the new BOSS in town

I wrote the following post yesterday, but I fell asleep before I could do anything with it. It’s about the first set of results from the Baryon Oscillation Spectroscopic Survey (BOSS), part of Sloan Digital Sky Survey-III project, which we announced to the science community and to the press yesterday. How this whole project was picked up by the press in a way I hadn’t anticipated is the matter for another post. What really matters is the science, and the science – if you don’t mind my exceedingly biased opinion – is just excellent.


I’m now making my way back home from this year’s National Astronomy Meeting (NAM) 2012 in Manchester. I love NAM. It’s always a chance to see old friends and listen to good science, to catch up on gossip and long-promised pints. This year, I did almost none of these things. The reason is that one of the teams I’m working with announced a pretty exciting set of science results today, in the form of six papers. This involved submitting the papers to the journals and to the arXiV yesterday. And that means that yesterday we panicked about coordinating submission of 6 papers across the globe, and today we panicked about press conferences, press interviews and science talks.

So what was all the fuss about? In essence, it all comes down to a map. You see, 14 years ago a number of astronomers announced a pretty remarkable observation: that the Universe’s expansion is accelerating. If the Universe is made only of matter (dark or bright), and the only force acting on large scales is gravity, then the Universe’s expansion should be doing exactly the opposite. This discovery, based on the observation of distant supernovae type Ia explosions (awarded with the Nobel Prize for Physics last year, and that I’ve covered here before), shook our cosmological model to the ground; the job of the science community was to build it back up.

There are a number of proposed explanations for this phenomenon, which astronomers named Dark Energy. The simplest from a theoretical point of view (but not from a conceptual point of view – more on that later) is the idea of a Cosmological Constant – a simple constant term in Einstein’s equations of General Relativity that would naturally explain the acceleration. In physical terms, this would equate to a rather bizarre property of empty space, a sort of vacuum energy that would cause space itself to expand. Einstein originally included this constant in his equations to ensure that the Universe was static. When Hubble announced his observation that the Universe was expanding, Einstein removed this constant from his theory and allegedly referred to it later as is ‘biggest blunder’. It’s ironic that we now put it back in – albeit with the opposite sign – to explain an observation that probably never crossed Einstein’s mind.

At the other extreme of possible explanations for Dark Energy we have modified gravity models. Einstein’s theory of General Relativity has passed some stringent tests at solar-system scales. But if this theory breaks down on very large scales, or at very low densities, then perhaps this acceleration is just really an illusion that stems from a ill-conceived theory.

So which is it? How can we tell? That’s where this map comes along. It turns out that one of the most effective ways to learn more about Dark Energy is to map out how matter is distributed in the Universe. As most of the matter out there is in the form of Dark Matter, and because Dark Matter doesn’t lend itself to an easy detection, we map galaxies instead. Galaxies are faithful (if sometimes cheeky) tracers of Dark Matter, so by mapping their positions we are essentially mapping the matter in the Universe. We map their position in the sky, and we use spectrographs to measure a redshift to each individual galaxy – resulting in a three-dimensional map of our Universe.

This map gives us two main ways with which to study Dark Energy.

Firstly it gives us the means with which to measure the expansion rate of the Universe. If we want to learn about this new phenomenon, then we best start by carefully measuring its effects. We do this by finding a measuring rod in the Universe, something whose size is known. Thankfully, Nature was kind enough to provide us with such a measuring rod, and this is how it happened: shortly after the Big Bang, the Universe was so hot and dense that all matter was in the form of a plasma and the Universe behaved very much like a fluid. This fluid wasn’t all of uniform density – some parts were denser than others, and these small gradients set up pressure waves in the fluid that propagated at a speed related to the density of the fluid (that in turn depends on its composition, temperature, etc). They set up ripples if you like, of a characteristic wavelength, that can still be detected in the distribution of galaxies today.

These are called Baryon Acoustic Oscillations. ‘Baryon’ because they exist only due to the interaction of baryonic (normal, i.e., not dark) matter with radiation at extreme temperature and densities; Acoustic because they are pressure driven, just like sound is; and Oscillations because they are waves. BAO, for short.

We know this wavelength – or size – pretty well by looking for example at the cosmic microwave background. So if we can measure it at different times of the Universe’s history, we can map out its expansion in a very clean and robust way. And we can measure it, by looking at the distribution of galaxies at times in the past (or equivalently, at galaxies at different distances from us – thanks to the finite speed of light). This BAO size shows itself in the distribution of galaxies as a distance at which galaxies are preferentially distanced from one another. The magic number is around 500 million light-years. Let me quote Daniel Eisenstein on this, the director of SDSS-III,

“Because of the regularity of those ancient waves, there’s a slightly increased probability that any two galaxies today will be separated by about 500 million light-years, rather than 400 million or 600 million.”

And that is the whole point – to measure the distance at which we find a slight excess of finding pairs of galaxies. As it has been expanding with the size of the Universe, tracking it means tracking the cosmic expansion.

BOSS mapped the positions of 250,000 galaxies that are roughly 6 billion light years away for Earth. We measured the BAO scale with a phenomenal 1.7% precision. When we combined it with other BAO scale measurements, we found an expansion rate consistent with a Universe dominated by Dark Energy in the form of a cosmological constant, and otherwise consistent with the General Relativity.

But this isn’t the whole story. The trouble is that you can always modify the laws of gravity to match any expansion history measurement in such a way that it becomes almost impossible to tell the difference between a GR + Dark Energy combo, from a Modified Gravity (MG) and no Dark Energy model if we just measure the Universe’s expansion. There’s one more test we can do, however, that tests gravity directly.

The galaxy map we measured was shaped by the continual tug of war between the two dominant forces in the Universe – on one hand we have Dark Energy accelerating things apart, and on the other we have gravity, pulling everything together.

Gravity causes structure to form – small fluctuations in matter density in the early Universe gave rise to the largest structures in the Universe today (I’m talking about galaxies, and clusters of galaxies) simply due to gravity. Importantly for the question at hand, this growth allows us to test our understanding of the laws of gravity. If gravity behaves as Einstein suggested it did, even at these colossal scales, we know how fast structure should be growing in the Universe.

Gravity and dark energy affect the motions of galaxies in different ways. By large, the motion of a galaxy is set by the expansion of the Universe (hence the relevance of the BAO measurements). But if you look closely enough, if you have enough galaxies, you can also detect the velocity component due to growth; due to gravity.

Now, remember that our map is really a redshift map. On one direction and one direction alone – that is along our line of sight – it gives us a recession velocity, or redshift. As galaxies are infallling into clusters, as structure grows, galaxies appear to be closer than they really are along this direction. This distorts our map in a very peculiar way, squashing it along our line of sight. A faster growth of structure results in a more prominent ‘squashing’ signal. This gives the name to this technique – redshift space distortions.

My measuring these distortions we can measure the rate at which structure grows and we can test gravity. BOSS, with its large volume and number of galaxies, set a rigorous test of Einstein’s theory of gravity at these very large scales. And what is outstanding is that it still seems to work.

This is a profound statement about the physical nature of our Universe, and one that we don’t make lightly. Of course we need to continue to look ahead. As we gather more data (and BOSS is only one third of the way through collecting data), we can make more precise measurements and more interesting tests.

To quote the principal investigator for BOSS, David Schlegel: “If there are surprises lurking out there, we expect to find them.”

Lauren Anderson, Eric Aubourg, Stephen Bailey, Dmitry Bizyaev, Michael Blanton, Adam S. Bolton, J. Brinkmann, Joel R. Brownstein, Angela Burden, Antonio J. Cuesta, Luiz N. A. da Costa, Kyle S. Dawson, Roland de Putter, Daniel J. Eisenstein, James E. Gunn, Hong Guo, Jean-Christophe Hamilton, Paul Harding, Shirley Ho, Klaus Honscheid, Eyal Kazin, D. Kirkby, Jean-Paul Kneib, Antione Labatie, Craig Loomis, Robert H. Lupton, Elena Malanushenko, Viktor Malanushenko, Rachel Mandelbaum, Marc Manera, Claudia Maraston, Cameron K. McBride, Kushal T. Mehta, Olga Mena, Francesco Montesano, Demetri Muna, Robert C. Nichol, Sebastian E. Nuza, Matthew D. Olmstead, Daniel Oravetz, Nikhil Padmanabhan, Nathalie Palanque-Delabrouille, Kaike Pan, John Parejko, Isabelle Paris, Will J. Percival, Patrick Petitjean, Francisco Prada, Beth Reid, Natalie A. Roe, Ashley J. Ross, Nicholas P. Ross, Lado Samushia, Ariel G. Sanchez, David J. Schlegel Donald P. Schneider, Claudia G. Scoccola, Hee-Jong Seo, Erin S. Sheldon, Audrey Simmons, Ramin A. Skibba, Michael A. Strauss, Molly E. C. Swanson, Daniel Thomas, Jeremy L. Tinker, Rita Tojeiro, Mariana Vargas Magana, Licia Verde, Christian Wagner, David A. Wake, Benjamin A. Weaver, David H. Weinberg, Martin White, Xiaoying Xu, Christophe Yeche, Idit Zehavi, & Gong-Bo Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Release 9
Spectroscopic Galaxy Sample arXiv arXiv: 1203.6594v1

Ashley J. Ross, Will J. Percival, Ariel G. Sanchez, Lado Samushia, Shirley Ho, Eyal Kazin, Marc Manera, Beth Reid, Martin White, Rita Tojeiro, Cameron K. McBride, Xiaoying Xu, David A. Wake, Michael A. Strauss, Francesco Montesano, Molly E. C. Swanson, Stephen Bailey, Adam S. Bolton, Antonio Montero Dorta, Daniel J. Eisenstein, Hong Guo, Jean-Christophe Hamilton, Robert C. Nichol, Nikhil Padmanabhan, Francisco Prada, David J. Schlegel, Mariana Vargas Magana, Idit Zehavi, Michael Blanton, Dmitry Bizyaev, Howard Brewington, Antonio J. Cuesta, Elena Malanushenko, Viktor Malanushenko, Daniel Oravetz, John Parejko, Kaike Pan, Donald P. Schneider Alaina Shelden, Audrey Simmons, Stephanie Snedden, & Gong-bo Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: Analysis of potential systematics arXiv arXiv: 1203.6499v1

Rita Tojeiro, W. J. Percival, J. Brinkmann, J. R. Brownstein, D. Eisenstein, M. Manera, C. Maraston, C. K. McBride, D. Duna, B. Reid, A. J. Ross, N. P. Ross, L. Samushia, N. Padmanabhan, D. P. Schneider, R. Skibba, A. G. Sanchez, M. E. C. Swanson, D. Thomas, J. L. Tinker, L. Verde, D. A. Wake, B. A. Weaver, & G. Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: measuring structure growth using passive galaxies arXiv arXiv: 1203.6565v1

Marc Manera, Roman Scoccimarro, Will J. Percival, Lado Samushia, Cameron K. McBride, Ashley Ross, Ravi Sheth, Martin White, Beth Reid, Ariel Sánchez, Roland de Putter, Xiaoying Xu, Lauren Anderson, Andreas A. Berlind, Jonathan Brinkmann, Bob Nichol, Francesco Montesano, Nikhil Padmanabhan, Ramin A. Skibba1, Rita Tojeiro, & Benjamin A. Weaver (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: a large sample of mock galaxy catalogues arXiv arXiv: 1203.6609v1

Ariel G. Sanchez, C. G. Scoccola, A. J. Ross, W. Percival, M. Manera, F. Montesano, X. Mazzalay, A. J. Cuesta, D. J. Eisenstein, E. Kazin, C. K. McBride, K. Mehta, A. D. Montero-Dorta, N. Padmanabhan, F. Prada, J. A. Rubino-Martin, R. Tojeiro, X. Xu, M. Vargas Magana, E. Aubourg, N. A. Bahcall, S. Bailey, D. Bizyaev, A. S. Bolton, H. Brewington, J. Brinkmann, J. R. Brownstein, J. Richard Gott III, J. C. Hamilton, S. Ho, K. Honscheid, A. Labatie, E. Malanushenko, V. Malanushenko, C. Maraston, D. Muna, R. C. Nichol, D. Oravetz, K. Pan, N. P. Ross, N. A. Roe, B. A. Reid, D. J. Schlegel, A. Shelden, D. P. Schneider, A. Simmons, R. Skibba, S. Snedden, D. Thomas, J. Tinker, D. A. Wake, B. A. Weaver, David H. Weinberg, Martin White, I. Zehavi, & G. Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: cosmological implications of the large-scale two-point
correlation function arXiv arXiv: 1203.6616v1

Beth A. Reid, Lado Samushia, Martin White, Will J. Percival, Marc Manera, Nikhil Padmanabhan, Ashley J. Ross, Ariel G. Sánchez, Stephen Bailey, Dmitry Bizyaev, Adam S. Bolton, Howard Brewington, J. Brinkmann, Joel R. Brownstein, Antonio J. Cuesta, Daniel J. Eisenstein, James E. Gunn, Klaus Honscheid, Elena Malanushenko, Viktor Malanushenko, Claudia Maraston, Cameron K. McBride, Demitri Muna, Robert C. Nichol, Daniel Oravetz, Kaike Pan, Roland de Putter, N. A. Roe, Nicholas P. Ross, David J. Schlegel, Donald P. Schneider, Hee-Jong Seo, Alaina Shelden, Erin S. Sheldon, Audrey Simmons, Ramin A. Skibba, Stephanie Snedden, Molly E. C. Swanson, Daniel Thomas, Jeremy Tinker, Rita Tojeiro, Licia Verde, David A. Wake, Benjamin A. Weaver, David H. Weinberg, Idit Zehavi, & Gong-Bo Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: measurements of the growth of structure and expansion
rate at z=0.57 from anisotropic clustering arXiv arXiv: 1203.6641v1

Reading Douglas

Today it’s international towel day. If you don’t know what that means, you’re ill prepared for what lies ahead of you – meaning life, in general terms, which has a way to throw the unexpected at you. Hence your need for a towel, even if you don’t know it yet.

Today is also a good day to take 5 minutes and remind yourself and the rest of us of a passage, a sentence, a moment that Douglas Adams pulled out of his head at some point in time and that it never fails to make you smile. There are many for me, but this morning this one is the first that came to my head. Twitter’s limitations meant that I decided to read it instead, and Emma challenged me to put it up here. As she rightly points out – it’s space related. Kind of. I now challenge my fellow gutter dwellers to follow suit. And I challenge you.

I also want to say just how great a book ‘Last Chance to See’ is. It brings out the best of the world, even in some seriously dire situations. Sometimes something happens and I wish Douglas Adams was there to write it down, and crystallise something that my brain recognises as funny and ironic and insightful, but which my brain can’t quite bring into words. This is why I love this passage. I’m so much like that megapode, and yet it took Douglas’ genius to make me realise that, and endlessly smile at the world as I do so. Chances are,  I would see that megapode nest, write code to compute its volume, and simply leave with the nagging feeling that there was something else to the whole situation than what meets the eye.

Douglas Adams was great at these something elses. I wish he was around for more.

Supernovae – setting the standard, part II

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In part I of this blog post I told you how supernovae type Ia have proven to be so important in defining today’s standard model of Cosmology. I did, however, leave out some important details so let’s get stuck right in.

Type Ias don’t always explode with the same brightness. There seems to be some intrinsic variation – some are a little dimmer, and some are a little brighter. It’s important to note that this variation is relatively small when it comes to astrophysical events, but it still gets in the way of precision cosmology. We’re trying to measure the acceleration of the Universe, and that’s not an easy task. Nature, however, can sometimes be kind to us and it turns out that type Ia supernovae provide themselves the means to compensate for this small variation.

The commonly used trick relies in measuring how the brightness of a given supernova changes with time. Right after the explosion, the brightness increases very rapidly until it peaks, and then it decays more slowly. The following animation shows a real example of a supernovae light curve. In the image on the left, you can see a bright spot getting brighter before it gets dimmer again – that’s the supernova. The curve on the top right shows how that brightness changes with time – we call this a light-curve – and the curve on the bottom right show the supernova spectrum. The timescale  is only of a few days, or perhaps a couple of weeks – but well within something what can be tracked with our current technology:

What is neat (and lucky!) about light-curves, is that they are systematically different for supernovae that are dimmer or brighter. Brighter supernovae have broader light curves, and fainter ones have narrower, or shorter light-curves. In practice, what this means is that if we can measure the light curve of a supernova and how broad, or stretched it is, then we can correct for the small variations I mentioned at the start of this post and infer its real brightness – handy! It works pretty well, as you can see in these two images: the first one shows the light-curves for a bunch of supernovae and you can clearly see how some and brighter and some are dimmer. The second image shows how you can use the width of the light-curves alone to calibrate all supernovae to a single, intrinsic brightness.

Supernova light curves before correction.

Supernova light curves after correction.

We now have the so called standard candles, although many Astronomers would rightly point out that supernovae should rather be called standardisable candles.

So is the problem solved? Once again – it depends on how well you want to play the game. Corrections like the ones I’ve shown you are pretty much standard right now, and they work to the precision required of present-day Cosmology experiments. They are definitely sufficient to establish the need for Dark Energy with a high level of confidence! Nonetheless, as we gear up to the next era of Cosmology experiments, Astronomers need to match technological advancements with new ways to analyse and interpret data.

There are two main potential drawbacks in the light-curve approach. One is that this correction may be too simplistic in detail, and another is that light-curves are still not trivial to measure (they require a follow up of the explosion for days after the event). And this brings me to the paper I read last week, and which prompted this post (you thought I’d forgotten, uh? In truth I’ve been wanting to introduce Type Ia cosmology for a while!).

Jordin et al., focus not on the light-curve of the supernova, but rather on its spectrum. I have a soft spot for this sort of approach for a variety of reasons, but primarily am I attracted by the possibility that a single shot of a supernova spectrum (instead of multiple images taking during the course of days to make up the light-curve) has the same information, the same potential to calibrate supernovae brightness. In practical terms this would be some serious advantage for future cosmology experiments. This is not the first time I hear about it, although peer reviewed papers are just starting to come out on this. As others have found, Jordin et al. find that a particular chemical signature in the spectrum of a type Ia – the Silicon II absorption feature – seems to be related with how stretched a light-curve is. Further study will be needed to find out if this Silicon II feature could in the future replace – or better, improve – light-curve corrections, but the results on this paper are at least enticing.

Another thing I find attractive in this approach, is the fact they find in the supernova spectrum information about the galaxy in which it comes from. The work done to date (and I’ve done a fair bit on this, too) involves using the spectrum of the host galaxy to infer the likely properties of the environment that gave origin to the supernova, but this is not without trouble too. One of the reasons is because we can often only extract average information about a galaxy as a whole, and not about the specific bit of the galaxy the supernova comes from. The spectrum of the supernova has the potential to tell a far more direct tale, and I find that pretty exciting.

The truth is, type Ia supernova are a truly empirical probe of the Cosmos. By this I mean that the way in which we use them is based purely in empirical laws – not on any analytical or computational modelling – and that means that we must be that little bit more careful, and smart, about how we deal with the data. But there is where a lot of the fun lies…

J. Nordin, L. Ostman, A. Goobar, R. Amanullah, R. C. Nichol, M. Smith, J. Sollerman, B. A. Bassett, J. Frieman, P. M. Garnavich, G. Leloudas, M. Sako, & D. P. Schneider (2010). Spectral properties of Type Ia supernovae up to z~0.3 Astronomy and Astrophysics arXiv: 1011.6227v1

Supernovae – setting the standard, part I

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I know.. it’s been a while. I’ll briefly mumble some apologies for having been so quiet here at the gutter (they’re heartily felt, though!) and quickly move on to the science. I realised, however, that there is a fair bit of background to cover before I can explain just how neat I think this paper is, so let’s do this one in parts, shall we?

The paper that broke this silence is related to supernovae – mighty stellar explosions that are so bright they can often outshine the entire galaxy (made of hundreds of millions of stars!) in which they occur. Their extraordinary brightness means they can be spotted even when they occur a long way away – turns out this is very useful and we’ll get back to this later.

A supernova explodes on the outskirts of a spiral galaxy.

Most supernovae are directly related to the death of very massive stars, as they reach the end of their lives. These supernovae are traditionally called core-collapse supernovae, a term which recalls the physical process that drives the explosion – a collapse of the core of the star, caused by a lack of fuel that stops the nuclear heart form pumping. Stars of different sizes and chemical composition will ignite slightly differently, and some explosions will be brighter whereas others will be dimmer.

But the type of supernova explosion this paper is concerned with has a very different origin, and they are called Type Ia supernovae. Their most important property, and one which rolled out the latest revolution in Cosmology, is the fact that  they always seem to explode with the same brightness, or luminosity. Astronomers have nicknamed them standard candles, and we will see  just how important this property came to be.

To create one, Astronomers think you need a white dwarf – in itself the remnant of another star that reached the end of its life, but which wasn’t massive enough to create a core-collapse supernova. White dwarfs do not generate any energy through nuclear processes, and are extremely dense – so dense that matter has arranged itself in a state called electron-degenerate matter. We can go onto the details some other time, but the important thing to note is that there is a physical limit to how massive a body held up by electron-degenerate matter can be – any more massive and gravitational collapse will occur. This mass is around 1.44 times the mass of the Sun, and naturally white dwarfs have to sit below this limit. Imagine, however, a situation where a white dwarf may be close enough to a companion star such that mass transfer from the companion star onto the white dwarf can occur. What will happen is that as soon as the white dwarf reaches 1.44 solar masses, gravitational collapse with begin and the star will go supernova. The intrinsic limit of 1.44 solar masses means this mechanism will naturally produce explosions of similar brightness. We have never directly observed the progenitor of a type Ia supernova, but the fact that their brightness is incredibly uniform, coupled with the presence of certain chemical elements in these explosions that are known to also be present in white dwarfs bring incredible strength to this theory.

So let us take a step back, and appreciate what we have here: a set of astrophysical events that are so bright that we can detect them a long way away, and for which we know the intrinsic brightness. For the rest of this post we will assume that we know how bright these explosions are exactly. We don’t – and that’s part II of this post – but it helps to assume we do for now.

The great advantage is that you can compute how far light has travelled by measuring how much fainter a supernova appears to you, if you know how bright it is to begin with. This is very intuitive – if I was to hold two identical light bulbs at different distances from you and asked you which is nearest you, you’d be able to answer correctly just by seeing which appeared brighter. Going back to the supernovae – by taking the spectrum of the supernova itself or of its host galaxy, we can also measure its redshift. The redshift, in turn, tells us how fast an object is receding from us. Finally, the relationship between redshift and distance depends on the rate of expansion of the Universe. So by measuring redshift and distance independently, Astronomers can constrain the rate of expansion.

In a Universe that is dominated by matter and that is currently expanding, we expect the rate of expansion to decrease with time. Think of how you’d expect the acceleration of a ball you threw up in the air to behave – it would set off with an initially velocity, and this velocity would slowly decrease until the moment it stopped and reversed its course to fall back down. Or, if you have unusually strong arms and could give it an initial velocity greater than the escape velocity, it would never turn back down but continue its journey into space, perpetually slowing down as it went.

What Astronomers discovered, when they measured the rate of expansion of the Universe using the distances (from their apparent brighntess) and the redshifts (from their spectra) of type Ia supernova, was something quite different. They found that these explosions were significantly dimmer than what you’d expect from a matter-dominated Universe. The Universe, between us and these explosions, had expanded a lot faster than expected – to the point that the only explanation is that the expansion of the Universe is, in fact, accelerating. Going back to our previous analogy – it’s a little like our test ball brought out a little jet pack and shot off into space. This was truly unexpected – Astronomers call whatever is causing the expansion of the Universe to accelerate Dark Energy.

Observations of type Ia supernovae in 1998 were the first evidence of Dark Energy, and it’s worth noting that other physical probes have since proved completely consistent with an accelerating Universe. And, to this day, type Ia supernovae are one of the most powerful probes to measure Dark Energy. The field has now moved on to trying to understand what Dark Energy is (rather than establishing its existence), which means measuring this acceleration to exquisite detail, and as a function of cosmic time.

For that, we need incredibly precise standard candles – but that, is the matter of part II.

The end of an era

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Coming out of lurking mode for a very brief post that notes the end of one of the most important experiments to cosmologists, astronomers, scientists and just about anyone who has an interest in how our Universe came to be.

After 9 years of peering through billions of years of cosmic history, the WMAP satellite has now stopped taking data – you can read the press release here.

The importance of WMAP to the confidence we have in the standard cosmological model today can’t easily be over-stated. We look forward to Planck taking over, and launching us into yet another era of precision cosmology and new and exciting discoveries!

The Universe according to now

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Following on the footsteps of giants, it was my turn this month to spend an evening looking through entries for the Astronomy Photographer of the Year competition run by the Royal Observatory Greenwhich and creating my own gallery.

It was a joy, thanks to all who made me smile! Endlessly.

Happy birthday Hubble, from rita

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So Emma got there first, straight to my all time favourite Hubble moment. Then Stuart stole my second favourite! You know.. great minds..

But nevermind, because with Hubble everything comes a close 2nd, or 3rd, or 1st. But here’s an image that has been making its way into my presentations since the very first time I saw it:

It’s disk galaxy NGC 5866, which faces us pretty much edge on. I love the way this image shows a galaxy from a slightly less known perspective. You can see the striking dust lanes going along the disk, and the slightly yellowish bulge of older stars in the middle – just beautiful. And if you can, for a moment, take your eyes off NGC 5866 you can see a number of galaxies in the background and it always amazes me how many you can see. I can have endless fun with the zoomable version of this image – go on, have a go and have a good look around.

And I do mean a good look around. Spare as much time as you can. Because every one of those galaxies has such a complexity to it, such a history and a future that it almost pains me to dismiss it as ‘seen’ at any given moment. There are billions of stars there, with numerous planetary systems and civilisations that almost certainly surpass our imagination; endless physical phenomena that we don’t yet understand or probably even know exist; and – I like to think – endless renditions of the blues. This stuff is real and sometimes, for as beautiful as the Hubble legacy is and for all it has done for us, I can’t help thinking that no image and no finite amount of staring can ever do this Universe any justice.

Image credit: NASA

A spot of Cold

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I read 3 papers! Three beautiful, science-packed, revolutionary and mind-blowing papers! Ok, maybe not – but trust me, after being so busy with things like measuring redshifts, fixing codes and mountains of admin/conference organising, almost any science is pure beauty for the old brain.

But one of these papers did tap into something I’m quite interested in, and it’s related to the Cosmic Microwave Background (CMB). I’ve briefly mentioned the CMB before, but here’s a more decent introduction: when we say CMB we are talking about radiation that was created when the Universe was very very young  – around 300,000 years old. At that time the Universe was hot and radiation (or photons) were the dominant component of the Universe. Because it was so hot at that time, the Universe was actually opaque – matter was ionized, meaning electrons were bobbling about not really being attached to any nuclei because of the high temperature. What this means is that photons could not get very far without bumping into something – they could not travel in a straight line for any decent sort of time, and were perpetually scattered around. That’s essentially what opaque means. However, something special happened around 300,000 years into the Universe’s lifetime, and that was a decrease in temperature that allowed these electrons to settle into atoms, effectively setting the photons free. We call this the time of last-scattering.

These photons were then free to travel unhindered. They were travelling then, and they are still travelling now! Straight into our telescopes, carrying information from around 13 billion years ago, virtually unaltered! This, for Cosmologists, is like Christmas – almost too good to be true. Anyway, I say virtually because some things affect these photons as they travel along. A big one is the fact that the Universe is expanding, and this causes their frequency to change. This is the reason why we see them in the Microwave part of the spectrum today – they were a lot hotter 13 billion years ago. Had we lived earlier on during the lifetime of the Universe, the CMB would have peaked in the visible band and the sky would be rather pretty! (of course, whether a planetary system with life could even exist then is another matter) But the Universe, in spite of being mostly empty, still has a lot of stuff around. You know, galaxies and things. And as these photons go through expanding regions with more matter (clusters) or less matter (voids) they see their frequency slightly changed.

Now, what I didn’t say and should have done is that not all CMB photons are the same. They are all very similar, but depending on the density of the region of space they were in at the time of last-scattering, today they are either a little bit hotter than the average, or a little bit colder. It’s a very small little – around 1 part in 10,000! But as I said before, a good enough experiment is able to pick up these tiny differences. When we look at the sky, we obviously only see the CMB photons travelling towards our telescope (they are travelling uniformly in every direction), but these tiny differences are projected in the sky in what is now a very familiar pattern, and that I need to show time and time again because it really is beauty:

The CMB, as seen by the WMAP satellite

What you’re seeing here is the whole sky, as seen in the microwave band (after having radiation from our own galaxy cleverly removed). Red patches are slightly hotter photons, blue patches are slightly colder photons. These patches tell you about the density distribution in the early Universe. It’s these fluctuations that seed the density fluctuations that give birth to stars and galaxies and you and I, but I’ll leave that to another time.

Right now, I want to focus on a small patch of these fluctuations. In you look at the bottom right corner rim, around 4:30pm if that map was a clock (oooohhhhhh!! That’s an idea! Can I have a CMB clock anyone?), you’ll find a cold patch that has been nick named The Cold Spot (one day I’ll write a rant about how Astronomers, being a rather clever and creative bunch of people, are rubbish at coming up with good names for things. mm.. maybe I just have.). Now, by eye the Cold Spot doesn’t look any different from other cold spots around the map. However, statistically, and given our model for the early Universe (which describes things pretty well, including the distribution of galaxies we see today), a spot of that shape, size and temperature has a very low probability to exist. ‘Very low’ here means something around 0.1% to 5%, depending on different estimates. This is slightly uncomfortable and Astronomers have spent a significant amount of time studying the Cold Spot.

There are a few options here:

1) The Cold Spot was formed at the last-scattering.

2) The Cold Spot was formed during the photon’s path to us.

3) The Cold Spot is an instrumental artifact.

4) The Cold Spot is a data-reduction artifact.

There are papers studying all of the above. 3) and 4) seem unlikely at this stage but should not yet be discarded. Having data from another experiment like Planck, with a whole new reduction pipeline, will help. 1) is fascinating because it could potentially mean there is something amiss from our early-Universe theory. But the paper that prompted this ridiculously long post actually focuses on 2).

Bremer et. al investigate the hypothesis that actually, the reason why we see a particularly cold spot in that region of the sky, is because there is a large void (a region of space with much less galaxies than the average) along that line of sight. The trick here is to note that the Universe is expanding – i.e., the shape and size of clusters and voids changes with time. The frequency (or temperature) of photons is affected by this change because the energy they loose or gain when they go into these structures is not completely recovered when they come out. There is a net effect, with is to gain a little bit of energy going through clusters, and loosing a little bit going through voids. There’s a little animation here that may make it clearer.

So Bremer and collaborators chose 5 regions of the sky, all inside the Cold Spot, and took redshift surveys in these 5 regions. This allowed them to see how the distribution of galaxies changed with distance between us and the region of last-scattering, and they looked for a deficit of galaxies that would be significant enough as to imprint the Cold Spot in the observable CMB.  They did this by comparing these redshift distributions with those from other regions of the sky (outside the Cold Spot) and did not find any sign of such a deficit, or void. They could only look up to redshift of 1 (or around 7.8 billion years ago) because of instrumental limitations, and they didn’t have enough galaxies below redshift of 0.35 (around 3.8 billion years ago), but they covered a significant chunck of time when this effect is more likely to happen and did not find what they were looking for. What this means, is that they discarded at least some theories that could lead to point 2), although not all of them.

So the jury is still out on the Cold Spot. Personally, I’m sometimes tempted to add a 5th option:

5) The Cold Spot was formed at last-scattering, but its significance is being over-estimated.

But that, I’m afraid, is another post (I’m late for work!!). M. N. Bremer, J. Silk, L. J. M. Davies, & M. D. Lehnert (2010). A redshift survey towards the CMB Cold Spot Submitted to MNRAS arXiv: 1004.1178v1

Our Universe

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I’m not going to fulfil my pledge just yet, mainly because I haven’t been doing much science recently – instead I was asked to push hard on this (and it turned out that 1500 was much too optimistic, there was a lot more that needed done!). But I got something a little different for you today.

Quite a long time ago, myself and a few colleagues from the ICG decided we’d have a go at making a videocast, called Our Universe, aimed at not only telling people a little bit about our wonderful Universe but also at trying to somehow share what is like to be a researching Astronomer. So we had a go at a pilot episode.

In the meantime, it became clear that none of us has the time to take this any further and decided not to pursue the project. It would be a shame, however, if the pilot never saw the light of day – if only because so many wonderful people contributed their time and energy to talk to us. Unfortunately, much of the footage was being saved for future episodes so most people’s contributions are under-represented in the pilot. That is a shame on itself, but we are genuinely thankful to everyone who spoke to us, or gave us ideas, or feedback, or money. This pilot was fully funded by the ICG, but the views represented on this podcast are not necessarily those of the ICG.

So here it is, as an exclusive to weareallinthegutter:

A pledge

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A few people have recently asked me what makes me write about one paper and not the other. There seems to be some expectation that I (or most science bloggers) would write about the most significant or controversial papers in their area, which I guess is a fair first assumption to make. And whereas in some cases these papers are instantly recognisable, personally I tend to concentrate on papers that are on my desk, and that are likely to have a direct impact on my work even if not raise press releases.

The neat thing about my job is that most papers I read do have some interesting aspect that makes me want to sit down and tell you about it. And more importantly, I think we should be telling you more about day-to-day papers which would normally go unnoticed by the press and therefore by the non-astronomer. They’re more likely to give you an unbiased view of what we (as in each individual scientist) do for a living, and perhaps even a more accurate perspective of the personal scientific process of discovery in astronomy.

Lack of time means I don’t get to read as many as I’d like. And lack of time squared means that I don’t get to blog about as many as I read, but let me make a pledge that for every three papers I read in detail from now on, I’ll blog about one. My guess is that this should result in one post every one or two weeks (I skim through a lot of papers, I actually read very few!), and that seems doable for me and not overwhelming for you.

As it turns out, I was away all of last week, so you may have to wait a little while for the first one to come along..