the new BOSS in townPosted: March 31, 2012
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.”
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