Saturday, 28 February 2015

Shooting relativistic fish in a rational barrel

I need to take a breather from grant writing, which is consuming almost every waking hour in between all of the other things that I still need to do. So see this post as a cathartic exercise.

What makes a scientist? Is it the qualification? What you do day-to-day? The association and societies to which you belong? I think a unique definition may be impossible as there is a continuum of properties of scientists. This makes it a little tricky for the lay-person to identify "real science" from "fringe science" (but, in all honesty, the distinction between these two is often not particularly clear cut).

One thing that science (and many other fields) do is have meetings, conferences and workshops to discuss their latest results. Some people seem to spend their lives flitting between exotic locations essentially presenting the same talk to almost the same audience, but all scientists probably attend a conference or two per year.

In one of my own fields, namely cosmology, there are lots of conferences per year. But accompanying these there are another set of conferences going on, also on cosmology and often including discussions of gravity, particle physics, and the power of electricity in the Universe. At these meetings, the words "rational" and "logical" are bandied about, and it is clear that the people attending think that the great mass of astronomer and physicists have gotten it all wrong, are deluded, are colluding to keep the truth from the public for some bizarre agenda - some sort of worship of Einstein and "mathemagics" (I snorted with laughter when I heard this).

If I am being paid to lie to the public, I would like to point out that my cheque has not arrived and unless it does shortly I will go to the papers with a "tell all"!!

These are not a new phenomenon, but were often in shadows. But now, of course, with the internet, any one can see these conference in action with lots of youtube clips and lectures.

Is there any use for such videos? I think so, as, for the student of physics, they present an excellent place to tests one knowledge by identifying just where the presenters are straying off the path.

A brief search of youtube will turn up talks that point out that black holes cannot exist because
is the starting point for the derivation of the Schwarzschild solution.

Now, if you are not really familiar with the mathematics of relativity, this might look quite convincing. The key point is this equation

Roughly speaking, this says that space-time geometry (left-hand side) is related to the matter and energy density (right-hand side, and you calculate the Schwarzschild geometry for a black hole by setting the right-hand side equal to zero.

Now, with the right-hand side equal to zero that means there is no energy and mass, and the conclusion in the video says that there is no source, no thing to produce the bending of space-time and hence the effects of gravity. So, have the physicists been pulling the wool over everyones eyes for almost 100 years?

Now, a university level student may not have done relativity yet, but it should be simple to see the flaw in this argument. And, to do this, we can use the wonderful world of classical mechanics.

In classical physics, where gravity is a force and we deal with potentials, we have a similar equation to the relativistic equation above. It's known as Poisson's equation
The left-hand side is related to derivatives of the gravitational potential, whereas the right-hand side is some constants (including Newton's gravitational constant (G)) and the density given by the rho.

I think everyone is happy with this equation. Now, one thing you calculate early on in gravitational physics is that the gravitational potential outside of a massive spherical object is given by
Note that we are talking about the potential is outside of the spherical body (the simple V and Phi are meant to be the same thing). So, if we plug this potential into Poisson's equation, does it give us a mass distribution which is spherical?

Now, Poisson's equation can look a little intimidating, but let's recast the potential in Cartesian coordinates. Then it looks like this

Ugh! Does that make it any easier? Yes, let's just simply plug it into Wolfram Alpha to do the hard work. So, the derivatives have an x-part, y-part and z-part - here's the x-part.
Again, is you are a mathphobe, this is not much better, but let's add the y- and z-parts.

After all that, the result is zero! Zilch! Nothing! This must mean that Poisson's equation for this potential is
So, the density is equal to zero. Where's the mass that produces the gravitational field? This is the same as the apparent problem with relativity. What Poisson's equation tells us that the derivatives o the potential AT A POINT is related to the density AT THAT POINT! 

Now, remember these are derivatives, and so the potential can have a whole bunch of shapes at that point, as long as the derivatives still hold. One of these, of course, is there being no mass there and so no gravitational potential at all, but any vacuum, with no mass, will above Poisson = 0 equation, including the potential outside of any body (the one used in this example relied on a spherical source).

So, the relativistic version is that the properties of the space-time curvature AT A POINT is related to the mass and energy AT A POINT. A flat space-time is produced when there is no mass and energy, and so has G=0, but so does any point in a vacuum, but that does not mean that the space-time at that point is not curved (and so no gravity).

Anyway, I got that off my chest, and my Discovery Project submitted, but now it's time to get on with a LIEF application! 

Sunday, 25 January 2015

The Constant Nature of the Speed of light in a vacuum

Wow! It has been a while, but I do have an excuse! I have been finishing up a book on the fine-tuning of the Universe and hopefully it will be published (and will become a really big best seller?? :) in 2015. But time to rebirth the blog, and what a better way to start that a gripe.

There's been some chatter on the interweb about a recent story about the speed of light in a vacuum being slowed down. Here's oneHere's another. Some of these squeak loudly about how the speed of light may not be "a constant", implying that something has gone horribly wrong with the Universe. Unfortunately, some of my physicsy colleagues were equally shocked but the result.

Why would one be shocked? Well, the speed of light being constant to all observers is central of Einstein's Special Theory of Relativity. Surely if these results are right, and Einstein is wrong, then science is a mess, etc etc etc.

Except there is nothing mysterious about this result. Nothing strange. In fact it was completely expected. The question boils down to what you mean by speed.
Now, you might be thinking that speed is simply related to the time it takes for a thing to travel from here to there. But we're dealing with light here, which, in classical physics is represented by oscillations in an electromagnetic field, while in our quantum picture it's oscillations in the wave function; the difference is not important.

When you first encounter electromagnetic radiation (i.e. light) you are often given a simple example of a single wave propagating in a vacuum. Every student of physics will have seen this picture at some point;
The electric (and magnetic) fields oscillate as a sin wave and the speed at which bumps in the wave move forward is the speed of light. This was one of the great successes of James Clark Maxwell, one of the greatest physicists who ever lived. In his work, he fully unified electricity and magnetism and showed that electromagnetic radiation, light, was the natural consequence. 

Without going into too many specific details, this is known as the phase velocity. For light in a vacuum, the phase velocity is equal to c.

One of the coolest things I ever learnt was Fourier series, or the notion that you can construct arbitrary wave shapes by adding together sins and cos waves. This still freaks me out a bit to this day, but instead of an electromagnetic wave being a simple sin or cos you can add waves to create a wave packet, basically a lump of light.

But when you add waves together, the result lump doesn't travel at the same speed as the waves that comprise the packet. The lump moves with what's known as the group velocity. Now, the group velocity and the phase velocity are, in general, different. In fact, they can be very different as it is possible to construct a packet that does not move at all, while all the waves making up the packet are moving at c!

So, this result was achieved by manipulating the waves to produce a packet whose group velocity was measurably smaller than a simple wave. That's it! Now, this is not meant to diminish the work of the experimenters, as this is not easy to set up and measure, but it means nothing for the speed of light, relativity etc etc. And the researchers know that!

And as I mentioned, understanding the difference between phase and group velocity has been known for a long time, with Hamilton (of Hamiltonian fame) in 1839, and Rayleigh in 1877. These initial studies were in waves in general, mainly sound waves, not necessarily light, but the mathematics are basically the same. 

Before I go, once of the best course I took as an undergraduate was called vibrations and waves. At the time, I didn't really see the importance of of what I was learning, but the mathematics was cool. I still love thinking about it. Over the years, I've come to realise that waves are everywhere, all throughout physics, science, and, well everything. Want to model a flag, make a ball and spring model. Want to make a model of matter, ball and spring. And watch the vibrations!

Don't believe me? Watch this - waves are everywhere.

Thursday, 6 November 2014

Scientists Have Figured Out What Colour The Universe Is

What's old is new again?

Over at Business Insider Australia we are told (with some lovely language) that Scientists have figured out what colour the Universe is. You've got to love a new and interesting astronomy story, but alas, the result is rather, well, beige (I refuse to say latte).
But that's not the point of this post. Now, I am not a young man any more, and my memory is not what it was, but I know I had heard this story before, somewhere in the past.

The story doesn't name any scientists or cite an original article, and so I turned to google. Hmmm - virtually the same story appeared in the UK Telegraph in 2009! Unsurprisingly, the Daily Mail carry very similar stories.

So this "news" is at least 5 years old! But now we have some names! Karl Glazebrook and Ivan Baldry!

A little more detective work leads to New Scientist article entitled The Universe is not turquoise – it's beige. This story from 2002!!!!!!!! corrects and earlier story in which the colour of the Universe was found to be turquoise! So, we're now looking at a story from 12 years ago!

Deep in the depths of my brain I remembered the entire saga (cards on table, I used to work at the AAO with Karl and Ivan) about how they averaged the colours of galaxies, initially finding the average colour to be bluish and then finding with the correct colour interpretation its actually whitish. In fact, Karl has a nice write up on the who thing here.

BUT why has this story resurfaced? There is no new data, no new analysis, nothing. In fact, seven seconds of thought and clicks takes you to a wikipedia page that points out the original press release with the incorrect colour was in 2001, 13 years ago.

So, is there really no new science news interesting enough to be published this week? Do we have to dig up stories from the past? What do we expect to see next? "Scientists discover the universe is expanding"? "Scientist link the orbit of the moon to the falling of an apple"? or even "Proto-scientists discover this water stuff does not burn"? Come one media, you can do better than this!

Sunday, 26 October 2014

The Redshift Drift

Things are crazily busy, with me finishing teaching this week. Some of you may know, that I am writing a book, which is progressing, but more slowly than I hoped. Up to just over 60,000 words, with a goal of about 80 to 90 thousand, so more than half way through.

I know that I have to catch up with papers, and I have another article in The Conversation brewing, but I thought I would write about something interesting. The problem is that my limited brain has been occupied by so many other things that my clear thinking time has been reduced to snippets here and there.

But one thing that has been on my mind is tests of cosmology. Nothing I post here will be new, but you might not know about it. But here goes.

So, the universe is expanding. But how do we know? I've written a little about this previously, but we know that almost 100 years ago, Edwin Hubble discovered his "law", that galaxies are moving away from us, and the further away they are, the faster they are moving. There's a nice article here describing the importance of this, and we end up with a picture that looks something like this
Distance is actually the hard thing to measure, and there are several books that detail astronomers on-off love affair with measuring distances. But how about velocities?

These are measured using the redshift. It's such a simple measurement. In our laboratory, we might see emission from an element, such as hydrogen, at one wavelength, but when we observe it in a distant galaxy, we see it at another, longer, wavelength. The light has been redshifted due to the expansion of the universe (although exactly what this means can be the source for considerable confuddlement).

Here's an illustration of this;
Relating the redshift to a Doppler shift we can turn it into a velocity. As we know, the Hubble law is  what we expect if we use Einstein's theory of relativity to describe the universe. Excellent stuff all around!

One thing we do know is that the expansion rate of the universe is not uniform in time. It was very fast at the Big Bang, slowed down for much of cosmic history, before accelerating due to the presence of dark energy.

So, there we have an interesting question. Due to the expansion of the universe, will the redshift I measure for a galaxy today be the same when I measure it again tomorrow.

This question was asked before I was born and then again several times afterwards. For those that love mathematics, and who doesn't, you get a change of redshift with time that looks like this

(taken from this great paper) where z is the redshift, Ho is Hubble's constant today, while H(z) is Hubble's constant at the time the light was emitted from the galaxy your observing. 

The cool thing is that last term depends upon the energy content of the universe, just how much mass there is, how much radiation, how much dark energy, and all the other cool things that we would like to know, like if dark energy is evolving and and changing, or interacting with matter and radiation. It would be a cool cosmological probe.

Ah, there is a problem! We know that Hubble's constant is about Ho = 72 km/s/Mpc, which seems like a nice sort of number. But if you look closely, you can see that it actually had units of 1/time. So, expressing it in years, this number is about 0.0000000001 per year. This is a small number. Bottom.

But this does not mean that astronomers pack up their bags and head home. No, you look for solutions and see if you can come up with technologies to allow you to measure this tiny shift. I could write an entire post on this, but people are developing laser combs to give extremely accurate measurement of the wavelength in spectra, and actually measure the changing expansion of the Universe in real time!

Why am I writing about this? Because these direct tests of cosmology have always fascinated me, and every so often I start doodling with the cosmological equations to see if I can come up with another one. Often this ends up with a page of squiggles and goes no where, but some times I have what I thing is a new insight.

And this gives me a chance to spruik an older paper of mine, with then PhD student, Madhura Killedar. I still love this stuff!

The evolution of the expansion rate of the Universe results in a drift in the redshift of distant sources over time. A measurement of this drift would provide us with a direct probe of expansion history. The Lyman alpha forest has been recognized as the best candidate for this experiment, but the signal would be weak and it will take next generation large telescopes coupled with ultra-stable high resolution spectrographs to reach the cm/s resolution required. One source of noise that has not yet been assessed is the transverse motion of Lyman alpha absorbers, which varies the gravitational potential in the line of sight and subsequently shifts the positions of background absorption lines. We examine the relationship between the pure cosmic signal and the observed redshift drift in the presence of moving Lyman alpha clouds, particularly the collapsed structures associated with Lyman limit systems (LLSs) and damped Lyman alpha systems (DLAs). Surprisingly, the peculiar velocities and peculiar accelerations both enter the expression, although the acceleration term stands alone as an absolute error, whilst the velocity term appears as a fractional noise component. An estimate of the magnitude of the noise reassures us that the motion of the Lyman alpha absorbers will not pose a threat to the detection of the signal.

Catching the Conversation

Wow!!! Where has time gone! I must apologise for the sluggishness of posts on this blog. I promise you that it is not dead, I have been consumed with a number of other things and not all of it fun. I will get back to interesting posts as soon as possible.

So, here's a couple of articles I've written in the meantime, appearing in The Conversation

One on some of my own research: Dark matter and the Milky Way: more little than large

And the other on proof (or lack of it) in science: Where’s the proof in science? There is none

There's more to come :)

Wednesday, 27 August 2014

Sailing under the Magellanic Clouds: A DECam View of the Carina Dwarf

Where did that month go? Winter is almost over and spring will be breaking, and my backlog of papers to comment on is getting longer and longer.

So a quick post this morning on a recent cool paper by PhD student, Brendan McMonigal, called "Sailing under the Magellanic Clouds: A DECAm View of the Carina Dwarf". The title tells a lot of the story, but it all starts with a telescope with a big camera.

The camera is DECam, the Dark Energy Camera located on the 4m CTIO telescope in Chile. This is what it looks like;
It's not one CCD, but loads of them butted together allowing us to image a large chunk of sky. Over the next few years, this amazing camera will allow the Dark Energy Survey which will hopefully reveal what is going on in the dark sector of the Universe, a place where Australia will play a key-role through OzDES.

But one of the cool things is that we can use this superb facility to look at other things, and this is precisely what Bendan did. And the target was the Carina Dwarf Galaxy. Want to see this impressive beast! Here it is;
See it? It is there, but it's a dwarf galaxy, and is so quite faint. Still can't see it? Bring on DECam. We pointed DECam at Carina and took a picture. Well, a few. What did we see?
So, as you can see, we took 5 fields (in two colours) centred on the Carina dwarf. And with the superb properties of the camera, the dwarf galaxy nicely pops out.

But science is not simply about taking pictures, so we constructed colour-magnitude diagrams for each of the fields. Here's what we see (and thanks Brendan for constructing the handy key for the features in the bottom-right corner).
All that stuff in the labelled MW are stars in our own Milky Way, which is blooming contamination getting in our way. The blob at the bottom is where the we are hitting the observational limits of the camera, and can't really tell faint stars from galaxies.

The other bits labelled Young, Intermediate and Old tell us that Carina has had several bursts of star-formation during its life, some recent, some a little while ago, and some long ago (to express it in scientific terms), while the RGB is the Red Giant Branch, RC is the Red Clump and HB is the Horizontal Branch.

We can make maps of each of the Young, Intermediate and Old population stars, and what we see is this;
The Young and Intermediate appear to be quite elliptical and smooth, but the Old population appears to be a little ragged. This suggests that long ago, Carina has been shaken up through some gravitational shocks when it interacted with the larger galaxies of the Local Group, but the dynamics of these interactions are poorly understood.

But there is more. Look back up there at the Colour-Magnitude Diagram schematic and there is a little yellow wedge labelled LMC, the Large Magellanic Cloud; what's that doing there?

What do we see if we look at just those stars? Here's what we see.
So, they are not all over the place, but are located only in the southern field, overlapping with Carina itself (and making it difficult to separate the Old Carina population from the Magellanic Cloud stars).

But still, what are they doing there? Here's a rough map of the nearby galaxies.
As we can see, from the view inside the Milky Way, Carina and the LMC appear (very roughly) in the same patch of sky but are completely different distances. But it means that the Large Magellanic Cloud must have a large halo of stars surrounding it, possibly puffed up through interactions with the Small Magellanic Cloud as they orbit together, and with the mutual interaction with the Milky Way.

It's a mess, a glorious, horrendous, dynamically complicated mess. Wonderful!

Well done Brendan!

Sailing under the Magellanic Clouds: A DECam View of the Carina Dwarf

We present deep optical photometry from the DECam imager on the 4m Blanco telescope of over 12 deg[Math Processing Error] around the Carina dwarf spheroidal, with complete coverage out to 1 degree and partial coverage extending out to 2.6 degrees. Using a Poisson-based matched filter analysis to identify stars from each of the three main stellar populations, old, intermediate, and young, we confirm the previously identified radial age gradient, distance, tidal radius, stellar radial profiles, relative stellar population sizes, ellipticity, and position angle. We find an angular offset between the three main elliptical populations of Carina, and find only tentative evidence for tidal debris, suggesting that past tidal interactions could not have significantly influenced the Carina dwarf. We detect stars in the vicinity of, but distinct to, the Carina dwarf, and measure their distance to be 46[Math Processing Error]2 kpc. We determine this population to be part of the halo of the Large Magellanic Cloud at an angular radius of over 20 degrees. Due to overlap in colour-magnitude space with Magellanic stars, previously detected tidal features in the old population of Carina are likely weaker than previously thought.

Friday, 25 July 2014

A cosmic two-step: the universal dance of the dwarf galaxies

We had a paper in Nature this week, and I think this paper is exciting and important. I've written an article for The Conversation which you can read it here.