Saturday, 30 March 2013

Fun with Planck

Easter is here, and so a little weekend's diversion. I thought I would have a little fun with Planck maps of the CMB sky we looked at previously. As a reminder, here's what Planck saw
and what the blue and red spots indicate are structures in the Universe when it was only 400,000 years old.

Also remember that we describe these bumps and wiggles by the power spectrum which tells us how many blobs of particular sizes we should see in the image. Our theories tell us the power spectrum, but not the location of each individual blob in the image.

As I mentioned last time, we describe the structure we see in terms of spherical harmonics, which are particularly useful functions to use on a sphere, and the power spectrum basically tells us how much of a particular function we need to add in to get the map above.

But we have a freedom, called the "phase" of the spherical harmonics. Basically, while the power spectrum tells you much of a function you need to add, it doesn't tell you how to orient it. So we can mix up the phases and make another realisation of the CMB sky.

So, this is what I did. I again used healpix and grabbed a power spectrum from CAMB, and started making my own CMBs. Here's a few I made earlier :)

How cool is that? They look like the observed CMB maps (which, of course, they must!)

But what are these other realizations? Well, the view from each point in the Universe will have a differing view of the CMB, but the power spectrum must be the same, so the only thing that varies from place to place must be the phases. And given that the Universe is infinitely large, I figure that every combination of phases must be realised somewhere, and so these are views of the CMB from some locations out there in the Universe.

Which got me to thinking... There must be some randomly selected combination of phases that results in apparent structure in the CMB map. There must be places where the spherical harmonics line-up in such a way to make a recognisable image! What kind of thing could you get?

Well, a bit of mucking about on a Saturday morning, while watching Good Game Spawn Point, resulted in the following
Of course, I put the image in there, and kept the phases, but I changed the amplitude of the raw power spectrum to match that predicted by our theoretical models. And just to prove it, here's the input (theoretical) power spectrum over plotted on the power spectrum for the above map.
Can only see one curve? That's because the two sit exactly on the top of each other. We could add any picture to the CMB, such as the Mona Lisa, or David Bowie, and I'm sure we could play this game.

Realistically, most places in the Universe with have phases that give the random blobs seen in our view of the CMB skies, and combinations of phases that give us recognisable structures on the sky would be very, very rare.

But, again, given that the Universe is infinite, I would like to think that there is a spot in the Universe where the phases can combine to give us the "Cyberman in the Sky" map above.

I wish it was right here. That would have made for a very interesting Planck Press Release!

Saturday, 23 March 2013

Inflation deflation.... The Universe from Planck

I've been away for a few days at the Synthetic Universes for Future Survey at the University of Western Australia (phew - Perth is a long way away!). It was an excellent meeting, presenting just what we need to do with regards to making synthetic universes so we can understand what is going on in our own.

As well as coinciding with the equinox, we also watched the press conference covering the latest results  from the Planck Satellite. While this was a spectacular success, it also was a bit of a disappointment. You might wonder how this can be. Let me explain.

The Cosmic Microwave Background (CMB) was discovered in 1964, and it was realised that this was the ever-cooling radiation left over from the Big Bang. I have to show ab obligatory history of the universe to explain how it has trundled through the universe.
You can think that the CMB was born at the end of inflation, and for a few hundreds of thousands of years the universe was a plasma soup, with protons, electrons and radiation all bouncing around. When the universe cooled enough, when it was around 380,000 years old, the electrons were able to join with the protons, and the first hydrogen atoms were born.

But once the hydrogen atoms form, then they don't play ball with the photons any more, and the "bouncing off each other" stops. The photons happily travel along, cooling as they go, until they are detected in our telescopes.

While the universe was very smooth just after inflation, in the 380,000 years until the first hydrogen forms, gravity had done its thing, and mass had started to move into the lumps that become the galaxies and clusters we see today. In human terms, it was still pretty smooth, but these very small density differences have an effect on the radiation. Some find themselves in the middle of baby clusters, and have to climb out, making them slightly cooler than the others. Others are in voids and gain energy as they fall towards the masses, making them slightly warmer.

This prediction that the CMB should have slightly different temperatures was made a long time ago, but it wasn't until the 1990s, with the observations of COBE that these temperature differences were observed. Here's what COBE saw

This is the entire sky view of the CMB, with the red being slightly hotter, and the blue being slightly cooler. This was proclaimed one of the greatest discoveries of all time, but to scientists it's not just the fact that the blobs are there, but the properties of the blobs. Not exactly where they are, but how many big blobs there are compared to smaller blobs. 

The distribution of blobs is normally expressed as a power spectrum (which those who are conversant with Fourier transforms may be familiar with). 

Above is the power spectrum for the CMB, with large scales on the left and small scales on the right. The black points are the results from COBE, and these are pretty flat, saying that a range of typical blob sizes are expected. But COBE was limited by the resolution of its instruments and it could not see smaller scale blobs (they were blurred out). 

After COBE, a range of other telescopes, some carried aloft by balloon, with sharper eyes, were able to see smaller and smaller blobs; these are the other coloured points in the plot. As you can see, there is a peak around 200, meaning that there are more blobs of that particular size than those around 10 or 1000.

However, if you look closely near 1000, there appears to be another peak. In fact, our theories of the early universe and inflation predict these peaks, and predict that there should be more beyond 1000. We needed even sharper eyes!

And that's that we got with WMAP, which was launched in 2001. Here's the view of the CMB sky as seen by WMAP;
If you look at the COBE map, you can see that there is a good correspondance, with big cool and hot spots seen in both, but there is soooooo much more resolution with WMAP. What about the power spectrum now?
The big peak is still there, as is the second people, and now another is clearly visible. It might seem like a lot of work, but the red curve is the fit to the data using our cosmological models, and it is incredible how well it work. This fit gives us the measure of the make-up of the universe; if you want to see how it works, check out Wayne Hu's superb animations.

And now to Planck, which was launched in 2009. How does the sky look to it?
A quick glimpse reveals more detail than WMAP, but here's a more direct comparison.
Clearly, the picture with Planck is so much clearer! And this is reflected in the power spectrum.
Again, the curve through the data is the best fit cosmological model, and it work. And it works well! Incredibly so. And here's where the problems begin....

Problems you ask? What problems? Surely this is beautiful?

Yes, it is. But what more have we learnt about the universe? Well, its make up and age etc have been tweaked a little. Here's the current energy budget
Small changes. Our prevailing model of the universe, a universe dominated by a dark energy which is consistent with a cosmological constant, works extremely well in explaining the cosmos we see around us.

While this is an incredible discovery, and you might think that this should fill us with elation, we are left with a problem of asking "what do we do next?" While there are lots of theoretical ideas bubbling around out there, some robust, lots less than speculative, what we are seeing is a particularly simple universe.

What would have been cool would have been if there were deviations from the expected signal, providing clues to where we should be looking next, pointing to what ideas need exploring, to what might provide the next big break through. 

But the universe as laid down in the 1920s, and honed in the last few decades, works extremely well. We have our "Standard Model" of cosmology, similar to the particle physicists are scrabbling for ideas to get beyond their "Standard Model". 

However, both have small "anomalies", not really overly significant (which means they could go away) but might be a starting point. We'll have to wait and see...

At the Perth meeting, we had a little sweep stake on what we thought the outcome of the Planck results may be. I was the tic-tac man and here were the votes on what the room (of around 40-50) thought the results would be (taken from AstroKatie). 
Virtually nobody thought that cosmology was about to be over turned, and the vast majority wanted "a hint of something" that would point us in a new direction. While there was a hint, it had been seen in the WMAP result,  "Vanilla Lambda-CDM" won. Well, it was the one I voted for, and I was the adjudicator :)



Saturday, 16 March 2013

A kinematic study of the Andromeda dwarf spheroidal system

Argh!! Another couple of weeks have flown by like a huge fast flying thing! So fast, I almost missed my birthday (which I share with another random person). And the pressure ain't off, so a quick post.

This time, a great paper by Michelle Collins.

As you will have guessed by now, we've been doing a lot of work on the dwarf galaxy population that orbits around our nearest large cosmological neighbour, namely the Andromeda Galaxy. We, and other groups, have been measuring lots of properties of these dwarfs, including their positions and distances. Over the last few years, we've also managed to get the spectra of many stars in the dwarfs which, for one thing, allows us to measure the velocities of stars (via the famous Doppler effect).

Michelle's paper focuses upon the velocities found in 18 of the 28 dwarfs we know live near Andromeda, and basically tries to find out lots of the nature of the dwarf population - such as are they all the same, or different, and how?

The paper is a monster, more than 40 pages, so I can't go into depth on every thing we did, but here's a nice couple of illustrations.

This shows how we identify stars in the dwarf itself
These are Colour-Magnitude Diagrams, which basically plot the colour (a proxy for temperature) across the bottom, and brightness (in magnitudes) up the side. A look at the left-hand plot has lots of dots, and these are a mix of stars in the Milky Way (for us, annoying contamination) and stars in the dwarf we are looking at. In the red dotted lines, you can see there is a well defined sequence, and this is the Red Giant Branch (RGB) of stars in the dwarf - basically, the ones we are after.

Without going into too much detail, Michelle has developed a method to find out, in a probabilistic sense, which stars are most likely to be the RGB stars we want, and which are contamination (as seen in the right-hand panel).

But even when you plonk down your spectrograph and collect the velocities of stars, how do you know what is in the dwarf and what's not. Here's a picky of the distribution of velocities in one of the dwarf fields
As you can see, there is a broad spread in the velocities we see, from zero to almost -600 km/s. The big broad components come from our own Milky Way (the velocities closer to zero) and Andromeda, centred around -300 km/s, but quite clearly, in red, there is a strong spike in the velocities, and these are the stars in the dwarf. Cool eh?

So, we can get a velocity for each of the dwarfs (and collects some already published) and get a velocity maps for all the dwarfs we have.
The ugly shape in there is one we have seen before, the footprint of the PAndAS survey.

So, what have we learnt? I wish I could put it in a couple of sound-bites, but that's going to be tricky, and being time-squeezed means I don't have a lot of time to type more, but here's a couple of the highlights.
This is luminosity on the bottom and mass-to-light ratio (basically, how much dark matter is in the dwarf) in the side for a mix of dwarfs from Andromeda, the Milky Way and in the Local Group. What we can see is a very strong trend, such that the less luminous an object is, the more dark matter it has relative to the stars. The littlies are the most dark matter dominated things about (although there is a whole 'nother blog post in that statement).

Here's another trend
This is again luminosity on the bottom, and metallicity, or how chemically enriched the stars are, up the side. Clearly, the more luminous ab object is, the more chemically enriched it is. This also makes sense, as larger systems can hold onto their gas and have several generations of star formation, which continually enriches the gas, where as lower mass galaxies can more easily lose gas in supernovae explosions.

In summary, we've learnt a lot, but there is a lot to learn.

Well done Michelle!

A kinematic study of the Andromeda dwarf spheroidal system

Michelle L. M. Collins, Scott C. Chapman, R. Michael Rich, Rodrigo A. Ibata, Nicolas F. Martin, Michael J. Irwin, Nicholas F. Bate, Geraint F. Lewis, Jorge PeƱarrubia, Nobuo Arimoto, Caitlin M. Casey, Annette M. N. Ferguson, Andreas Koch, Alan W. McConnachie, Nial Tanvir
We present a homogeneous kinematic analysis of red giant branch stars within 18 of the 28 Andromeda dwarf spheroidal (dSph) galaxies, obtained using the Keck I LRIS and Keck II DEIMOS spectrographs. Based on their g-i colors (taken with the CFHT MegaCam imager), physical positions on the sky, and radial velocities, we assign probabilities of dSph membership to each observed star. Using this information, the velocity dispersions, central masses and central densities of the dark matter halos are calculated for these objects, and compared with the properties of the Milky Way dSph population. We also measure the average metallicity ([Fe/H]) from the co-added spectra of member stars for each M31 dSph and find that they are consistent with the trend of decreasing [Fe/H] with luminosity observed in the Milky Way population. We find that three of our studied M31 dSphs appear as significant outliers in terms of their central velocity dispersion, And XIX, XXI and XXV, all of which have large half-light radii (>700 pc) and low velocity dispersions (sigma_v<5 km/s). In addition, And XXV has a mass-to-light ratio within its half-light radius of just [M/L]_{half}=10.3^{+7.0}_{-6.7}, making it consistent with a simple stellar system with no appreciable dark matter component within its 1 sigma uncertainties. We suggest that the structure of the dark matter halos of these outliers have been significantly altered by tides.

Tuesday, 5 March 2013

Cutting through the spin on supermassive black holes

It's the week for submission of Discovery Projects, our main funding route through the Australian Research Council. Our current proposal is almost 150 pages long, and I feel like we've been working on it since the Cosmological Dark Ages. It will be good to submit it, but don't think that means I get a break. No, it's time to catch up on all the stuff that has been on the back-burner when grant writing :)

So, a quick post today on an article I wrote for The Conversation called "Cutting through the spin on supermassive black holes". As the name suggests, I describe how astronomers can not only measure the mass of black holes, but can also calculate their spin.

I must admit that the article is a little long compared to others I've written for The Conversation, and what is published is the shortened version. It's a topic that I think is pretty cool and I put in too much detail.

I'll let you read the article, and will happily answer any questions below. The crux is that supermassive black holes spin, and often appear to be spinning as fast as they can.
Just I'll just mention part of the article that was cut due to space limitations. Essentially, the modern idea of black holes was born firstly by Einstein in his Theory of Relativity, and then properly launched by Schwarzschild working on the Eastern Front in 1917, with the spinning black hole metric being found by Kerr almost half a century later. We can now use this mathematical framework, which was initiated by a young man pondering what would happen if you could move at the speed of light, to calculate what we would see if we had gas whizzing around a spinning, billion solar mass black hole many millions of light years away.

And it works!

And to me, that shows the power of science and the human mind. Amazing.