level is more massive than the one in its ground state. This, of course, makes sense. We know from special relativity that mass and energy are interchangeable, and so the excited nucleus has more energy and so more mass.

OK - finally, let's get back to the announcement of bottomium. Bottomium is a meson, meaning that it is make of a quark and an anti-quark, held together by the strong force. Looks something like this -

Now, there are only 6 different quark (up, down, top, bottom, charm and strange) and so there are only a finite number of combinations you can have to make mesons; the one above is a pion Bottomium is a quarkonium state, made of a bottom quark and an anti-bottom quark.

So, did the LHC discover bottomium in the previous week? Well, no. The ground-state bottomium particle was the η_b was discovered by the BaBar experiment in 2008. So what is the bally-hoo coming from the LHC.

Well, mesons (and they close companions, the baryons) obey the rules of quantum mechanics, and like atoms and nuclei, they have quantized states. They can be in their ground state, but if you give them energy, they can move into a higher energetic state. Just like nuclei, the more energy means more mass, and so the new particle, the χ_b(3P) is just the same old bottom-anti-bottom, but just in a higher energy state (in fact, the 3P gives it away, as those chemically minded would remember the P from SPDF). So, it's not really a new particle in the sense that it is made of something new, but is the same old particle but now with added energy. I guess that would not have been such an amazing press release :)

One last thing. As we've noted above, nuclei and mesons (and baryons) have more mass when they absorb energy and move to higher level. But the same must be true of atoms, and an excited hydrogen atom has more mass than an atom in its ground state. But the amount of energy held in an excited electron system is quite small compared to the overall atomic mass. Cool eh!

Note added in proof: Due to a convoluted history, the first excited bottomium state was discovered in 1977, with the ground state discovered in 2008. Here's the energy level diagram from the 2008 article above.

and while the press has been going on about bottomium, the correct name is bottomonium :)

What shape is the ocean on a cubical planet?

2011-12-20T13:37:00.000+11:00

Clearly, they are circular!

The question is, however, how do you calculate this?

The first step is to realise that the surface of a liquid is an equipotential surface. This means nothing more than the value of the potential energy over the surface is the same. This is easy to think about if you remember your classical mechanics as if there were differences in the potential energies in the water, so one bit of water was higher than the other, then that potential can be converted to kinetic as the water flows downwards. When could all this be static? When the surface of the water is at the same potential, so no one bit is "higher" (in terms of potential energy) than another.

So, to work out the shape of the ocean, we just need to calculate surfaces of the same potential energy. But how do we do that for our cubical world?

In the following, I've made a few simplifying assumptions. I assume my planet is not rotating (that adds a further set of energy terms) and that I can ignore the mass of the water (so ignore its potential energy). All I need to do is work out the potential energy of the cubical planet!

Many will remember from school that the potential energy of a point-like mass is given by

where M is the mass, G is Newton's gravitational constant and r is the distance away from the mass.

One thing that often confuddles students is that this expression diverges as r goes to zero. But what's the potential of an extended mass distribution? Well, then we need to add up all the little mass bits to give the total potential which, when we have a continuous mass distribution, we can represent as an integral. So the potential now looks like this

Let's not worry about the details, but those in the know will know that such an integral can be done quite efficiently as a convolution using Fourier transforms. If you are not in the know, just put it down to mathemagic.

Once we know the potential, we just plot up up the surface of the planet and the surface of the water (and equipotential) and voila. I choose to use povray for this.

Let's add a little more water. What do we get?

Notice that the water bulges out significantly. So what would life be like on the cubical planet? Well, it should be clear that the corner of the cubes would be like mountains, and so if you lived in the centre of a cube face, you would be able to stand upright, but as you walks towards the edge you would feel the orientation of the gravitational field changing. As you move away from the centre, it would feel that the climb was getting steeper and steeper, event though you are on an apparently flat plane!

But remember that the surfaces of the water are equipotentials, so you can happily sail over the ocean with little effort. In fact with a little more water, we can sail right around the planet. The view would be cool!

But what about other planets? What would the ocean be like on a dumbell planet? Easy

An uneven dumbell

Slab planet

The possibilities are endless :) I'll play some more with this in the future.

In closing, I just want to note that I didn't design the planetary textures in povray, but I have lost the reference to where I got it from (although it is called Cheap World). Thanks to the original author!

The Sydney-AAO Multi-object Integral field spectrograph (SAMI)

2011-12-18T10:40:00.000+11:00

When I say modern astronomy, what do you think of? Large telescopes peering into the sky, looking to unlock the secrets of the Universe? Of course, telescopes drive astronomical research, but what is often forgotten is the business end of the telescope, the instruments that collect the light, are the things that really define the science we can do. And such instruments can be extremely complex, and extremely expensive.

Here at Sydney we have a group working on astronomical instrumentation and a new instrument, The Sydney-AAO Multi-object Integral field spectrograph (SAMI), has recently be commissioned and the first paper has been accepted.

So, what does this new instrument do? Well, spectroscopy is an essential part of astronomy. Basically, it just means collecting light and dispersing it into a rainbow. Over to Pink Floyd

We don't use prisms any more (we use volume phase holographic gratings, which sounds much more science fiction). Looking at the light tells us lots and lots about the object we are looking at; the velocity, the chemistry, the star-formation history etc.

The problem has been that spectroscopy is that often you can only collect light from a single object at a time (long-slit spectroscopy). However, more recently we have been able to multi-object spectroscopy, using instruments like 2dF

Each little red dot here is little prism connected to a fibre, and so when pointed at the sky we can collect the light from lots of objects (and in this case, around 400 at a time).

The problem is that the prism is just a single `hole' and so if plopped onto a complex object with lots of structure, like a galaxy, will not see any of the complexity, it will just collect all the light in a smush.

But what if we want to get spatially resolved spectroscopy, to measure the chemistry and velocity from little bits of a galaxy? Enter SAMI!

What is it? Well, put simply, instead of single fibres, each fibre is made of a bunch of tightly packed fibres, and each collects light over a small patch of sky. You get a picture like this

This field is 1.6 arcseconds across and has been plonked on a galaxy. What we are seeing is the velocity field of the galaxy, measured from the Doppler shift (the numbers on the side are km/s, so we can see a clear rotation curve for the galaxy.

I've been working with a student on measuring cosmology by looking at the correlations between galaxy spins and where the galaxies live. So with SAMI (which is on the Anglo-Australian Telescope), which can collect the light from 13 objects at a time, we have the prospect of surveying a large number of galaxies and actually undertaking the measures we propose. Exciting stuff!

But as I said at the start, the first SAMI paper is accepted for publication, with the SAMI team as authors and Scott Croom as head author. Well done Scott!

The Sydney-AAO Multi-object Integral field spectrograph (SAMI)

Scott M. Croom (1 and 2), Jon S. Lawrence (3 and 4), Joss Bland-Hawthorn (1), Julia J. Bryant (1), Lisa Fogarty (1), Samuel Richards (1), Michael Goodwin (3), Tony Farrell (3), Stan Miziarski (3), Ron Heald (3), D. Heath Jones (5), Steve Lee (3), Matthew Colless (3 and 2), Sarah Brough (3), Andrew M. Hopkins (3 and 2), Amanda E. Bauer (3), Michael N. Birchall (3), Simon Ellis (3), Anthony Horton (3), Sergio Leon-Saval (1), Geraint Lewis (1), A. R. Lopez-Sanchez (3,4), Seong-Sik Min (1), Christopher Trinh (1), Holly Trowland (1) ((1) University of Sydney, (2) ARC Centre of Excellence for All-sky Astrophysics, (3) Australian Astronomical Observatory, (4) Macquarie University, (5) Monash University)

(Submitted on 14 Dec 2011)

We demonstrate a novel technology that combines the power of the multi-objectspectrograph with the spatial multiplex advantage of an integral fieldspectrograph (IFS). The Sydney-AAO Multi-object IFS (SAMI) is a prototypewide-field system at the Anglo-Australian Telescope (AAT) that allows 13imaging fibre bundles ("hexabundles") to be deployed over a 1-degree diameterfield of view. Each hexabundle comprises 61 lightly-fused multimode fibres withreduced cladding and yields a 75 percent filling factor. Each fibre corediameter subtends 1.6 arcseconds on the sky and each hexabundle has a field ofview of 15 arcseconds diameter. The fibres are fed to the flexible AAOmegadouble-beam spectrograph, which can be used at a range of spectral resolutions(R=lambda/delta(lambda) ~ 1700-13000) over the optical spectrum (3700-9500A).We present the first spectroscopic results obtained with SAMI for a sample ofgalaxies at z~0.05. We discuss the prospects of implementing hexabundles at amuch higher multiplex over wider fields of view in order to carry outspatially--resolved spectroscopic surveys of 10^4 to 10^5 galaxies.

Gravitational Waves and the Wild Wild West

2011-12-16T17:41:00.000+11:00

I started this post at Perth airport, but the wireless was too slow to complete it. So here goes (again).

I'm back in Sydney after a few days in Perth. I gave a talk at ICRAR and the The Australian International Gravitational Research Centre at the University of Western Australia. Perth clearly is a boom town, with plenty of money flowing from the mining (a bottle of house white cost $58!), and it still has a slight feel of the wild-wild western - the police were touring the lounge where I was sitting at the airport as, as you can imagine, several hundred miners were heading out on a Friday afternoon and, apparently, there has been trouble in the past.

I had several really interesting meetings in Perth, especially with regards to writing grants in the next ARC round (which has come round really quickly again). I'll write about those at a later date. On Thursday I was heading up to Gingin, north of Perth, for a BBQ at the Gravitational Wave Centre, and I was invited to take a look inside the research part of the centre. So, here's some piccies.

This is the view from the top of the 13 storey leaning tower (and it's quite an exciting climb!). The big building off in the distance is the research centre, and the smaller buildings are the ends of the arms of the interferometer (the length is about 80 m).

The tower leans do you can recreate Galileo's mythical dropping of canon balls from the leaning tower of Pisa. No canon balls here tho, only water filled balloons.

So, what does gravitational research, at the experimental end look like. Here's some examples.

There's a lot of plastic sheeting, clean rooms, optical benches, vacuum chambers etc. The optical benches are amazing as they look chaotic, but each optical element has a purpose, and the laser light makes its way through all of them.

The thing about research labs (in physics at least) is that they often look like a nightmarish mish-mash of cables and equipment, dewers, liquid nitrogen, helium tanks, random computers, etc, and there will often be people squirreling about in various corners apparently oblivious to what is going on in the rest of the lab. But amazingly, it is in this environment that great advances and discoveries are made. Neatness gets you nowhere (at least that is the excuse I use to explain my office).

They have a pretty cool visitors centre at Gingin, with lots of hands on things for kids to crazy with. But one thing caught my eye.

On the wall is the Einstein equation. To me, it is barse-ackwards as I would want to calculate the gravitational field (G) from the mass and energy distribution (T), but being gravitational wave research, where they want to detect G, they then want to calculate T (the source of the gravitational waves). Cool eh!

Scary monsters (and supermassive black holes)

2011-12-10T10:29:00.001+11:00

A quick post this morning as I have spent a few hours plumbing in a new dishwasher (the previous one decided it would like to be like the Nile, and so flooded every so often), and have a children's party this afternoon. But, I've had a new article published on The Conversation titled Scary monsters (and supermassive black holes).

It's a review of the discovery of the most massive supermassive black holes yet. As I note in the article, the discovery itself is not such a surprise as we know that there is a well known relation, the M-sigma relation which shows that larger galaxies have larger black holes (the astronomers in the article weren't just blindly looking for black holes - they knew where to look).

Also, in the article, I touch on another article in The Conversation called Black holes might exist, but let’s stay sceptical, which was in response to a previous article I published on black holes. This article seemed to suggest that astronomers believe black holes are real, and so think that there is little point to supporting experiments to test general relativity, especially the search for gravitational waves. Here's a quote from the article;

"And, hence, you’re less likely to support gravitational wave astronomy. General relativity predicts unique patterns of the gravitational waves produced in collisions between event horizons."

As I noted in the comments, this is completely wrong. Any perceived lack of support is purely financial, not scientific. In fact, I wrote

"There is a finite pot of money, and astronomy is big science. If you ask an optical astronomer which should I fund, the Square Kilometre Array, LIGO or the next generation of optical telescope, then the response will be "in a perfect world, with an infinite amount of cash, funding all would be excellent, but given that you have asked me to choose, I will support the one that has the direct impact on my research, the optical telescope", and I am sure you will get different answers from the radio astronomers, and, of course, the gravitational wave astronomers."

In fact, astronomers are searching for the signal of gravitational waves using pulsar timing, wanting to attempt to snatch a potential Nobel from the LIGO teams.

Finite pots of money are the source of many problems, but, after mentioning Nobel prize winners, I just want note that Brian Schmidt of the ANU, one of this years winners of the physics Nobel prizes, is donating some of his prize winnings for a primary school science program the federal government has stopped funding. What an exceedingly noble thing for a Nobel to do. Well done Brian!

More dark matter shenanigans

2011-12-07T08:25:00.001+11:00

The internet is alive again with another cry that dark matter is dead (again) and the slashdot-eratti are getting themselves into the usual lather and "DM is BS" claims.

I've written my views on slashdot commenting previously, and will not reiterate them here, but will comment on the paper and the "meaning" of dark matter to astronomers.

OK. The paper can be read here and here's the press image that goes with it.

The picture is correct. If we just considered the gravitational attraction of stars, then their rotation speed should follow the red curve, but when we measure it (and we can measure it far outside the stellar disk by looking at the rotational velocities of HI gas) it actually follows the white curve.

So, the big question is why? The prevailing hypothesis is that there is more mass there than we can see, i.e. dark matter. But others suggest that dark matter is not there, and there is some other influence, usually by modifying the laws of physics (i.e. MOND) which accounts for the extra acceleration which is needed to give the larger speeds.

The current paper suggests that the extra acceleration comes from the attraction of mass in the local universe *outside* of the galaxy. Those that remember their classes on Newtonian physics will remember that if you sit inside a spherical shell of mass, you do not feel any gravitational pull from the mass. But this paper says "well matter is not smooth but is lumpy" and the author, Carati, tries to calculate the influence of this lumpiness.

This is all quite legit, but it turns out the calculations are very difficult, so instead of directly calculating the influence, he calculates some average effect. What happens is the "average" effect modifies the gravitational attraction in galaxies and so looks like this

It's the second term that modifies the gravitational attraction due to this average effect. Take a couple of rotation curves of galaxies, and viola

The left-hand panel has the data (the dots and error bars) and the effect of two components; the disks is due to the mass we see, and the halo is the influence of dark matter. The right-hand panel is the result of this new model, which only has the disk we see and then the correction due to the cosmological mass. Excellent. Let's have a press release!

But, hold your horses. Is everything as excellent as it seems? Well, no. Firstly, we knew we could get this model to work in some galaxies as it basically looks extremely similar to the mathematical form of MOND, and we know that works in some galaxies (and so, in some sense, we knew the answer beforehand). And remember that we have taken some sort of averaging effect, rather than calculating the actual effect, and, as it is stochastic should be stochastic and I can't imagine that it will give a smooth influence on the galaxy.

Are the religious, dark matter zealots up in arms, calling for Carati's head for daring to suggest that the god of dark matter may not exist? Well, no. While the majority of scientists will be nowhere near convinced by this one paper, Carati is free to do whatever research they want to, and I am sure they know that if they want to convince us that their model is viable, the maths needs to be worked out, and then they must show that their model explains everything that dark matter does; from gravitational lensing, big-bang nucleosynthesis, hot gas in clusters etc etc.

Again, scientists don't believe in dark matter. Currently all the evidence points to a material substance explaining what we see out there in the Universe, and so people are heavily weighted into using this particular model. But if we woke up tomorrow and some one has convincing proved that dark matter as a substance can be conclusively ruled out, there will be no wailing and gnashing of teeth, and most scientists will say "oh, that's interesting! what more does that tell us about the Universe". Science will move on. And I am sure the Slashdotters will tell us "we told you so!".

How many tanks?

2011-12-03T13:22:00.001+11:00

The German tank problem is a fav of mine. The wikipedia page on it is a little long winded, but I think it can be looked at a lot faster with a little numerical mucking about.

The problem is quite simple. The enemy are producing tanks, and each has a sequential serial number (for simplicity, let's assume that the numbers are reset every month). You encounter this scene on the battle field;

and we see that this is tank number, say, 15 of a particular months production. How many tanks were produced in that month? Can we even answer the question?

This is the problem that faced the Allies in WWII; you really wanted to know how many panzers are out there. Intelligence officers were reporting productions of more than a 1000 tanks per month, but based on statistics, the predicted number was significantly fewer than that, in the hundreds. After the war, the numbers were checked against records and the statistical answer was amazingly correct (read the wikipedia page for more details).

But let's see the how we can calculate this. We'll adopt the Bayesian approach (because that's the correct thing to do it :). So, let's assume that the number of tanks actually made is a number N, and let's assume that we guess that the maximum number of tanks that could be possibly be made is M (we'll insert some real number in here soon).

On the battle field, we find a tank with a serial number, A. What is your estimate of the number of tanks made in that months (let's call this X)? We want to make a probability distribution, and where this peaks, this is our best estimate for the number of tanks build.

Clearly, the minimum number of tanks is A (because you have the serial number you have). What about the rest of the probability distribution? If you think about it, if the total number of tanks is X, then the probability of randomly selecting tank A is simply 1/X. So the probability distribution look like this

This is the case where the actual number of tanks produced was 274, and the maximum we think they could produce is 1000, and the serial number of the one tank found was 217. So the most likely number of tanks is 217, but there is still a lot of probability that there could be 900 or 1000.

Now for the cool part. You hear a report that another tank has been knocked out, this time serial number 91. You might think that tells you nothing new, as you know the minimum number is 217, but 91 has a similar probability distribution to 217, and to get the resultant distribution for the total number of tanks you multiply these together.

I've brushed over some of the key Bayesian words and concepts here, but this is basically what it boils down to; we get more evidence and we update our beliefs. So, what's the result of now finding tank 91? The result is the red curve below.

Notice that the most likely number of tanks is still 217, but knowing 91 as well has really started to suppress the numbers up near 1000.

Reports come in that three more tanks have been knocked out, 256, 248 and 61. What's the resultant distribution look like?

Again, each of the blue curves is the probability distribution for each tank, whereas the red is the total. Notice that the peak is now at 256, and the chances of more that 600 tanks being produced per month is pretty small, and 1000 is negligible.

Report come in of 5 more tanks, number 250, 172, 189, 29 and 170. What's the distribution now?

For clarity, I've left out the blue curves, but you can see that with just 10 tanks, we know the number produced is more than 256, but quite probably less than 400.

We can continue to play this game, and with 25 tanks knocked out, we get

Notice that I've changed the scale on the x-axis. We can be quite confident that less than 300 were made.

Now I think that is cool. And that's how information should be used.

The Beast with Four Tails

2011-12-02T16:09:00.001+11:00

The Sagittarius Dwarf galaxy is a pretty cool object. It was discovered by a very close collaborator of mine, Rodrigo Ibata of the Strasbourg Observatory, way back in the ancient past (well, 1993), and it is clearly a little galaxy in trouble. It's orbit brings it dangerously close to the Milky Way, where its tidal gravitational pull is ripping it apart.

During its destruction, stars have been continuously pulled from the dwarf and are now wrapped around the Milky Way. These tidal streams are really interesting, and can be used to work out how the dwarf has been pulled apart. More importantly, if we can work out the orbit of the streams, then we can measure the amount of dark matter surrounding the Milky Way, a very important thing to do.

But look at this

This is a map of the sky in coordinates where the tidal stream of Sagittarius wraps around the equator. You can clearly see the Milky Way galaxy. The colour are fields from the Sloan Digital Sky Survey where stars in the halo of the Milky Way have been isolated.

A couple of years ago, when there was less Sloan data, the right-half of the image was called the field of streams. Here's the patch

and it was clear that the stream of Sagittarius seemed to split into two. This is exceedingly weird, we really didn't expect it to do this, and none of the models we had predicted this. What the new data does is double the trouble as we now see that the forked stream continues into the South.

So, we have an object like this

a real Beast with Four Tails. Honestly, this is very bizarre, and as of yet we have no real explanation on why it looks like this. I remember when Sagittarius was discovered, and it was going to tell us what the dark matter halo of the Milky Way looked like, and it would all be wonderful, but what has happened is that every observation seems to make thinks more and more complex. But it keeps us in a job :)

A new paper on this, written by Sergey Koposov and Vasily Belokurov, and me, has been submitted for publication. Well done Sergey and Vasily!

The Sagittarius Streams in the Southern Galactic Hemisphere

Sergey E. Koposov (1,2), V. Belokurov (1), N. W. Evans (1), G. Gilmore (1), M. Gieles (1), M. J. Irwin (1), G. F. Lewis, M. Niederste-Ostholt (1), J. Peñarrubia, M. C. Smith, D. Bizyaev, E. Malanushenko, V. Malanushenko, D. P. Schneider, R. F. G. Wyse ((1) Institute of Astronomy, Cambridge, UK, (2) Sternberg Astronomical Institute, Moscow, Russia)

(Submitted on 30 Nov 2011)

The structure of the Sagittarius stream in the Southern Galactic hemisphereis analysed with the Sloan Digital Sky Survey Data Release 8. Parallel to theSagittarius tidal track, but ~ 10deg away, there is another fainter and moremetal-poor stream. We provide evidence that the two streams follow similardistance gradients but have distinct morphological properties and stellarpopulations. The brighter stream is broader, contains more metal-rich stars andhas a richer colour-magnitude diagram with multiple turn-offs and a prominentred clump as compared to the fainter stream. Based on the structural propertiesand the stellar population mix, the stream configuration is similar to theNorthern "bifurcation". In the region of the South Galactic Cap, there isoverlapping tidal debris from the Cetus Stream, which crosses the Sagittariusstream. Using both photometric and spectroscopic data, we show that the bluestraggler population belongs mainly to Sagittarius and the blue horizontalbranch stars belong mainly to the Cetus stream in this confused location in thehalo.

Probing planetary mass dark matter in galaxies: gravitational nanolensing of multiply imaged quasars

2011-12-01T08:27:00.001+11:00

One of the big questions of modern astrophysics is "what is dark matter?" Of course, we all know that there is a possibility that it is not a material substance at all, and the extra gravitational influence we need to explain observations may be due to a modification in the laws of physics, but it being matter is the simplest of hypotheses and it seems to work very well (but, of course, to the media and rabid slashdotters/internet trollers, we are nothing but religious fanatics pushing our wheelbarrow of dark matter, blinkered to the geniuses out there!).

If it is a material substance, then we have ruled out a number of candidates (in terms of stellar mass black holes etc), and the weight of evidence is pointing towards a subatomic particle. As the Universe evolves, dark matter clumps together to make dark matter halos within which galaxies like our own Milky Way form.

How small do dark matter halos get? Well, some think it continues down to planetary mass scale. If it does, out galaxies dark matter halo is made of a myriad of small mass halos whizzing about. How do we test if they are actually out there?

One method is to search for their gravitational lensing effect. The problem is that studying the effect is very difficult, as the number of little masses along the line of sight to a distant source is huge, and computing their influence is very hard.

Luckily, one of my PhD students (who recently submitted his thesis), Hugh Garsden, is a computing expert and he developed a brand new supercomputer code to address such a problem, and, with postdoc, Nick Bate, we just had a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society.

The paper look at nanolensing, the effect of small masses in galactic halos on our view of distant quasars. To do that, we need to calculate a magnification map, tracing the paths of billions of rays through hundreds of millions to billions of lensing masses. Here's an example of the maps we produce

The little wibbles and wobbles in the map are due to the effect of the small masses. We can use these to work out how the brightness of a source will fluctuate as these masses move in front of it. You end up with the follow

As you can see, the presence of the small masses cause quite rapid, small time-scale fluctuations which, if you try hard enough you can actually observe. Of course, if we don't see them, then we can rule out this kind of dark matter clumping, and science can move on.

Well done Hugh and Nick.

Probing planetary mass dark matter in galaxies: gravitational nanolensing of multiply imaged quasars

H. Garsden, N. F. Bate, G. F. Lewis

(Submitted on 29 Nov 2011)

Gravitational microlensing of planetary-mass objects (or "nanolensing", as ithas been termed) can be used to probe the distribution of mass in a galaxy thatis acting as a gravitational lens. Microlensing and nanolensing light curvefluctuations are indicative of the mass of the compact objects within the lens,but the size of the source is important, as large sources will smooth out alight curve. Numerical studies have been made in the past that investigate arange of sources sizes and masses in the lens. We extend that work in two ways- by generating high quality maps with over a billion small objects down to amass of 2.5\times10-5M\odot, and by investigating the temporal properties andobservability of the nanolensing events. The system studied is a mock quasarsystem similar to MG 0414+0534. We find that if variability of 0.1 mag inamplitude can be observed, a source size of ~ 0.1 Einstein Radius (ER) would beneeded to see the effect of 2.5\times10-5M\odot masses, and larger, in themicrolensing light curve. Our investigation into the temporal properties ofnanolensing events finds that there are two scales of nanolensing that can beobserved - one due to the crossing of nanolensing caustic bands, the other dueto the crossing of nanolensing caustics themselves. The latter are very small,having crossing times of a few days, and requiring sources of size ~ 0.0001 ERto resolve. For sources of the size of an accretion disk, the nanolensingcaustics are slightly smoothed-out, but can be observed on time scales of a fewdays. The crossing of caustic bands can be observed on times scales of about 3months.

Morphogenesis

2011-11-27T13:33:00.001+11:00

I've been interested in chaos since reading Gleick's book back in the 1980s. I was first introduced to fractals when I was a summer student at the Rutherford Labs in the late 1980s; this was when colour printers were rare and expensive and I spent a lot of time convincing the guardian of the printer that printing out large colour fractals for my bedroom wall was essential for my studies of proton-anti-proton scattering.

But that's another story.

A little while ago, I caught an excellent documentary called "The Secret Life of Chaos" by the equally excellent presenter, Jim Al-Khalili. This linked a lot of topics, including chaos and complex systems, which is when a group of things following simple rules results in complicated (and sometimes difficult to predict) behaviour.

One of the key things I learnt was the importance of this man

in some of the earliest work in the field. I'm sure a number of you recognise him as Alan Turing. I first came across him in his work on cracking the Enigma code in World War II, and then discovered his founding work on modern computing.

But it turns out that in 1952 (and this is what I learnt from the Chaos doco), he published this paper

and it's an amazing look by a mathematician into the world of biology. I am not an expert in this area, or a historian of science, but I know of Schrodinger's attempt to talk about information in genes in his book What is Life?; Schrodinger was wrong in many aspects, but I like the fact that he was thinking about these questions.

Turing's paper addresses a very interesting question, namely how does a complex being like a human, arise from a pretty uniform and symmetric sack of cells soon after conception. How do cells know to become liver, heart, nerve and skin cells? What triggers the process?

My reading of Turing's paper (and I need to read it in more depth) is that the feeling of the day was that any inhomogeneities in the sack of cells would diffuse out, and the sack would remain essentially featureless. What Turing did, however, was to create a mathematical model for such a sack. It was not intended to be strictly biological, and much of the paper discusses an unrealistic ring of cells, but the fact that he could write down equations means he could start to predict what happens to the cells.

His basic model was that the local chemistry decides what a cell will do, and chemical can diffuse through the ring, from higher to lower concentration. So far, so standard, as you would expect the net chemistry of the sack of cells to be effectively uniform.

But then the stroke of genius. Get the chemicals to interact with one another, and get the reaction rates to depend on the amount of chemical locally. What happens? As noted by Turing, things do not just settle down to a uniform, but you get waves and pulses of chemicals moving through the cells.

This is taken from Turing's paper

which is the 2D distribution of chemicals in one of his simple models. As pointed out, this patten doesn't look too different to

and perhaps cows patterns, or that of a leopard, zebra or my old Jack Russell, are simply the indication of the chemical mathematics in the skin of the forming animal.

Anyway, it was pouring with rain yesterday morning, and so I thought I would try and make a simple Turing model. I decided to make a ball of cells, laying them out using HealPIX to put them on a sphere (this means each cell occupies the same area and so I don't have to worry about correcting for that). I chose three chemicals (let's call them Red, Green and Blue) and allowed each cell to have effectively the same amount at the start, except for a few overdoped cells.

After this, I allowed diffusion between nearest neighbour cells, and also put in interaction terms at each point, depending on the amount of chemical in that cell. Of course, this is just a simple set of differential equations, which takes little time to code up in MatLab.

Without the interaction terms, the ball of cells becomes a uniform bland colour, but once we have the interaction terms turned on, cool things start to happen. I actually dumped the output of MatLab into a rendering format (PovRay, fiendishly difficult - but free) and make some pictures. So, this is just one output,

Pictures are nice, but we want to see some action. So, here's a couple of movies I made

I must admit that I cheated slightly and used the Lorenz equations to ensure I got chaotic behaviour, but with the diffusion terms as well we get to see the chemical signatures pulsing through the cells. Well, I like it.

Now, some might say "why bother?" To you, my monomath cousins, I say that the answer is that it is interesting and continues to teaching me things beyond my area of expertise, and new skills that I might need to use one day. Anyway, there are worse ways to spend a rainy Saturday morning :)

The Star Formation History and Dust Content in the Far Outer Disc of M31

2011-11-23T20:18:00.001+11:00

You wait for a bus, and then two come along at once.

Postdoctoral researcher, Edouard J. Bernard, working with Annette Ferguson at Edinburgh's Institute for Astronomy, and myself, has had his paper on the star formation history in a couple of fields observed with the Hubble Space Telescope. Before continuing, I want to say that the IfA has the best, thickest custard in the entire world!

The focus of the paper is deep Hubble Space Telescope fields in the outer parts of the Andromeda galaxy. The absolutely wonderful thing about Hubble is that being above the atmosphere, we can accurately measure the brightness of faint stars, but the annoying thing about Hubble is that the field o view is tiny. Here's the fields we got

The grey area is the sky that we've observed as part of the PAndAS program with Canada-France-Hawaii Telescope, whereas the tiny squares are the bit covered by Hubble; all telescope time is quite competitive, but getting lots of Hubble time is difficult (although the PHAT team is getting much of the disk of M31).

So, what can you do with these observations? Well, you can make a fantastic picture like this

These are colour-magnitude diagrams, extremely powerful tools in astrophysics. I should note that the F606W and F814W are the filters on the Hubble Space Telescope, and these deep images get way down into the stellar populations. Here we can clearly see the Red Giant Branch (RGB) and the Red Clump. Overlaid are theoretical isochrones which allows us to unravel lots, like the star formation history of each of the regions looked at.

After a significant amount of work, you end up with pictures like this

that chart the star formation history and element enrichment over time. Great stuff! What is so fantastic is that in this history we see a burst of star formation when M33 and M31 last interacted, a few billion years ago. Isn't astrophysics simply wonderful?

Well done Edouard & Annette!

The Star Formation History and Dust Content in the Far Outer Disc of M31

Edouard J. Bernard, Annette M. N. Ferguson, Michael K. Barker, Sebastian L. Hidalgo, Rodrigo A. Ibata, Michael J. Irwin, Geraint F. Lewis, Alan W. McConnachie, Matteo Monelli, Scott C. Chapman

(Submitted on 22 Nov 2011)

We present a detailed analysis of two fields located 26 kpc (~5 scalelengths)from the centre of M31. One field samples the major axis populations--the OuterDisc field--while the other is offset by ~18' and samples the Warp in thestellar disc. The CMDs based on HST/ACS imaging reach old main-sequenceturn-offs (~12.5 Gyr). We apply the CMD-fitting technique to the Warp field toreconstruct the star formation history (SFH). We find that after undergoingroughly constant SF until about 4.5 Gyr ago, there was a rapid decline inactivity and then a ~1.5 Gyr lull, followed by a strong burst lasting 1.5 Gyrand responsible for 25% of the total stellar mass in this field. This burstappears to be accompanied by a decline in metallicity which could be asignature of the inflow of metal-poor gas. The onset of the burst (~3 Gyr ago)corresponds to the last close passage of M31 and M33 as predicted by detailedN-body modelling, and may have been triggered by this event. We reprocess thedeep M33 outer disc field data of Barker et al. (2011) in order to compareconsistently-derived SFHs. This reveals a similar duration burst that isexactly coeval with that seen in the M31 Warp field, lending further support tothe interaction hypothesis. The complex SFHs and the smoothly-varyingage-metallicity relations suggest that the stellar populations observed in thefar outer discs of both galaxies have largely formed in situ rather thanmigrated from smaller galactocentric radii. The strong differential reddeningaffecting the CMD of the Outer Disc field prevents derivation of the SFH.Instead, we quantify this reddening and find that the fine-scale distributionof dust precisely follows that of the HI gas. This indicates that the outer HIdisc of M31 contains a substantial amount of dust and therefore suggestssignificant metal enrichment in these parts, consistent with inferences fromour CMD analysis.

Conservation Laws

2011-11-23T10:44:00.001+11:00

With exam marking and committee meetings, it has been a slow week, but here's a pop quiz.

What does this woman

have to do with this video?

(the video has a nasty crack at the end of it, and so don't watch if squeamish). The answer is not that the woman in the photograph is the girl in the video.

Of course, what we are looking at here is a collision, and due to the use of the yoga balls, it is a pretty elastic collision, and so energy is almost conserved (and if you account for the energy that goes into heat and noise, it's completely conserved).

But what we all remember from our high school physics is that the thing called momentum, the sum of mass times velocity, is always conserved in collisions, and so if, before the collision, we take the mass of the boy and multiply it by his velocity, and then do the same for the girl, and then add the two quantities together, this sum is the total momentum. If we do the same after the collision (assuming no external forces), the total momentum remains the same (this is why it is a conservation law).

But you knew that. It still, however, leaves a question. Why is the sum of mass times velocity conserved? Why isn't the sum of mass divided by velocity conserved? Some of you will undoubtedly think this is a silly question, and the answer is "because a teacher told me so".

This brings us back to the woman in the top picture. Some of you will recognise her as Emmy Noether, and she is one of the most important (and yet unknown) people in physics (even though in reality she was a mathematician).

What's the link? Well, Noether discovered something amazing. She discovered that symmetries in physics give us conserved quantities. Without getting into the technical details, what she discovered is that if the laws of physics here are the same as the laws of physics over there (translational symmetry) then the quantity we call momentum (strictly linear momentum) must be conserved.

Also, if the laws of physics remain the same if I rotate myself around (rotational symmetry), then we have a quantity known as angular momentum conserved. And, if the laws of physics are the same in the past and future as they are now (temporal symmetry) then we have a quantity called energy that is conserved.

Ohh!! You might say, that's interesting. But then you might say that isn't energy conservation a bit obvious. I mean, clearly "energy cannot created or destroyed" is obviously true. After all, you were told it in school, and they would never lie to you.... would they.

This is where it starts to hurt a little. Emmy Noether's original work on conservation laws considered a Newtonian universe, and Newtonian laws of mechanics. Things were pushed into 4-D space-time to accommodate special relativity and electromagnetism. This is cool as we now have quantities such as relativistic energy (and kinetic energy) which are conserved. Furthermore, there is a symmetry you can pull out, there is no change in the laws of physics is you shift the phase of the quantum mechanical wave-function, which implies that charge is conserved.

The last one will, I'm sure, annoy some. Charge being conserved is obvious, electrons are little balls and charge is a little minus side painted to their point-like side. But alas, no. Without the appropriate symmetry, charge would not have to be conserved.

In fact, symmetries in the quantum groups give us lot of other quantities which are conserved in interactions.

What about non-conserved things? Well, there are. Mass does not have an associated symmetry and is not conserved in interactions (i.e. an electron and a positron annihilate to give two photons - mass is not conserved, but momentum, spin, energy etc are.)

Things get really hairy when we consider general relativity. Space-time in special relativity is the same everywhere and everywhen, and so we have the outcomes of physics experiments being the same and symmetries and conserved quantities we are happy with. But in general relativity, the underlying geometry of space-time changes with position and time and so the symmetries we were happy with previously are gone, and so are the conserved quantities.

The one that gets most people is the expanding universe and the loss of the time-symmetry; the space-time in the future will be different to the space-time now. No t-symmetry, no conservation of energy. Energy is not conserved in an expanding universe.

Photons lose energy as they travel through the universe. Where does that energy go? The equations of relativity tell us; nowhere! Don't worry, energy is not conserved in an expanding universe.

High speed masses slowly grind to a halt, as the universe expands, with respect to the the local environment. Where did all that energy go? Nowhere! Don't worry, energy is not conserved in an expanding universe.

Don't just listen to me. Listen to Sean Carroll over at Cosmic Variance. Energy is not conserved in an expanding universe.

I know some of you will not like this, as the conservation of energy is clearly obvious, and makes perfect common-sense. All I can say is that common-sense is a pretty poor guide to the last 100 years of physics.

Black hole noms: planetary treats for the galactic monster

2011-11-09T07:24:00.000+11:00

I didn't write the title, and had to check the dictionary on what a nom is (and was surprised that it was actually a word), but I wrote a brief article for The Conversation

It summarizes a paper by Kastytis Zubovas of the University of Leicester on the continual burps of energy from the black hole at the centre of the Milky Way. You can read the original paper here

Sgr A* flares: tidal disruption of asteroids and planets?

Kastytis Zubovas, Sergei Nayakshin, Sera Markoff
(Submitted on 31 Oct 2011)
It is theoretically expected that a supermassive black hole (SMBH) in thecentre of a typical nearby galaxy disrupts a Solar-type star every ~ 10^5years, resulting in a bright flare lasting for months. Sgr A*, the residentSMBH of the Milky Way, produces (by comparison) tiny flares that last onlyhours but occur daily. Here we explore the possibility that these flares couldbe produced by disruption of smaller bodies - asteroids. We show that asteroidspassing within an AU of Sgr A* could be split into smaller fragments which thenvaporise by bodily friction with the tenuous quiescent gas accretion flow ontoSgr A*. The ensuing shocks and plasma instabilities may create a transientpopulation of very hot electrons invoked in several currently popular modelsfor Sgr A* flares, thus producing the required spectra. We estimate thatasteroids larger than ~ 10 km in size are needed to power the observed flares,with the maximum possible luminosity of the order 10^39 erg s^-1. Assuming thatthe asteroid population per parent star in the central parsec of the Milky Wayis not too dissimilar from that around stars in the Solar neighbourhood, weestimate the asteroid disruption rates, and the distribution of the expectedluminosities, finding a reasonable agreement with the observations. We alsonote that planets may be tidally disrupted by Sgr A* as well, also veryinfrequently. We speculate that one such disruption may explain the putativeincrease in Sgr A* luminosity ~ 300 yr ago.

Basically, what he is saying is the energy is released as small objects, planets and asteroids, as ripped apart on their final death plunge into the black hole.

This is not a new idea, as I remember people suggesting that such flares could be due to comets etc being ripped apart, but he has a little twist on the story that some of these objects are effectively recycled from objects that were smashed in the frenetic orbits close to the black hole.

Anyway, you can read my article here - questions and comments below :)

How was the Universe born?

2011-10-31T09:11:00.000+11:00

Just a quick note to say that I had an article published in Australasian Science on the birth of the Universe. As you can see, I also got the cover. Unfortunately, you have to pay to subscribe, but I think this appears in many Australian schools. Anyway, here's the abstract

Modern cosmology tells us that the universe as we know it arose 13.7 billion years ago in the fiery birth of the Big Bang, but our understanding of the laws of physics is incomplete and we are currently unable to answer the questions of where the universe actually came from. Cosmologists have many ideas, ranging from the reasonably strange to the extremely outlandish.

Gravitational Lensing with Three-Dimensional Ray Tracing

2011-10-26T15:00:00.000+11:00

One of the cool things about the universe is that light rays don't travel in straight lines. As they pass through the cosmos, lumps of mass (stars, galaxies, clusters, black holes etc) tug on the path of light rays and so they follow a wiggly path.

It looks something like this

The colours here represent density in the universe, where yellow is high density, purple middling and black low density, and you can see the cosmic web of mass which has come from a computer simulation of structure formation.

The green line is a light path travelling through the universe, and as you can see, it wiggles.

Here's another version of the picture

The result is that view of the distant universe is distorted, and a considerable focus of future telescopes is to measure the amount of distortion that we see. This will allow us to measure a couple of key things, namely the distribution of matter (which is good, because a lot of it is that pesky dark matter that we can't see), and also the underlying cosmology (and so will be a probe of dark energy).

But to understand all of this, we need some theoretical models to compare to the observations. How do we do this? Well, it's not easy to follow light rays and typically people use what's known as the multi-plane approximation. It's easy to visualize - you take your continuous mass distribution and chop it into chunks, and then squash the chunks onto flat planes. Light rays then travel in straight lines between the planes, but as they pass through a plane, they feel the mass in the plane and get deflected.

This looks something like this

which I took from the paper by Hilbert et al.

But the question is, is this a good approximation. People generally shrug and go "I think so".

I'm please to announce that my ex-student, Madhura Killedar, who got her PhD earlier this year and is now a postdoc in Trieste, has just had one of her thesis papers accepted where she tests this, by comparing the multiplane method with a more "correct approach", actually integrating the geodesic equation through the simulations.

The task should not be underestimated, as it took three years of her PhD to do this. There are lots of technical issues which I will not go into here, but involved big simulations, Fourier transforms, multi-dimensional integrals, resolution scales etc etc, So this paper is the first of a series presenting her thesis work. This one got a pretty sweet referees report too :)

I encourage you to have a look. Well done Mud!!

Gravitational Lensing with Three-Dimensional Ray Tracing

Madhura Killedar, Paul D. Lasky, Geraint F. Lewis, Chris J. Fluke

(Submitted on 21 Oct 2011)

High redshift sources suffer from magnification or demagnification due to weak gravitational lensing by large scale structure. One consequence of this is that the distance-redshift relation, in wide use for cosmological tests, suffers lensing-induced scatter which can be quantified by the magnification probability distribution. Predicting this distribution generally requires a method for ray-tracing through cosmological N-body simulations. However, standard methods tend to apply the multiple thin-lens approximation. In an effort to quantify the accuracy of these methods, we develop an innovative code that performs ray-tracing without the use of this approximation. The efficiency and accuracy of this computationally challenging approach can be improved by careful choices of numerical parameters; therefore, the results are analysed for the behaviour of the ray-tracing code in the vicinity of Schwarzschild and Navarro-Frenk-White lenses. Preliminary comparisons are drawn with the multiple lens-plane ray-bundle method in the context of cosmological mass distributions for a source redshift of $z_{s}=0.5$.

A sad C day

2011-10-17T22:19:00.000+11:00

I understand a lot of people were moved by the death of Steve Jobs, but IMHO a gianter (if that's a word) person of the computer world also died last week. Sadly, Dennis Ritchie died on the 12th October.

While Steve gave us shiny toys (and yes, I am typing this on a MacBook, and I have an iPad), Ritchie gave us (well, me) some of the vital tools of the trade. With Ken Thompson, he developed the best operating system in the world, namely

Yes, UNIX. When I started my PhD, we used VMS on VAXes, but these were superseded by these beauties -

Sun machines running SunOS, my first experience of UNIX. Soon after, we had the birth of penguin, and we could have real computing in the home with Linux.

In the old days, we were spending a fortune on maintenance contracts for Vaxes and Sun machines, so the prospect of running a free proper operating system on cheap hardware was very appealing (and no, windows of any variety is not a proper operating system - good for games, but not for my research).

People realised that that you can string computers together to build cheap supercomputers and computational astrophysics forged ahead. Many of the spectacular cosmological simulations were run on such supercomputers.

But if that was not enough, Ritchie give us one more thing, the C programming language. I'm going to moan and gripe like an old man that students today don't know how to code, and perhaps they can get away with Python, but I'm a fan of basic languages, C and fortran, for coding in supercomputer environments (and I'm not going to get all religious on programming language).

However, I started to learn C from Ritchie's own book

While I now realise this is an excellent book, it is not the book to learn C from. Many times I ended up shouting at the book "Why!!! Oh why doesn't it work!!" Ah, the joy of being a research student. Thank you Dennis Ritchie :)

Faster than the speed of light

2011-10-10T08:58:00.000+11:00

After a weekend of great rugby (from the Welsh, the English and Australians were rather blah), I have responded to a good question posted over at The Conversation on distances in the Universe. It's a question that gets raised quite a bit and basically put it goes something like

If the Universe began 13.7 billion years ago, when the distance between any pair of points was zero, how can anything be more than 13.7 billion light years away?

The answer is the difference between local motions and global motions. Here's the response I posted

An excellent question, and one which may not make sense to start with. We know from special relativity, nothing can travel faster than light (recent neutrino claims excepted). But in reality, special relativity says that nothing can go faster than the speed of light **locally**, so in a small box, if I try and race an electron and a photon across the box, the photon will win.

With the expanding universe, the question we are asking is a little different. We are asking how fast something is moving "over there", rather than locally. It turns out that, if you crank the handle of general relativity, that something over there can be moving faster than the speed of light here. But anyone in the universe who does the local test on the speed of light, by racing photons and electrons, will always find that the photons will win.
If you think about it, what it means that, relative to the speed of light here, light out there is moving faster than the speed of light.

This is a very important point. It seems that everyone takes the statement from special relativity, namely that you can't travel fast than light, and then tries to apply it into a global picture. But that is not correct.

One key feature of the curved space-time of general relativity is that you map any patch into the flat space-time coordinates of special relativity, where you only need to worry about the physics of special relativity (and gravity vanishes). At this point, all massive objects sit within their future light-cones - which basically means their local motion is less than the speed of light.

But there is no simple way to compare a patch here to a patch there, and talking about how fast something is moving "over there" really does not have a unique answer in relativity. It depends on the coordinates you choose to us (and if something depends on coordinates, it ain't physically observable).

Here's an excellent example (taken from Tamara Davis's website).

The dotted lines are are the motions of objects. In the top one, in "physical" distance, the more distance objects are very "tilted over" and are moving faster than the speed of light locally. But then again, light out there is moving fast than the speed of light locally.

Moving down the pictures, the bottom one is in comoving coordinates, and now distant objects are stationary with respect to us. How fast a distant something is moving with respect to us depends on which coordinates you choose.

It's actually messier than that. I can chose to mix up time and space into new coordinates, and this demonstrates that superluminal verses subluminal motions get even more confusing. But I won't write about it here, but let you have some reading.

Coordinate Confusion in Conformal Cosmology

Geraint F. Lewis, Matthew J. Francis, Luke A. Barnes, J. Berian James

(Submitted on 13 Jul 2007)

A straight-forward interpretation of standardFriedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies is that objects moveapart due to the expansion of space, and that sufficiently distant galaxiesmust be receding at velocities exceeding the speed of light. Recently, however,it has been suggested that a simple transformation into conformal coordinatescan remove superluminal recession velocities, and hence the concept of theexpansion of space should be abandoned. This work demonstrates that suchconformal transformations do not eliminate superluminal recession velocitiesfor open or flat matter-only FRLW cosmologies, and all possess superluminalexpansion. Hence, the attack on the concept of the expansion of space based onthis is poorly founded. This work concludes by emphasizing that the expansionof space is perfectly valid in the general relativistic framework, however,asking the question of whether space really expands is a futile exercise.

Accelerated Expansion

2011-10-05T09:28:00.000+11:00

The Science Blogosphere is going to be full of comments on the winning of the 2011 Nobel Prize by Perlmutter, Riess and Schmidt, and so I am not going write a lot on this topic, other than to say that I know Brian well and I am very happy for him. The fact Riess is younger than me suggests that I may not be personally on the Nobel Prize trajectory :)

Anyway, I quite like the Nobels. Not the prize itself, as it rewards individuals from what are typically large groups, but guessing who the next winner will be is fun.

This year I was correct, and I have evidence over at Uncertainty Principles, and it looks like I win a prize myself :)

Still in the Dome

2011-09-29T17:41:00.001+10:00

I'm still at Kitt Peak, on the 5th night of a 6 night run at the Mayall 4-m Telescope. Observing sometimes make me think of life in a nuclear shelter, post-apocalypse. No windows, no daylight, being very tired and drinking lots of coffee, which is not good given the nearest toilet is two floors down, down a very nuclear-shelter-like steel stairway. When the mighty dome moves, it's like another nuclear strike. In the control room, it's a little dark, with music thumping, and science being done.

Bed at about 6am with sleep to the early afternoon, followed by an hour or two of absorbing Sun-shine. Today we had a trundle around Kitt Peak to see the other telescopes. Here's a couple of snap-shots:

After a little instrument failure, we're back on the sky, but one thing I think people don't know is that we typically finish the night when it is still dark outside. Why? Well, even when the Sun is well below the horizon, it still lights up the sky and, while imperceptible to the human eye, there is enough light to swamp out detector. So, we normally push things as hard as we can (15 minutes into astronomical twilight) and then call it quits.

Of course, we still have a 4-m telescope and a wide-field camera (with a 36'x36' field of view) to play with, and while we can't do science, we can still get some pretty images. Yesterday, we decided to go after the Crab Nebula, with exposures of no more than a few seconds. We got three bands (B, V & R), which let's you make a false colour image of the Crab, and this is what I got.

The colours aren't perfect because I had to tone down the blue (for the aficionados, I didn't flat field the blue and so adjusted the colours so we wouldn't see the results), but you must admit, it's rather pretty :)

Long way to the chemist’s: a rough guide to distances in the universe

2011-09-28T11:38:00.001+10:00

My article, Long way to the chemist’s: a rough guide to distances in the universe, was published in The Conversation (in fact, it's lead article :). Following on from my article on stars, I was asked to think about distances in the Universe.

My guiding idea is the marvelous quote by Douglas Adams from The Hitchhiker's Guide to the Galaxy;

"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space."

I've spend quite a bit of time thinking about scale in the Universe, and remember animated discussions as a young PhD student with a good colleague on the question of "If the Milky Way was a 1p coin, how far away would the edge of the [Observable] Universe be?", and the distance (of order a couple of km) always struck me as being exceedingly close!

Also, in these articles, while you can convey the relative distances, it's hard to really demonstrate the underlying structure to the Universe. Things aren't thrown about randomly, but stars, of course, live in galaxies, and most galaxies live in groups. As an example, here's the Hickson Group

Remember, those are pretty large galaxies in there, with many hundreds of billions of stars. The Milky Way is no different, sharing this bit of the Universe with Andromeda and more than 70 dwarf galaxies. Here's a schematic of the Local Group

You can see that the Milky Way and Andromeda are the centres of attention, surrounded with a little bunch of dwarf galaxies, with a few more scattered throughout the Local Group.

As we move up in scale, then we get bigger agglomerations of galaxies, into spectacular clusters. This is Abell 1869

The orange blobs are galaxies. Unlike the Milky Way, which is a spiral, these are generally elliptical, and it is thought they get this way through repeated collisions. If they started off as spirals, all of that structure is lost and we are left with a blob of stars.

Of course, this is the future of our Milky Way, as in roughly three billion years it will collide with Andromeda. John Dubinski of Toronto shows us what will happen

These groups and clusters are not randomly scatter through space, but sit together embedded in a cosmic web. One of the great Australian astronomical successes is the 2dF Galaxy Redshift Survey which charter the positions of almost a quarter of a million galaxies using the 2dF spectrograph on the Anglo-Australian Telescope and produced a map of the (relatively nearby) structure of the Universe.

What did they find? Here's on of their maps (and look at the distance scale)

And the cosmic web is revealed. Isn't it wonderful!

But science is more than just making pretty pictures. What do our theories predict? I don't have time to describe the various steps involved, but here's a patch of a simulated universe taken from Shaun Coles' (of Durham University) website

I am always amazed by how good the match is. On large scale, our ideas of the cosmos seem of work fine. Where they start to wobble is on the smallest scale. But more about that in another post.

With regards to my The Conversation article, I spent quite a bit of time ensuring the scales are correct, which can be tricky to do when you juggle metres, with light years and giga-parsecs, but, of course, that's what it's all about.

Not everyone is concerned with accuracy. Case in point, the new background for Mac os X is a lovely picture of a galaxy I will be observing again, namely the Andromeda Galaxy. Here's Mac's picture:

Quite lovely. Until you compare to a "real" picture of Andromeda, such as this

by Rob Glender and we see that Mac have excised one of the lovely dwarfs orbiting Andromeda, namely NGC 205 (also known as M110). Maybe they thought that users might think it's a smudge on the screen, or maybe it just isn't pretty enough, but not everyone seems to be that interested in accuracy.

Intermission: View from the 4m

2011-09-25T16:47:00.000+10:00

After a rushed week, it was time to hop on a plane and head for the US. After passing through LA Airport (not my favorite, especially given the time taken to fingerprint me and the several hundred people who arrived with me, but the passport checking guy was friendly and chatted about astronomy - holding up the hundreds behind me :), I got into Tucson yesterday where it was 37C.

Today was the drive up to Kitt Peak, through very stark desert, with amazing cacti and people seemingly living in the middle of nowhere. Definitely cooler, the view from almost 7000 ft over the desert is very cool. Here's the view from the bottom of the 4m Mayall Telescope, overlooking the rest of Kitt Peak.

The clouds, of course, aren't exactly a good sign, and we've lost a little time this evening to them, but things look like they will be running a soon. I will write a little more about the science later in the week.

Before going back to the clearing skies, a couple of interesting things about the trip so far.

Firstly, it was very interesting switching from the Qantas AirBus 380, with its hundreds of passengers, to the much smaller American Eagle plane, with less than 40 seats, for the LA to Tucson leg.

And secondly, heading out of LA, we flew over the sea, and, as I had a window seat, I gazed at the huge city and the boats on the ocean. We were still climbing and one boat looked a bit weird, sort of long and blue, but with bits sticking out at the back. I wondered a bit more, and as it dawned on me that I was looking at, it broke the surface and there was a gush of water vapour. Now, I'm not say it was a blue whale (but it was blue), but apparently they being seen off the coast of California.

Blue whale or not, it was pretty cool.

Peer Review: The Fallacy of Fine-Tuning

2011-09-12T07:44:00.001+10:00

A quick post today, as I am running behind on a couple of things, but I have had a book review published in The Conversation. The book is The Fallacy of Fine-Tuning: Why the Universe Is Not Designed for Us by Victor Stenger, and is on a topic I have thought a lot about over the year, but never published on, namely "why are we here, in this Universe"?

For those of a religious inclination, the answer is apparently obvious, the Universe was fine tuned to be habitable. For those grounded in a more scientific approach to understanding the Universe, the answer is not so clear and straight forward. If we start to play around with the fundamental physical parameters, such as the strength of forces etc, it appears we end up with mainly dead Universes.

Heck, even messing with the content of the universe, in terms of the amount of matter, ends up with universes that collapse too quickly, or expand so fast that stars (and hence people) cannot form.

I think it is safe to say that most astronomers (and physicists) don't really think about this stuff too deeply, and there aren't a lot of papers out there on the topic. Why? Well, I think in the first case there is the "fringe" feeling to the entire Anthropic Principle, especially some of the comments about it in the book by Barrow and Tipler

while the string theorists think it's all in the bag with the idea of the multiverse

But I am not particularly satisfied by it all, although this book does demonstrate that some of the claims of fine tuning are not as fine-tuned as originally thought. In fact, the author has a little web interface called MonkeyGod which allows you to make your own universe and see if you will get stars and heavy elements (the ingredients of life).

In summary, this is something we should probably give some serious thought to, and this book is not a bad place to start.

Could physics predict a giraffe?

2011-09-09T09:18:00.000+10:00

I have a cosmological post brewing, so I thought I would touch on a slightly different topic, namely the question of "could physics predict a giraffe?" The following has the usual "buyer beware" clauses; I am a physicist, an astrophysicist at that, and not a chemist, or a biologist, and definitely no a philosopher of science, although I may end up annoying all of them. To start with, let's look at the subject, to wit, a giraffe.

The reason for the post is because of an article over at The Curious Wavefunction titled Why biology (and chemistry) is not physics. The basic argument is this;

Physics is a fundamental science, and identified the basic workings of the Universe. How do nuclei hold themselves together, how does the Universe expand, why do electrons flow through conductors etc etc. That's physics. Now, physics is "reductionist", in that all complex processes can be broken down into a the application of relatively simple underlying physical laws. An example; the thing that is the source of friction, which stops your car when you apply the breaks, and the force stopping you from falling to the centre of the Earth, both the electromagnetic force, the same force that gets electrons to flow through wires. And, of course, with just gravity and electromagnetism, we can explain the physics of everyday experiences.

However, apparently biology and chemistry are somehow different, and that if you try to reduce biological and chemical processes to the basic bits and pieces (i.e. into physics) you somehow lose something.

The issue with the article that kicked this off is that, even given all the basic rules of the Universe, the laws of quantum mechanics and general relativity, physics would be unable to "predict a giraffe". Why? Because somewhere along the line, there were random events (that mutated the genetic code of proto^n-giraffes) and physics just would not take this into account. The final paragraph of the article hammers home the point:

"This role of contingency and accident is one of the most important reasons why the reduction of chemistry and biology to physics won't work. In addition as I have described before, reductionism cannot account for variety in chemistry. Yet another reason why chemistry and biology are not physics."

The "variety in chemistry", after a little reading, suggests that chemists have broader intuition based on experience, and that is lost when you try and reduce chemistry to basic physics.

Well, I grew up in the country and often worked on farms, and I can tell you what all of this smells like (to me). Before I continue, an intermission.

Right, I'll start with something controversial. There is no such things as physics, chemistry and biology. Well, of course we have departments of Physics, Chemistry and Biology, and people who label themselves as physicists, chemists and biologists, and even journals dedicated to further, ultra-fine refinements of subsets of these areas. But where is the boundary between chemistry and physics, or chemistry and biology?

Take a look at this image

The caption for this image reads "A pictureof the simplest ionchannelknown - antibacterial gramicidin A. It conductsmonovalent cations at near diffusion rates, causing collapse of themembrane potential and killing hapless bacteria. HereGramicidin A(helical dimer in red) is embedded in lipid bilayer (only the phosphatehead groups in green are shown) and solvated with a KCl solution (Kblue, Cl red and water is in the background). Because of itssimplicity, gramicidin offers an ideal channel structure for testingnew methods and ideas."

Now, is this biology, chemistry or physics? It has a number of buzz-words from all fields, but it is actually research being done by the Computational Biophysics group at the University of Sydney. Why is this physics? Because it is being done in a department with a big sign saying Physics across the front door, by a person who labels themselves as a physicist as they did a degree in a different large building with Physics written across the door.

You don't have to look far to see that, at the edges, these subjects are blurred, and the labeling of physics, chemistry and biology can seem a little arbitrary. But people like to put things in boxes with rigid walls, even then the walls do not exist.

Back to the matter in hand. As ever, xkcd makes a deep comment on the issue;

Note, mathematics, especially pure mathematics, is not science. But this encapsulates some of the key points (although, as I pointed out, there are not rigid divides between the "fields") and even touches on physics envy.

So, could physics predict a giraffe? I think here it is important to realise that much of science is the science of complex systems, where lots of simple underlying processes interact to produce a more complex outcome. Some people think complex systems are somehow magical, but they aren't really, they produce unexpected results, yes, but the underlying processes are simple. Also, some seem to think that because a system is complex, it is somehow unpredictable. Again, this is not really the case. If we know the initial conditions well enough, we can evolve a system using the simple rules and see what happens (and yes, I do know what chaos is).

One of my fav complex systems, cosmological n-body simulations.

Lot's of little masses, following simple rules, producing complex outcomes. What's the limiting issue? Computational power. More computational power, the more things we can follow, the more detailed outcomes we can determine.

When we break it down, a giraffe really is just a bunch of fundamental (i.e. physical) processes interacting in a complicated way. With enough computational power, we could simulate all the processes going on in a giraffe, and hence an entire giraffe. With enough computational power, we could simulate the evolution of a giraffe, including all of the random possible events that happened along the way, and as well as all the giraffes that exist, we could find all the other animals that could have evolved instead of giraffes.

Now, I'm not saying this is computationally easy. We definitely don't have the computational power today to do this, and we may never really have it (it may take an actually universe to compute such things), but fundamentally it is possible.

I'll say it again, even if you hate reductionism, all processes, at the end of the day, however complex, boil down to physical interactions. You may wave your hands and go on about the variety of chemistry and complexity of biology, but this does nothing but reinforce the imaginary walls between the fields. Sure, you have intuition and ideas guided by your experience, but physics is at the basis of everything. If you don't believe this, then at some point you have to inject magic. And that just isn't science.

OK. On a lighter note, seeing we are talking about giraffes, why do they have such long necks? I'm sure you'll say something about them browsing the leaves high in the trees, but surely giraffes have long necks because their legs are so long, and without it, they would not be able to drink?

2D Universe - Calculting the force

2011-09-03T12:27:00.000+10:00

You'd think as a teacher of relativity, I would understand time a little better, but I seem to have little clue to where it all goes every week (luckily Sean Carroll over at Cosmic Variance points out that time is a more slippery customer than you may expect).

It's been a little while, so I thought I would catch up on my 2D Universe. Those who have been following closely will have seen that we have derived our equations of motion over the surface of a sphere, and now all we need at the acceleration terms. This is where it starts to get a little sticky.

The first part is the easy point. If you remember, we want a gravitational-like force, and this depends on the distance between the two objects. Now, again, there is more than one way to skin a rabbit (is there?) but I am going to take the computationally simple approach.

Any point on a sphere is denoted by our two coordinates, (θ,φ); remember, it's a 2D surface, so no radius to worry about. But let's pretend it's a unit sphere, so we have r=1, then we can convert these polar coordinates to Cartesian with the usual transformation. Matlab has an inbuilt function for doing this, sph2cart, although one has to be careful with which angle is which (remember, there is some ambiguity in the definition of which angle is θ and which is φ). So, any point on the sphere gets mapped to a Cartesian point, (x,y,z).

Now, we can treat these coordinates as components of a unit vector, and then all we need to do is to take the dot product of two such vectors to give

The angle, ξ, is just the distance between the points on the sphere. Excellent!!

So, in my 2D spherical universe, the strength of gravity varies as ξ^-1 (unlike gravity in our 3D universe which goes as the inverse square of distance). However, given we are on a sphere, I have made two components of the force, one on the shortest distance between two point, which is just ξ, and one on the longest, which is 2π - ξ. Why? So, if I have two objects at rest at opposite poles, then there is no net force acting and they just sit there.

Now, as I said, the magnitude of the force is the easy bit. The hard bit is working out which direction the force points in. Hmmmm. This needs a little more thought, but the key thing is that we are asking for the bearing you must set off from point one to travel to point two. Of course, jolly old navigators sorted this one out long ago.

There a lots of approaches, but I want a vector bearing, so I followed this little set of recipes, which work very well. The good thing is that you end up with a vector, W, in a tangent plane, which is, as the name suggests, a plane which is tangent to the surface of the sphere at the point of interest.

The bad part is that, because I was working with Cartesian coordinates, I now have a 3D vector with components in (x,y,z), but what I really need is components in the angular coordinates on the sphere. Here I call on the magic of tensors, especially the rules that let you convert from one coordinate system to another, so my acceleration in angular coordinates is related to that in Cartesian coordinates via

Those in the know will recognise this as the Jacobian. Looks messy, but how do we know this has worked? Well, our vector is in the tangent plane, and so a^r should be zero (there should be no vector pointing in the radial direction). And there isn't, so it is all wonderful!

So, that's it. We now have our gravitational attraction in the polar coordinates of the sphere, so that completes the equations of motion. We integrate and we get

Wonderful! (Well I think so). Right, I think this is now done and dusted, and I promised to get back to zombies soon. I'll also put together some notes on the shapes of oceans on non-spherical worlds, but that's for later.

Relativity Homework

2011-08-21T14:20:00.000+10:00

A busy weekend, so here is some weekend thoughts (and associated reading) for you. It's all to do with relativity and are linked with some blog posts.

Time does not flow: To paraphrase Shakespeare, All the world is a 4D manifold, and all its players are but worldlines. Basically, General Relativity tells us that the Universe is a 4D canvas and our paths are traced out. It's all there, past, present and future (from any individual's perspective). The Universe does not unfold as we progress into the future. It's all already written out (like a path on a map). Enter the philosophers and discussions on freewill, but taking relativity at face-value, there isn't any. Heck, it doesn't even tell us which direction our experience of time flows in, we set that with a decision on dt/dτ.

Energy is not conserved: In general, energy is not conserved in General Relativity. This one usually freaks out the students, but it caused Einstein a whole host of headaches as well. When the Universe expends, photons are redshifted. They "lose energy". Where does this energy go? You can do some groovy handwaving and say "into curvature", but the answer is "Don't worry about it, energy is not conserved".

Space does not expand: This is one I have been involved with for a long time, and, again, it freaks people out, especially with the wealth of popular science and textbooks that talk about expanding space. You have to be careful here, and a paper I wrote a number of years ago now, Expanding Space: the Root of all Evil?, discusses this in some detail. It comes as a shock to students of relativity to realise that, almost a hundred years after the discovery of the expanding Universe, that the meaning of the expansion is still a matter of debate. I'll close this with a quote from Steven Weinberg and Martin Rees, reported in New Scientist a few years ago;

Popular accounts, and even astronomers, talk about expanding space. But how is it possible for space, which is utterly empty, to expand? How can ‘nothing’ expand?
“Good question,” says Weinberg. “The answer is: space does not expand. Cosmologists sometimes talk about expanding space – but they should know better.” Rees agrees wholeheartedly. “Expanding space is a very unhelpful concept,” he says. “Think of the Universe in a Newtonian way – that is simply, in terms of galaxies exploding away from each other.”

Has physics become cool again?

2011-08-20T08:11:00.001+10:00

It's Friday, it's pouring with rain, and time for a grumpy post.

The BBC is running a story Has physics become cool again?. While the overall story is good news, that the number of people people doing A-Level physics in the UK is on the way up, it also high-lights what I see as a big problem, basically the media's portrayal of physicists.

Words like "nerd" and "geek" to generically describe scientists makes my blood boil, as does their portrayal as socially inadequate, Star Trek obsessed, comic-book reading dweebs. Case in point:

Apparently "smart is the new sexy".

And our own Beauty and the Geek had this bunch

Not to mention Nerds FC. In fact, each year we get an email circulating around our physics department that Beauty and the Geek are looking for new contestants, but from physics they are only looking for geeks, not beauties.

Harmless fun you may say. But is it? We moan about falling numbers in science, and in physics, the number of women in science is a continual source of concern. We know that the teaching of science in school can be problematic, being too boring or too much work, but what message does the above send to kids?

(Pop psych warning) My feeling is that they say that to be a scientist, you must wear the geek/nerd badge. That you must be "uncool". No wonder young people would rather be lawyers or accountants (are there Big Bang Theory equivalents of these?). The portrayal of women in Beauty and the Geek is not particularly flattering either.

The BBC article continues with some mixed messages, with the "Cox Effect" (who can coolly stare from a mountain top in some remote location while make a deep comment about the Universe) inspiring a new generation of physicists. But there is also there is a comment from the excellent Professor Jim Al-Khalili (he of the fantastic Atom tv-series) apparently stating

"The geeks are on the march again!”

A quick read of Al-Khalili's wikipedia page points out that he

"has been a supporter of Leeds United football club ever since the Revie days of the early seventies"

a football tragic and very un-geeky passtime. I wonder if he means other people are geeks?

In truth, physicists are people who do physics. A look at the lives of physicists reveals the same angsts, woes, joy and excitement as most other people. It doesn't take a lot of detective work to find out the very human activities of Schrodinger and Feynman (or even Einstein for that matter), as well as the tragedies in the lives of Boltzmann, Curie and Ehrenfest. They were people, doing physicsy things, but living peopley lives.

I'm not saying that there are not some nerdy and geeky people in science - there are. But there are nerdy and geeky people in all professions and walks of society, and there are very non-geeky and non-nerdy people in science (such as Cox and Al-Khalili). So, here's the new slogan:

and let's have a few more positive portrayals of physics in the media, doing the kind of things that we do best (like calculating the influence of evolving dark energy on the recent expansion history of the Universe), while hosting cool and sophisticated cocktail parties (well, perhaps at least a fun BBQ and a game of triv). Maybe this will have a positive influence on young people considering a career in science?

I'll finish on a footnote; in a couple of weeks, I will be attending Speed Meet a Geek in the city. When invited to participate, I said I was not interested due to the geek label (which, apparently, is a term of endearment rather than derision), and now my invite says I will be at Speed Meet a Scientist. However, it's still being advertised as Speed Meet a Geek to the kids; Sigh, I'll be sure to wear my polyester trousers, brush up on my Star Trek trivia, and bring some Spiderman comics along......

Dark matter may be an illusion

2011-08-14T16:18:00.007+10:00

Again, with teaching, PhD reviewing, public talks etc, things have been very busy, but should calm down a little in the next few weeks. So, again, a brief post this week. An as I am feeling grumpy, I'm going to comment on

Is dark matter an illusion created by the gravitational polarization of the quantum vacuum? (original paper here).

We are all aware that we don't know what dark matter is, but there are a couple of things we do know; 1) we know that we need something to explain the way we observe the Universe 2) that something may be something material (like a dark matter particle, or left over black holes) or even a modification to our laws of physics, such as modified Newtonian Dynamics or (cue weird music) leaky dimensions, or other such stuff.

Now, I will say this again. We know this. IMHO, most astronomers don't worry too much about this and they go with the simplest assumption, that dark matter is just that, matter which is dark. Most papers report very simple numbers, such as a Mass-to-Light ratio, or if there are more direct probes, such as gravitational lensing, you may get the shape of the dark matter.

But if someone said tomorrow that Dark Matter is definitely not material, but is something else, something weird, most people would say "OK" and get on understanding their results within this framework. I don't think people will be really shocked.

So most astronomers don't really bat an eye-lid at papers like this one. They are quite common, maybe dark matter is this, or dark matter is that, or unicorns, or whatever. But they often claim to explain everything, but have very few (if any) predictions that can truly differentiate the new idea from plain old vanilla dark matter. And so on we carry, quoting ML and other simple quantities.

To be clear, I am not attacking this current paper. The author, at the end, states that;

"In conclusion, we have revealed the first indications that what we call dark matter may be consequence of the gravitational repulsion between matter and antimatter and the corresponding gravitational polarization of the quantum vacuum by the existing baryonic matter. Of course, this is not a claim, just possibility. A lot of work would be needed before such a claim would be eventually possible."

Basically, the paper is one of a myriad of speculations of what dark matter might be.

No, my reason for posting is what happens when this stuff gets into the media. Of course, the speculation often gets firmed up in the press release. And then the discussion on blogs etc begin. Just check out SlashDot! The first comment is

"I hope so. Dark matter is the ugliest kludge to the standard model ever."

and so on and on. Apparently,

"Average Slashdotters have been more skeptical of they dark matter theory than physicists, from what I've seen."

This is the kind of comment that really gets to me. No, what is actually happening is that average slashdotters like ideas with lost of mysterious sounding, but actually extremely skeptical, ideas, than the simplest working model.

It's actually the opposite of Occam's razor, the wish to employ the most complex and whizz-bang over the straight-forward. Of course, this actually happens in science, when we have to let go of old ideas and adopt new ones, but things like quantum mechanics were not adopted because they were whacky and out-there, but because the weight of observational evidence crushed classical ideas.

Currently, all observations are consistent with dark matter being just that. When we have the observational evidence that does not agree with this simplest of hypotheses, then it will be discarded. And this will not happen until the alternatives can tell us precisely what tests we need to do to single out their theories as the best. Today is not the day that this is going to happen.

Anyway, looking over the comments, it's clear that many slashdotters actually understand this. But to the others, this is for you;

They might be giants: a mind-blowing sense of stellar scale

2011-08-05T13:23:00.000+10:00

Well, after saying that I am not much of a star person, my article on the sizes of stars was just published in The Conversation. It's titled They might be giants: a mind-blowing sense of stellar scale and covers the size of stars from red dwarfs to hypergiants. The mini-cusps appear in the article as well :)

For a quick review, I fully recommend this video on YouTube;

It makes my head hurt a little every time I watch it.

A New Candidate Magnetar Located Outside the Galactic Plane

2011-08-05T09:36:00.000+10:00

Magnetars!! I'm not a star person (they are useful kinematic tracers and stellar evolution [being on the Red Giant Branch] allows us to isolate stars in specific systems), but with ARC Postdoctoral Fellow, Sean Farrell, I'm an author on a paper on the detection of candidate magnetar outside the galactic plane.

The study uses mainly X-ray observations, plus a lack of an optical source (from the PAndAS survey; my contribution to the paper) to identify one of these weird objects. Basically, these are neutron stars which huge magnetic fields, and this results in some dramatic bursts of energy.

Co-author, Bryan Gaensler, and I gave some talks on "Music and the Cosmos" over the last could of years, and he recounts the story of SGR 1806-20; and I'm surprised that more people don't really know about it. Basically, this magnetar is 50,000 light-years away, and 50,000 years ago, it had a major energy burst, releasing, in one tenth of a second, the same amount of energy as the Sun outputs in 100,000 years (thank you wikipedia).

This outburst hit the Earth's atmosphere on December 27th, 2004. To directly quote wikipedia

The gamma rays struck the ionosphere and created more ionization which briefly expanded the ionosphere.

Remember, this is an outburst on a star on the other side of the Galaxy!! Pretty cool. On this new candidate magentar, well done Sean!!!

A New Candidate Magnetar Located Outside the Galactic Plane

Joseph R. Callingham (1), Sean A. Farrell (1,2), Bryan M. Gaensler (1,2), Geraint F. Lewis (1), Matthew J. Middleton (3) ((1) The University of Sydney, Australia, (2) ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Australia, (3) University of Durham, UK)

(Submitted on 2 Aug 2011)

In this paper we present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM-Newton in January 2008 during a survey of M31. This source has previously been identified as a black hole X-ray binary in M31, but here we argue that the X-ray spectra and timing data are inconsistent with this conclusion. We instead argue that 2XMMi J003833.3+402133 may be a new addition to the rare class of magnetars. The X-ray spectrum is well fitted by either a steep power law plus a blackbody model or a double blackbody model. Prior observations with XMM-Newton, Chandra, Swift and ROSAT spanning 1991 to 2007, as well as an additional Swift observation in 2011, all failed to detect this source. No counterpart was detected in deep optical imaging with the Canada France Hawaii Telescope down to a 3sigma lower limit of g = 26.5 mag. The transient behaviour, X-ray spectrum, and lack of an optical counterpart are all consistent with a magnetar. The derived luminosity and black body emitting radius at the distance of M31 argue against an extragalactic location, implying that it is located within the Milky Way but 22 deg out of the plane. The high Galactic latitude could be explained if 2XMMi J003833.3+402133 were an old magnetar, or if its progenitor was a runaway star that traveled away from the plane prior to going supernova.

Gravitational Microlensing of a Reverberating Quasar Broad Line Region - I. Method and Qualitative Results

2011-08-03T14:59:00.000+10:00

PhD student, Hugh Garsden, postdoc, Nick Bate, and I just had a paper accepted for publication in MNRAS. As you can see by the title, we combine two separate astrophysical techniques to probe the inner regions of quasars, namely gravitational microlesing.

I don't have much time this week (too much teaching and paperwork to do), but will expand on these in the near future. In summary, microlensing accounts for the gravitational lensing due to the myriad of compact objects, be they stars, planets or black holes, as they pass through the line of sight. Unlike the relatively boring microlensing within the galactic halo, this microlensing results on beautiful and complex magnification maps;

Reverberation mapping happens when a flare from the central engine of a quasar, typically thought to be an accretion disk orbiting a supermassive black hole, propagates through surrounding clouds of high velocity material, the Broad Line Region. I pinched the below picture from Brendon Brewer's recent paper on using Bayesian methods to measure the black hole mass from reverberation mapping;

In combining these two effects, we now have a source which varies its structure over time, with the image of the broad line region changing as the flare travel through, and various regions begin selectively magnified by the microlensing. Here's an example of the BLR sources superimposed on the magnification map;

You should look at the paper to see what the result is when they are combined; it's pretty interesting! But for now, I just want to say, Well Done Hugh and Nick!!

Gravitational Microlensing of a Reverberating Quasar Broad Line Region - I. Method and Qualitative Results

H. Garsden, N. F. Bate, G. F. Lewis

(Submitted on 2 Aug 2011)

The kinematics and morphology of the broad emission line region (BELR) of quasars are the subject of significant debate. The two leading methods for constraining BELR properties are microlensing and reverberation mapping. Here we combine these two methods with a study of the microlensing behaviour of the BELR in Q2237+0305, as a change in continuum emission (a "flare") passes through it. Beginning with some generic models of the BELR - sphere, bicones, disk - we slice in velocity and time to produce brightness profiles of the BELR over the duration of the flare. These are numerically microlensed to determine whether microlensing of reverberation mapping provides new information about the properties of BELRs. We describe our method and show images of the models as they are flaring, and the unlensed and lensed spectra that are produced. Qualitative results and a discussion of the spectra are given in this paper, highlighting some effects that could be observed. Our conclusion is that the influence of microlensing, while not strong, can produce significant observable effects that will help in differentiating the properties of BELRs.

My journey into GPGPUs

2011-07-30T17:17:00.001+10:00

The semester has begun here, Down Under, and that means two things. Firstly, I am teaching General Relativity from Monday onwards to our Honours class (this is my favorite course and I'll blog about it a little more, as I have a particular view of the teaching of this subject), and I have become a student again.

Why? Well, because of these;

Those that have braved opening up a computer may recognise this as a GPU, or Graphics Programming Unit, and it's the engine that make high level graphics possible, especially for computer gaming. The explosion in their development came around because of

Not the flash and manic grin, but the hair (and my understanding is that it is long, flowing female hair that is the goal, which may tell us more about those who write computer games!).

The result is that GPUs have become computationally very powerful, but the computer architecture is different to a CPU. Basically GPUs are massive parallel processors, many quite simple computation engines. This means that if you have a simple calculation that you want to perform many times, a CPU might have to step through each calculation, whereas the GPU can do them all at once.

This is precisely what we want to do in many astronomical (and generally scientific) applications. As an example, to calculate the gravitational force on an object, then you need to add up the force due to all the other objects. Typically, you do this one at a time, which can get quite slow for many (i.e. billions) of object, and so things would go much faster if we could do the summation at once.

There is a problem, however. The makers (e.g. NVIDIA and AMD) keep the details of the architecture close to their chests. And they have, in the past, not been as rigorous as CPUs as ensuring floating point arithmetic works as it should; if you are simulating hair, then 2+2=5 is not such a problem now and again, but it can render useless the output of a scientific simulation (would you fly on a plane whose wings had been tested on a machine that sometime got floating point arithmetic wrong?)

But this is changing, and more robust arithmetic is now the name of the game, as well as providing computing libraries, specifically CUDA and openCL to allow us to develop applications on GPGPUs (the first GP is now for General Purpose). There is some urgency on getting to grips with this, as we are starting to build GPU-based supercomputers (in Australia, we will soon have g-Star to undertake GPU-based supercomputing of theoretical astrophysics). So, I have enrolled in a programming course for CUDA in the School of IT here.

There is, however, a problem. The problem (and I know this is going to hurt) are generally not very good at coding. Some are, but the majority aren't. We rely on the fact that we don't have to worry about complicated stuff because things like memory management, order of processing etc are hidden in high level codes, typically C and fortran, although python seems to be getting a foothold. We are bad enough for me to chuckle at the fact that this book

has an astronomy picture on the front; is it an example of a field that is renowned for needing this book, or perhaps we are better than the rest (which is a scary thought).

Anyway, back to GPGPUs. They are difficult to program. I think it was best put by by lecturer, they are difficult to program because you are

"programming bare metal"

You HAVE to worry about memory, and what's computing what and when, and, and this will shock most astronomer, you can't debug your code by sticking write statements everywhere (this will cause your code to fall over in a heap.

Anyway, I have had my first lecture, which so far is fine, but I also got my first homework, essentially playing with memory management in C. Of course, the young IT students confidently read over the homework sheet as I replayed the opening script of Four Weddings and a Funeral in my mind; it's been a little while since I really programmed in C.

I'll keep the blog updated on my journey into GPGPUs.

Intermission: Australia vs South Africa

2011-07-24T17:30:00.001+10:00

No zombies or 2-d universes this weekend, as I have been busy with a few other things. This included heading to ANZ Stadium to see Australia play South Africa at rugby.

It is very easy for me to get to the stadium here in Sydney, much easier than it was to get to Arms Park when I was younger, and me and the family trundled down for the 8pm kick off. Trundling through the happy rugby cloud, we almost ended up at Acer Arena where Enrique Iglesias (whose crowd was as equally rowdy, and friendly, as the rugby crowd) was performing. It was interesting to see that also-Neath-born Katherine Jenkins will be performing there with Placido Domingo in September.

After finding the stadium, we watched a good game of rugby, especially given the Wallabies woeful appearance against Samoa last week. So good, in fact, that I only caught a couple of piccies.

As I said, a good game, although the crowd doesn't sing Delilah. One cool point was the mexican wave. Due to the torrential downpour over the week, we all got little foam squares to sit on, which became a spectacular part of the wave as it went round.

A New Collisional Ring Galaxy at z = 0.111: Auriga's Wheel

2011-07-22T11:35:00.001+10:00

A great week for the Conns! My ex-students, Blair Conn (who is no relation to Anthony), Richard Lane and I have just had a new paper accepted on the discovery of a Collisional Ring Galaxy at redshift z=0.111 (the most distant one known - correction, one of the most distant known).

The galaxy was found serendipitously in a Subaru survey of the galactic disk and looks quite interesting. Here's two images of the system, in g-band (left) and r-band (right).

Cool eh! But it looks a bit better when you combine these to make a colour picture (we can make a pretend in-between band by averaging the two pictures above). This is what you get (and is annotated as in the paper).

The Slits in the image correspond to where we pointed the GMOS spectrograph on the Gemini-North telescope, allowing us to measure the velocities of the various components. It also revealed that there are active galaxies hidden down in the middle.

So, what's going on here? Well, we have some local examples of these Collisional Ring Galaxies. Here's a pretty one as seen by the Hubble Space Telescope

As the name suggests, these are formed when two galaxies collide, where the collision is almost a bulls-eye, rather than a glancing blow. The collision causes gas to collapse and result in a burst of star formation in the ring. In Auriga's Wheel, gas also flows into the centre, pouring fuel onto the central black holes and resulting in the active galaxies we see. This suggest that the ring is very young, only 50 million years (a cosmic baby).

We've also done a preliminary set of computational modeling of the collision which confirms this:

Here we have two galaxies of about the same mass, one with red particles, and the other with black. Looks like what we see (isn't physics wonderful!). Rory Smith is working on some more detailed modeling, and I'll pop that here when it's done. Well done Blair and Richard!!

A New Collisional Ring Galaxy at z = 0.111: Auriga's Wheel

Blair C. Conn, Anna Pasquali, Emanuela Pompei, Richard R. Lane, André-Nicolas Chené, Rory Smith, Geraint F. Lewis

(Submitted on 20 Jul 2011)

We report the serendipitous discovery of a collision ring galaxy, identified as 2MASX J06470249+4554022, which we have dubbed 'Auriga's Wheel', found in a SUPRIME-CAM frame as part of a larger Milky Way survey. This peculiar class of galaxies is the result of a near head-on collision between typically, a late type and an early type galaxy. Subsequent GMOS-N long-slit spectroscopy has confirmed both the relative proximity of the components of this interacting pair and shown it to be the most distant spectroscopically confirmed collisional ring galaxy with a redshift of 0.111. Analysis of the spectroscopy reveals that the late type galaxy is a LINER class Active Galactic Nuclei while the early type galaxy is also potentially an AGN candidate, this is very uncommon amongst known collision ring galaxies. Preliminary modeling of the ring finds an expansion velocity of ~200 kms^-1 consistent with our observations, making the collision about 50 Myr old. The ring currently has a radius of about 10 kpc and a bridge of stars and gas is also visible connecting the two galaxies.

A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)

2011-07-19T23:00:00.001+10:00

PhD student, Anthony Conn from Macquarie University, has been working with me on measuring distances to dwarf galaxies and substructure in our nearest cosmological companion, the Andromeda Galaxy. I'm very pleased to say that Anthony's first paper, using Bayesian methods to measure these distances, has now been accepted for publication in the Astrophysical Journal. Excellent result, Anthony!!

A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)

A. R. Conn, G. F. Lewis, R. A. Ibata, Q. A. Parker, D. B. Zucker, A. W. McConnachie, N. F. Martin, M. J. Irwin, N. Tanvir, M. A. Fardal, A. M. N. Ferguson

We present a new approach for identifying the Tip of the Red Giant Branch (TRGB) which, as we show, works robustly even on sparsely populated targets. Moreover, the approach is highly adaptable to the available data for the stellar population under study, with prior information readily incorporable into the algorithm. The uncertainty in the derived distances is also made tangible and easily calculable from posterior probability distributions. We provide an outline of the development of the algorithm and present the results of tests designed to characterize its capabilities and limitations. We then apply the new algorithm to three M31 satellites: Andromeda I, Andromeda II and the fainter Andromeda XXIII, using data from the Pan-Andromeda Archaeological Survey (PAndAS), and derive their distances as $731^{(+ 5) + 18}_{(- 4) - 17}$ kpc, $634^{(+ 2) + 15}_{(- 2) - 14}$ kpc and $733^{(+ 13)+ 23}_{(- 11) - 22}$ kpc respectively, where the errors appearing in parentheses are the components intrinsic to the method, while the larger values give the errors after accounting for additional sources of error. These results agree well with the best distance determinations in the literature and provide the smallest uncertainties to date. This paper is an introduction to the workings and capabilities of our new approach in its basic form, while a follow-up paper shall make full use of the method's ability to incorporate priors and use the resulting algorithm to systematically obtain distances to all of M31's satellites identifiable in the PAndAS survey area.

Just as a passing note, this is another cool astro-ph date, with another Geraint on there. Have a read of;

An MCMC approach to extracting the global 21-cm signal during the cosmic dawn from sky-averaged radio observations

by Geraint Harker. I wonder if he is related to the famous Jonathan Harker (vampire slasher)? At some point, I'll write about the mathematical modelling of vampire outbreaks!! One thing I know, is that a vampire outbreak will not look like this!!

Dawn of the Dead

2011-07-17T12:35:00.001+10:00

Given my age, I was a young teenager when videos arrived in the UK, and horror movies were the rage. I'm pretty sure that the first zombie movie I saw was Dawn of the Dead (the original, although the recent remake wasn't that bad).

Dawn is one in a long line of zombie films by George A. Romero, colloquially known as the Living Dead Series, starting with the classic Night of the Living Dead in 1968. Some of the later movies are, well, not so good, but Dawn is an excellent movie (well, excellent zombie movie).

I'm not going to give the story away here, but as noted in Dawn (and especially the recent Zombieland), there will be survivors, and these survivors will get better at surviving by not being killed, and getting rid of more zombies. As noted in Zombieland;

The first rule of Zombieland: Cardio. When the zombie outbreak first hit, the first to go, for obvious reasons... were the fatties

So, in my zombie models, I have added a factor to account for the "hardening" of the population, by making the various parameters a function of time. As a reminder, here's the starting point, with all the parameters kept constant.

I've modified the plot and now, on the bottom, have the key parameters that control the zombie apocalypse. These are α, humans killed by zombies, β, humans infected by zombies, and δ, zombies killed by humans.

With the constant values given above, the population crashes.

OK, let's change these. What I've done is use a logistic function to change the values. The key points to this is a time which represents the mid-point of the change, a time scale and a size of change. Here's one where the population stiffens at around 300 days.

Again, the population collapses and the zombies take over. What we need to do is make the population stiffen a lot earlier. Let's get the population fighting back harder at about 250 days, during the collapse of the population.

Now this is more like it. What we see now is that the population drops, but the stiffening of the population halts the decline and the population flattens out and the zombies are basically eradicated.

Moving the stiffening back again, we should expect the population to do better, and to go back to 200 days, we find

So the rule is simple. Fight back, and the earlier we fight back, the better.

Keep your eyes peeled for the shuffling undead!

If I had a blank cheque I’d … trace the history of the Milky Way

2011-07-15T21:40:00.000+10:00

A busy day, zipping down to Melbourne for a meeting on computer infrastructure. In the meantime, I got another article published in The Conversation titled If I had a blank cheque I’d … trace the history of the Milky Way.

The article is part of a series on what would happen if you give scientists a blank cheque. I was very restrained (I'd get rid of my mortgage first), and I focused on things that are actually achievable with a reasonable (rather than infinite) pot of money.

Not to steal the thunder of the article, I said I would use $100 million to build WFMOS, a project which I was very involved with. Here's a random image

WFMOS was to be the next generation multi-fibre spectrograph, built as part of a consortium between Gemini and Subaru, and to be placed at the top-end of the mighty Subaru Telescope.

This project was a long time in development, a history which I won't recount here now. But there were a number of meetings to discuss the science, the focus of which was the nature of dark energy and galactic archaeology. In fact, I organized one of these meetings;

The meeting was very enjoyable, but alas, it was also the platform for Gemini to announce that WFMOS was cancelled. A rather depressing outcome after a huge amount of effort.

However, the Japanese have continued with a new project, called Sumire, which will attack the dark energy questions that WFMOS was intending to do. Alas, the galactic archaeology waits in the wings.

As I said, I will not reproduce The Conversation article here, and I will write more on galactic archaeology in the future. I will take a moment, however, to lament the memory of WFMOS.

A long night in the dome.....

2011-07-13T08:44:00.001+10:00

I've been observing at the AAT a couple of time over the last few months. It's a long drive from Sydney (almost 6 hours), for long winter nights in the dome. We were using the rather wonderful AAOmega spectrograph to chase stars in the Sagittarius Dwarf and its associated tidal stream (I'll write more about these later).

I'm not much of an Astronomer, in the sense that I am pretty clueless about constellations and the names of stars (but having Google Sky on my phone is really starting to help). But when I am observing, I take the chance to really look up. My first observing run (in 1991) was at the William Herschel Telescope in the Canary Island, and it was the first time that I **really** saw stars, and saw that they had colours. I have a valid excuse, having grown up in the not-so-clear skies of South Wales, followed by a few years in London.

However, there are more than things to look at other than stars. I really like catching Iridium Flares, a family of communication satellites which catch the Sun just before sunrise/sunset, and can become amongst the brightest things in the sky. Sometimes, we are luck to get a pair of flares within 30 seconds of each other, and on the last run, the legendary AAT TO Steve Lee photographed such a pair of flares. Here's the image;

For the keen-eyed, you should also see the disk of the Milky Way and a Magellanic Cloud.

One of the weirdest things about observing is that when the Sun is on it's way up, but still well below the horizon, we can no longer observe due to the sky brightness, even though it (to your eyes) looks dark outside.

After all the calibrations are done, and your body is yearning for bed (and it is important to get to sleep before the Sun really comes up), what else can you do? Well, if Saturn is up (as noted by Anthony Conn who was observing with us), perhaps we can look at it with the AAT? Remember, however, we have a spectrograph on, and what we really want is an image. In fact, we used something called the Focal Plane Imager (which allows us to do things like monitor the seeing when observing with the AAOmega), and if you cut the integration time down to a bare minimum (remember, we do have a 3.9m telescope here), what you get is

Again, we have Steve to thank for this excellent image (with 4 moons thrown in also, although I have a sneaking suspicion that the one at the top is a cosmic ray).

I guess, as an astronomer, I should look up a little more. It's all rather pretty up there.

Feyman on Computers

2011-07-10T09:18:00.000+10:00

Over at In the Dark, Peter Coles has been quoting Richard Feynman on various topics (and Feynman had a lot to say on a lot of topics, much of it quite eye-opening). One of his recent quotes was the following (which I creatively copied - i.e. pinched - directly from Peter's blog);

Well, Mr. Frankel, who started this program, began to suffer from the computer disease that anybody who works with computers now knows about. It’s a very serious disease and it interferes completely with the work. The trouble with computers is you *play* with them. They are so wonderful. You have these switches – if it’s an even number you do this, if it’s an odd number you do that – and pretty soon you can do more and more elaborate things if you are clever enough, on one machine.
After a while the whole system broke down. Frankel wasn’t paying any attention; he wasn’t supervising anybody. The system was going very, very slowly – while he was sitting in a room figuring out how to make one tabulator automatically print arc-tangent X, and then it would start and it would print columns and then bitsi, bitsi, bitsi, and calculate the arc-tangent automatically by integrating as it went along and make a whole table in one operation.
Absolutely useless. We *had* tables of arc-tangents. But if you’ve ever worked with computers, you understand the disease – the *delight* in being able to see how much you can do. But he got the disease for the first time, the poor fellow who invented the thing.

I remember reading this in Feynman's excellent book Surely You're Joking Mr Feynman, a book I read about the time I decided not to be a particle physicist and to be a astrophysicist instead. But I remember that when I originally read this quote, I *think* I misunderstood what it was saying. I think it was saying that "if you use computers, don't waste your time playing around with them".

Unfortunately, I already knew I had the disease Feynman talks about. Back in the early 1980s (I think it was 1982), I remember trundling on the bus to Swansea to spend almost £100 on a ZX81 and 16k RAM pack (on which the slightest breath would delete everything). Just for nostalgia sake;

While there were games available to buy (on cassette tape, which you could actually listen to as they loaded), they weren't too cheap. To make up for this, there was a number of magazines that published game source code (in Sinclair BASIC) which you could type in and play.

Games often weren't much cop, with basic graphics and game play - below is one you actually bought

and often when typing in the source code, there would often be a bug that you introduced, or a bug in the actual source code. To make the games work, you had to understand what the code was actually doing, such as mucking about with memory with peeks and pokes, and, even though I didn't realise it at the time, you had to learn how to debug code. Learning these skills helped me play around, write my own code, and actually start to use it. Again, I remember writing a code adding wave after wave to show that you could make a square wave, and this was before I was formally introduced to Fourier analysis.

OK, time for grumpy old man time. I'm told that the current generation of students coming into universities these days are computer-literate, having grown up with computers. While this is true, it is clear that many students have never played with computers in the sense of Feynman's statement. Web searching, email and Facebook just don't cut it.

Does it matter? If you want to do a degree in physics, both undergraduate and graduate, I think you need these skills. Many (most) research projects will require students to integrate/differentiate functions, find correlations, fit models to data etc etc, and I see students struggle having to learn these concepts from scratch. This is made worse as many students shun computational physics courses when doing their degree (probably because they don't realise how important these skills are).

I'll leave the final word to the wonderful Randall Munroe and xkcd. While it was BASIC for me, rather than PERL, I very much agree.

The last shuttle

2011-07-09T15:11:00.000+10:00

Atlantis has set off on the last shuttle flight, and so I thought I would put down a few of my thoughts on the shuttle program.

I was born a couple of months before Armstrong walked on the moon, and I effectively grew up expecting the Space Shuttle to change spaceflight. I don't really remember the Apollo-Soyuz link-up, but I remember Skylab flying, and then falling. I barely remember anything about the Russian space program as a child.

About the time of the first shuttle flight, I sent a letter to NASA asking if they had any information about the shuttle I could have (remember, this was in the stone age when there was no internet), and I received a deluge, more packages than I can remember on the shuttle program, and its future promise.

I've always enjoyed watching the shuttle fly, but looking back I realise that it didn't really reach the potential outlined at the starts. The Challenger accident, as well as introducing me to Feynman, revealed how dangerous the shuttle was (it still does not have a launch abort system, unlike every other manned rocket), and this was recognized by the astronauts.

But the shuttle has done some amazing things, and for me (and my research) repairing Hubble was amazing. If Hubble breaks down now, then that's it. With JWST looking a little shaky, we may have to say goodbye to large optical telescopes in orbit.

So, it's almost goodbye shuttle. I hope Atlantis has a safe flight and eventually finds rest in a museum somewhere. With the Americans now heading towards a glorified apollo capsule in the form of the Orion spacecraft, I can't but help think that a big step backwards in space travel is being made.

Before leaving, I should mention that I am a fan of manned spaceflight, but the shuttle is not my favorite spacecraft, or apollo, gemini or mercury. In fact, my favorite is not even american, and I would fly on it, if offered, in a heartbeat. My favorite is soyuz;

Two friends in my 2-d Universe

2011-07-09T13:06:00.000+10:00

So, I have now generalized the 2-d Universe a little more, and here are two particles interacting with each other within the surface of a sphere.

Cool, isn't it? So, how does one calculate such a pair of paths? As I mentioned previously, it's all standard non-Euclidean geometry and vectors and the like. So, let's go through the basics (and maths-types, please remember I am an astrophysicist and don't get grumpy about the words I use - it works :)).

Starting point is that we are on a sphere, and so it makes sense to use spherical polar coordinates. Now, one painful thing is that different people define which angle is ϑ and which is φ, so I will be following the convention shown in the top figure on Wikipedia. Remember, however, we are working in the surface of the sphere, and so we have no radial (r) coordinate.

If we have two infinitesimal displacements in our coordinate (and assuming a unit sphere for convenience), then the separation between the displaced coordinates is given by;

And this defines the terms in the metric of the surface. We can write this as a matrix (I can feel the mathmos blood starting to boil) of the form;

To calculate the motion, we need some equations of motion :) and for those, we use Christoffel symbols, which tell us the equations of motion are given by

where we are using the Einstein summation convention, and the greek characters are just our coordinates that we are using. The v-terms are velocities in our coordinate directions. If we plug all the terms in, calculating the Christoffel symbols, the equations of motion become;

Excellent. We can immediately see that if we plonk a particle down at rest (so its velocity terms are zero), these equations are zero. If the velocity is not zero, the particle will move over the sphere, but these equations are not zero, so the components of the velocity are changing. What does this mean for the path of the particle? It's not hard to show that the particle moves with a velocity of constant magnitude and covers a great circle path over the sphere, but here I will just show this graphically. Here's the same experiment with the magnitude of the interacting force set to zero.

As we expect, two great circles. But what about the components of the velocities? Let's just take an example path;

Here, the blue is the ϑ velocity, and the green is the φ, while the red is the magnitude of the velocity, which is constant. The important thing to remember is that we are not sitting on a flat, Euclidean surface with a cartesian coordinate system, and so the normalization of the velocity is not simply obtained using Pythagoras, but by

So, that all works. Particles happily travel through the spherical surface on effectively free-fall paths, covering great circles. But now we need to add the interaction between the particles, and for that we need to calculate a few things. Firstly, there is the distance between particles in the surface of the sphere! That's relatively straight-forward. Then we need to work out the vector components of the force, that is somewhat harder. Once we have these, we then use the modified form of the geodesic equation, which includes the influences of forces, that is

where the a terms are components of the acceleration. But I'll leave that for next time.

A Dynamical 2-d Universe

2011-07-03T16:29:00.000+10:00

One last post for today, as I have conquered movie making in matlab and handbrake (for small values of conquered) to make a movie of particle motion in my 2-d universe.

I have softened the interaction so that it is now 1/r (which seems more sensible as we are one dimension down here). Here's the path (again, with the blue dot being the primary mass).

And for your viewing pleasure, here's a movie of the orbit

Hmmmm. Works, but needs more work.But isn't maths great!

Two-Dimensional Universe

2011-07-03T11:15:00.000+10:00

What this?

If you said "That's a small mass orbiting a large mass in a 2-d spherical universe", then you're correct.

I am going to be teaching general relativity in a couple of weeks time, and every time I do I start thinking about geodesics, not just through 4-d space-time, but also standard 3-d geometry. One of the problems (IMHO) with current physics degrees is that we don't really touch on curvilinear coordinates and tensors until their final year, and (especially when it comes to general relativity), this all comes as a bit of a shock.

However, we can cast much of (all of?) physics in generalized coordinates, and we should be doing this from the start, showing how things like classical mechanics can be done in Euclidean or polar coordinates (or whatever), and the key thing being that the physical predictions come out to be the same.

I also think this will help students understand things like conserved quantities a bit more, and realize that there is nothing particularly magical about momentum, and that momentum and angular momentum are just conserved "things" in different coordinate systems.

Anyway, back to the above figure. I was thinking about geodesics on a 2-d spherical surface. It is quite simple to show that these are great circles. But how to you show some other things, like if you are a point A and want to travel to a point B, which direction should you set off in to travel along the great circle path (this, of course, is what aeroplanes have to do every day).

But this got me thinking about non-geodesic paths, paths which are pushed and pulled by forces. So, I plonked a mass at the pole (the blue dot) and added a 1/r^2 force (where r is now the great circle distance) and integrated the path. The key part is the force is a vector, and so you have to know which direction it points in the sphere. For the mass on the pole, it's relatively straight-forward. For a more generalized location, it's a bit harder, but doable with vectors and coordinate transformations (and without having to rotate coordinates back and forth). The goal is to have several particles interacting with each other on the sphere.

Why do it? Well, I don't think this 2-d universe has any practical uses; I'm not advocating that this is in anyway a real system. But you do get to use some very key concepts as outlined above, and it is an interesting problem. For me, learning complex techniques is very much helped by trying out toy models first.

I'll discuss how I worked out the orbits (they're numerically integrated, but I'll demonstrate how to set up the equations). Just to prove I can put the mass at any location, here's another figure.

Astro-Ph: The scatter about the "Universal" dwarf spheroidal mass profile

2011-06-30T08:29:00.000+10:00

PhD student, Michelle L. M. Collins, working with Scott Chapman (IoA, Cambridge) and members of the Pan-Andromeda Archaeological Survey (PAndAS) (including me), has had her paper on the study of the size of dwarf galaxies accepted for publication in MNRAS. This paper shows further evidence that the dwarf galaxies around the Milky Way are different from those around its sister, the Andromeda Galaxy, and suggests that there must have been some difference in the way these two similar galaxies formed and evolved. Well done Michelle!!!

The scatter about the "Universal" dwarf spheroidal mass profile: A kinematic study of the M31 satellites, And V and And VI

M. L. M. Collins, S. C. Chapman, R. M. Rich, R. A. Ibata, M. J. Irwin, J. Peñarrubia, N. Arimoto, A. M. Brooks, G. F. Lewis, A. W. McConnachie, K. Venn

While the satellites of the Milky Way (MW) have been shown to be largely consistent in terms of their mass contained within one half--light radius (M_{half}) with a "Universal" mass profile, a number of M31 satellites are found to be inconsistent with such relations, and seem kinematically colder in their central regions than their MW cousins. In this work, we present new kinematic and updated structural properties for two M31 dSphs, And V and And VI using data from the Keck Low Resolution Imaging Spectrograph (LRIS) and the DEep Imaging Multi-Object Spectrograph (DEIMOS) instruments and the Subaru Suprime-Cam imager. We measure systemic velocities of v_r=-393.1+/-4.2km/s and -344.8+/-2.5km/s, and dispersions of sigma_v=11.5{+5.3}{-4.4}km/s and sigma_v=9.4{+3.2}{-2.4}km/s for And V and And VI respectively, meaning these two objects are consistent with the trends in sigma_v and r_{half} set by their MW counterparts. We also investigate the nature of this scatter about the MW dSph mass profiles for the "Classical" (i.e. M_V<-8) MW and M31 dSphs. When comparing both the "classical" MW and M31 dSphs to the best--fit mass profiles in the size--velocity dispersion plane, we find general scatter in both the positive (i.e. hotter) and negative (i.e. colder) directions from these profiles. However, barring one exception (CVnI) only the M31 dSphs are found to scatter towards a colder regime, and, excepting the And I dSph, only MW objects scatter to hotter dispersions. We also note that the scatter for the combined population is greater than expected from measurement errors alone. We assess this divide in the context of the differing disc-to-halo mass (i.e. stars and baryons to total virial mass) ratios of the two hosts and argue that the underlying mass profiles for dSphs differ from galaxy to galaxy, and are modified by the baryonic component of the host.

Most Distant Quasar Found

2011-06-30T08:17:00.000+10:00

A quasar has been found at a redshift of 7.1. While this is not the most distant object discovered (there are gamma-ray bursts and galaxies that have been found at a higher redshift), it is considerably brighter, has a lovely spectrum and comes with some very pretty graphics;

The lead on the paper is Dan Mortlock. I've known Dan for years; he was a PhD student at Melbourne when I visited in in 1995 (I was finishing up my thesis at the time). Excellent result.

Astro-ph: Stellar Streams as Probes of Dark Halo Mass

2011-06-25T15:17:00.000+10:00

PhD student, Anjali Varghese, working with me and my close colleague, Rodrigo Ibata, has had her paper working on probing dark matter halos with stellar streams accepted. Congratulations Anjali!!

Stellar Streams as Probes of Dark Halo Mass and Morphology: A Bayesian Reconstruction

Anjali Varghese, Rodrigo A. Ibata, Geraint F. Lewis

Tidal streams provide a powerful tool by means of which the matter distribution of the dark matter halos of their host galaxies can be studied. However, the analysis is not straightforward because streams do not delineate orbits, and for most streams, especially those in external galaxies, kinematic information is absent. We present a method wherein streams are fit with simple corrections made to possible orbits of the progenitor, using a Bayesian technique known as Parallel Tempering to efficiently explore the parameter space. We show that it is possible to constrain the shape of the host halo potential or its density distribution using only the projection of tidal streams on the sky, if the host halo is considered to be axisymmetric. By adding kinematic data or the circular velocity curve of the host to the fitting data, we are able to recover other parameters of the matter distribution such as its mass and profile. We test our method on several simulated low mass stellar streams and also explore the cases for which additional data are required.

Fish

2011-06-20T20:56:00.000+10:00

My kids are interested in making a stop-motion movie, so I grabbed Framebyframe for a whirl, and it seems to work. The boys have made a couple of lego movies, but here's my first attempt;

I'm quite proud of it.

Time to fight back!

2011-06-19T09:33:00.000+10:00

Oh no!! There was a zombie outbreak in Leicester and it was clear that the city council was completely unprepared. Maybe they had not mathematically modeled the outbreak!!

Let's take the next step in the modeling and add a fight back factor. This means that there is a chance that, during a Healthy-Zombie interactions, the Zombies themselves can be truly killed and moved into the Dead population.

The dynamical equations become

where we now have a new parameter, δ, which controls the probability of killing Zombies.

OK, some runs. Let's try δ=0.003. This means that there is still a good chance of being bitten and becoming inflected, but you have a better chance of killing a Zombie than a Zombie killing you. What you get is

This is quite interesting. Clearly, the timescale for things to happen has been extended; it now takes a couple of weeks until the Zombies take over, but the population of Healthy humans still crashes very quickly.

OK - one more test (as breakfast is ready), but let's try δ=0.009, so there is almost as much chance of you killing a Zombie as you getting bitten. What happens now?

Notice that the timescale is much longer now, and we have half-a-year where things are OK, but the outcome is the same; the population of Health humans crashes and Zombies take over, but with a lot fewer Zombies in the end.

Right, that's enough for now. Next we will consider hardening of the population, where people become better at killing Zombies and avoiding infection. Then we will add a cure. but that's next.

When there's no room left in hell.....

2011-06-18T16:21:00.000+10:00

It's been a very productive week, and so I am going to take a little time to write up some Zombie Apocalypse code. As I noted previously, this is based on a earlier piece of work, but I am going to extend the model (and modify it to more accurately match zombies in movies).

The start point is the variables we will be following. These are H, the health humans, I, those bitten by a zombie and so on their way to zombiedom, Z, the star of the show, and D, the well and truly, non-zombie, dead.

Unlike the previous work by Robert Smith?, I don't assume hat any olf dead are revived. Rather, those that are bitten become infected, die and become zombies. Other dead remain dead, and as we know, a knock to the head moves a zombie into the permanently dead camp.

So, my starting dynamics equations are;

Where the ' are derivatives with respect to time. The H Z term is the number of Healthy and Zombies multiplied together. This is a measure of the number of interactions between the two populations.

For the parameters, α controls the rate that an encounter between a healthy and a zombie results and a dead human (D), whereas β controls rate that the outcome will be an inflected human (I). The remaining parameter, γ, controls the rate that infected humans become zombies.

So, let's do a test run. We'll start with 500,000 Healthy humans, and a solitary zombie. We are actually going to be working in units of 1000 individuals, and the parameters to start with are α = 0.001, β = 0.01 and γ = 0.5. What do we get;

Excellent. We have about a week where nothing really happens (the zombie population is growing very slowly), and then BAM!, the population of healthy humans crashes in a week as the number of inflected rapidly rises and them decays away as zombies grow in strength.

This is exactly what we should expect to happen. Everything is flowing out of H and into Z and D, and no matter how we muck about with the parameters (other than having no zombies, or no harm in interacting with them), this is going to be the outcome.

What's next? Well, there are two possibilities: we can see if we can find a cure to stop those who are infected becoming zombies (and this may include a severe case of lead poisoning), and having a population of healthy humans who are good at protecting themselves from zombies.

I'll investigate this later.

God, the Big Bang … next please …

2011-06-17T08:26:00.000+10:00

Another article of mine has been published on The Conversation. Titled God, the Big Bang … next please …, I discuss whether the Big Bang was the birth of the Universe. The reason for writing this is the catch call from those wanting to undermine modern cosmology, declaring its birth from nothing is illogical.

While we have no theory of everything, with no single picture encompassing gravity and the other forces (and be careful of what you hear from the hype known as superstrings), we cannot see "through" the Big Bang, and don't know what was before it.

But that does not mean we think there was nothing, literally nothing, no time and no space, before the Big Bang, and plenty of ideas are out there on what possible came before. We could be a daughter universe, born during the formation of a black hole in a universe before, or just one of a continuation of cycling universes, or something even weirder we haven't even thought of yet.

As I say in the article; I don't think any cosmologist really thinks that the Big Bang was the Beginning.

Anyway, I didn't originally have God in the title. I'm sure it will raise a few eye-brows.

Talking of Hubble!

2011-06-12T10:17:00.000+10:00

A quick post, as it is the Queen's Birthday Long Weekend here

Hubble Space Telescope Observations Awarded to Sydney Astronomers

9 June 2011

Two astronomers in the School of Physics, Professor Geraint Lewis and Professor Joss Bland-Hawthorn, have each been awarded individual observation time on the Hubble Space Telescope.

"Hubble observation time is internationally sought after and very competitive," says Professor Lewis who is known for his work on galactic clusters.

Professor Lewis' project is targeting old stellar systems, known as globular clusters and is part of a large international team, led by Dougal Mackey at ANU, and includes astronomers based in North America, the UK and Europe.

"These clusters are orbiting our nearest cosmic neighbour, the Andromeda Galaxy at very large distance. While Andromeda seems to have a number of these, our own galaxy, the Milky Way, does not. We really want to know why there is this difference."

Professor Lewis says that using Hubble the astronomers will be able to accurately see individual stars in these globular clusters, something that is impossible from the ground.

"We will be able to chart out the history of the globulars, and work out just where they came from, providing important clues to the formation and evolution of galaxies. So to get observing time on telescope as great as Hubble is fantastic for Australian science."

Professor Bland-Hawthorn, an ARC Federation Fellow, is part of a team of comprising five astronomers from the USA and one Australian scientist who have been granted "20 orbits" to use the COS ultraviolet spectrograph.

"We are studying the Magellanic Stream, which is a stream of gas discovered by Australian radio astronomers in the 1970s that wraps right around the Galaxy."

Earlier work by Professor Bland-Hawthorn showed that what can be seen with radio telescopes is only a fraction of the gas.

"Much of it is warm, or even hot, but you can only see this with optical and, especially, UV telescopes," he explains, "We want to confirm this claim from my work and my modelling, and demonstrate that much of the gas falling into the Galaxy is in the form of a warm rain."

The Hubble Law; or is it?

2011-06-09T08:28:00.000+10:00

One of the greatest discoveries of the 20th Century was the discovery of expanding Universe. Of course, what this means is that see the light from galaxies redshifted, with more distant galaxies having a larger redshift. Importantly, this is precisely what you expect from a Universe described by general relativity, in terms of the expanding space-time metric given by the Friedmann-Robertson-Walker metric, and governed by the Friedmann equations.

Converting a galaxies redshift into a velocity (by treating it as a Doppler shift), Hubble's law is v = Ho d, where v is the velocity, d is the distance, and Ho is the (in)famous Hubble constant. It is Ho that tells us how fast the Universe is expanding at the moment.

Measuring Ho was a big preoccupation of 20th Century astronomy, with us finally finding it is around 72 km/s/Mpc. But who was the first to measure Ho, essentially by plotting the distance against redshift for galaxies and measuring the slope? Credit is typically given to Hubble. But the situation is not so clear.

As mentioned in Letters to Nature, a recent paper on astro-ph suggests that Lematire beat Hubble to the "linear velocity–distance relationship" (i.e. Hubble's law) by two years.

Today's astro-ph paper by Sidney van den Bergh muddies the water even more. Directly quoting his abstract;

The 1927 discovery of the expansion of the Universe by Lemaitre was published in French in a low-impact journal. In the 1931 high-impact English translation of this article a critical equation was changed by omitting reference to what is now known as the Hubble constant. That the section of the text of this paper dealing with the expansion of the Universe was also deleted from that English translation suggests a deliberate omission by the unknown translator.

However, a recent paper by Jean-Pierre Luminet directly names the translator as the famous astronomer Arthur Eddington, leader of the expeditions to verify Einstein's general theory of relativity by examining the deflection of starlight by the Sun. As explained by Luminet;

Next, Eddington carried out an English translation of the 1927 Lemaitre article for publication in the Monthly Notices of the Royal Astronomical Society. Here took place a curious episode: for an unexplained reason, Eddington replaced the important paragraph quoted above (where Lemaitre gave the relation of proportionality between the recession velocity and the distance) by a single sentence: "From a discussion of available data, we adopt R'/R = 0,68x10-27cm-1 (Eq. 24)". Thus, due to Eddington's (deliberate?) blunder, Lemaitre will never be recognized on the same footing as Edwin Hubble for being the discoverer of the expansion of the universe.

So, we are left with the fact that at some level, Hubble's law should probably be known as Lemaitre's law. History is never as simple as the textbooks make out! At some point I'll write something about the strange case of Ollin Eggan and the vanishing Greenwich Observatory documents.

Zombies (and Differential Equations)

2011-06-07T08:46:00.000+10:00

A couple of years ago, Robert Smith? and collaborators published a paper on numerical modeling a zombie outbreak. The article got lots of press mileage, but I think that an important message did not shine through.

While I am sure that the authors are not getting ready for the coming zombie apocalypse (although others clearly are), the story is about how more realistic hazards, such as diseases, can be computationally modeled as they flow through a population, and this, as we all know, is governed by differential equations.

Why computational? Because (and this is not a fact we really make apparently to our undergraduate students) the vast majority of differential equations do not have analytic solutions, and we need to turn to the computer to model complex interactions.

And if I had the opportunity to study zombie outbreaks as an undergraduate, I am sure that learning about differential equations and computational approaches would have been a lot more fun.

Anyway, having watched a few zombie movies in my time, I felt there were a couple of problems with Smith?'s original model for the life-cycle (if that's the word) for zombies, and I coded up some models of my own, but as ever, time got the better of me.

However, others, such as here and here, clearly were thinking along the same lines. So, I'm going to use this blog to go through the model and check out the results. The goal is to see if we can come up with a scenario in which we will get some survivors (although, so far, this is not looking very likely).

The first real post will be coming soon, but for now, here's a picture;

Dark energy is "real"

2011-06-06T08:19:00.000+10:00

The "Dark Energy is Real" kerfuffle is continuing with interesting comment over at Universe Today on the issue. As Nerlich points out;

I mean how the heck did ‘dark energy’ ever become shorthand for ‘the universe is expanding with a uniform acceleration’?

I have to agree. It seems that, with the recent Missing Mass Hysteria, there are some issues to do with astronomical press releases, namely that the truth is somewhat stretched to make the story interesting to the media.

This is, in my opinion, a problem. Some feedback I have received on this is that any press is good press. But I don't think this is necessarily the case. What happens when dark energy is possibly shown not to be real? Or that the missing mass was not discovered by an undergraduate student during a vacation project?

Perhaps the assumption is that the general public has a short memory, or that telling the complete truth doesn't really matter. Continuing down this road, we can expect this to come back to bite us. We should not forget what happens to "sexed up" stories, and our colleagues in climate science are actively accused as liars in various areas of the press.

My feeling is, therefore, that the astronomers have to pull their heads in a little, and tell it like it is. Perhaps tempering their press releases to more realistically reflect the actual science, rather than pandering to the hyperbolic to get the press interested.

The press, on the other hand, has to realise that science is not all Nobel prize winning results, and that incremental science is still newsworthy. Media that actually reflects the doing of science is not a bad thing (and will potentially reassure those entering science that you don't need to be Nobel prize winner to make a realistic contribution - something we all learn in the end).

Closing with Nerlich, his final comment hits the mark;

Not saying it’s impossible, but no way has anyone confirmed that dark energy is real. Our flat universe is expanding with a uniform acceleration. For now, that is the news story.

Should we even have a "Cosmology" prize?

2011-06-05T08:29:00.000+10:00

As part of the discussion on the Gruber Prize being held over at Peter Coles's blog, the question whether a prize in one small part of astrophysics, namely cosmology, is a good idea? Not that I have checked in detail, but I doubt there are similar prizes in symbiotic stars, dwarf galaxies, intra-cluster gas etc. So why cosmology?

A quick squizz at wikipedia you can see that the Gruber Prize in cosmology is one of five international prizes awarded by the Peter and Patricia Gruber Foundation, the others being in Women's Rights, Genetics, Neuroscience and Justice, as well as another prize for Young Scientists. These are all worthy prize areas, and my feeling is that this is effectively private money and they can give it to whom ever they want.

Looking a little deeper (i.e. reading wikipedia a little more) that the cosmology prize is in fact given to "a leading cosmologist, astronomer, astrophysicist or scientific philosopher for theoretical, analytical or conceptual discoveries leading to a fundamental advances in the field", which is a broader definition of cosmology than would spring into many astronomers' minds.

In fact, looking at the list of recipients, we can see that the 2010 winner was (the very worthy) Chuck Steidel. Chuck's work has focuses upon faint, blue star forming galaxies at high redshift, which would clearly fall into the astrophysics camp, rather than what a lot of people call cosmology.

Rather embarrassingly, I seem to have missed the announcement of the 2010 prize, have a vague recollection of the 2009 prize to Freedman and Mould, and didn't know that Dick Bond got the 2008 prize. The last I really remember is the Supernovae teams getting it in 2007; perhaps this is because a local was the winner. I should really keep my eyes a little more open.

As George Efstathiou points out on Peter's blog, there are other prizes out there, such as the Kavli Prize for Astrophysics, that have a very broad coverage.

I guess we didn't get into this game to win prizes, and so I don't personally mind the Gruber Foundation for setting up a prize for Cosmology. It gives us, all astronomy, press coverage and hence visibility in the general population, which can only be a good thing (I have heard grumbles from other groups of physicists on what media mongrels astronomers are).

So good on the Gruber Foundation. I have a few of my own cosmological papers they can have a look at, if they are interested :)

2011 Gruber Prize for Cosmology

2011-06-03T09:41:00.000+10:00

The 2011 Gruber Prize for Cosmology has been awarded to the "Gang of Four", Marc Davis, George Efstahiou, Carlos Frenk and Simon White. The award is for their work on “their pioneering use of numerical simulations to model and interpret the large-scale distribution of matter in the Universe”.

As Peter Coles points out in his "blog", this group was undertaking simulations of cosmological structure in the mid-1980s, something we do routinely now. The startling thing is that they could only 32768 particles. These days, students will not get out of bed for less than a million particles, and often many more. This is because life has become easier, with prepackaged code, especially the wonderful "GADGET" by Volker Springel, and access to supercomputers, which has become 10-a-penny since we learnt how to cluster linux machines together. It's now hard to keep track of all of the high resolution simulations, such as "Millennium" and "Via Lactea", and the torrent of papers they produce.

And as computers get faster, with the advent of GPGPU-based supercomputers, it's only going to get worse (in a good way).

It should be remembered, however, that back in the (good-old) days, you often had to write simulation code yourself, whereas now, such coding skills are in the hands of a relative few. I'll save my grumpiness on the coding skills of many students to another day, and just finish by congratulating the Gang of Four.

Adventures in the dark side of cosmology

2011-06-02T14:57:00.000+10:00

And just to round off this first day of posting, here's a link to my other article in The Conversation, namely "Adventures in the dark side of cosmology".

This was in response to a press-release titled "Dark Energy is Real" which recently came out of the "WiggleZ", and is a bit of a comment on what "real" means in cosmology.

By the way, WiggleZ is not pronounced "Wiggle-Zee", or "Wiggle-Zed", but after an Australian Cultural Icon.

Will we ever see the Big Bang?

2011-06-01T13:37:00.000+10:00

As an opening post to this blog, I have very recently published an article in The Conversation entitled "Will we ever see the Big Bang?"