New Baryonic Acoustic Oscillation Measurements

A 2% Distance to z = 0.35 by Reconstructing Baryon Acoustic Oscillations (paper I, II, III)

Nikhil Padmanabhan, Xiaoying Xu, Daniel J. Eisenstein, Richard Scalzo, Antonio J. Cuesta, Kushal T. Mehta, Eyal Kazin

This is a summary of 3 papers released today by the above authors (who all shuffle in order dependent on the exact paper) but basically it's a way to improve the measurements of the Baryonic Acoustic Oscillation (BAO) using the Sloan Digital Sky Survey Data Release 7 sample of galaxies... So of course the first question is, what's the BAO? 

*ahem* {Cue Deep Voice} In the beginning the Universe was hot, smooth and devoid of interest. {Deep voice over}

The gas (i.e. baryonic) was sufficiently hot to be ionised (a plasma), yet dense enough that no photon of light could travel far before colliding with an electron/proton. This was essentially a 'fog' through which light couldn't travel. It also meant that the plasma was constantly being puffed up by the collisions from the photons (in the same way that the wall of a balloon is held taught by the collisions from air molecules inside). This relativistically coupled ionised plasma was unable to collapse under gravity thanks to the pressure support of the photons. In fact, this medium behaved a lot like an ideal gas in which sound waves (acoustic oscillation remember?) could travel within - although these sound waves travelled across the Universe at about the third of lightspeed.

Now, as the Universe expanded it cooled adiabatically (same way you use deodorant from a can and it cools as the volume of the spray increases) which eventually reached a low enough temperature that the plasma was able to recombine to form neutral gas. Effectively the fog precipitated out, the Universe was clear for the photons of light and they travelled from this point unimpeded (which we see at the Cosmic Microwave Background). The largest sound wave possible at this moment was essentially the size of the Universe which meant that the gas was distributed with a characteristic size. Since the galaxies then formed from this gas they would also be typically separated by this distance (about 100 Mpc or so). There are details related to the dominant dynamical effects of Dark Matter which I won't go into but essentially the idea is that if we see galaxies separated at a given distance more often then not, then this will tell us the size of the Universe when it cooled sufficiently to recombine and form the Cosmic Microwave Background.

Measuring this distance scale has now become almost routine BUT the problem with galaxies is that they aren't a perfect tracer of this scale as they tend to move around, especially when near huge concentrations of mass. This is actually a cosmological probe in an of itself, but for measuring the BAO scale it's a nuisance (it tends to broaden the error bar on the distance measurement). The clever bit in this work is to reverse the motion of the galaxies due to local concentrations of mass, and reconstruct the original separation distances (and hence sharpen the measurement on the BAO). This work has the effect of decreasing the distance error by a factor 2, to only 2% (with the old technique you would need a survey three times the volume to get a similar improvement!). 

Truly an exceptional effort... the cosmological constraints from this measurement (when combined with distance measurements of the Cosmic Microwave Background, and supernovae luminosity distances AND local measurements of the Hubble expansion) show that we live indeed in a Dark Energy dominated Universe, best modelled by the Cosmological Constant (with Dark Matter the most important gravitating component, then finally the baryons). So nothing new, but a stunning piece of work all the same!