Cosmology and the Spatial Distribution of Galaxies
Friday, April 4, 2014
Science Update - A look at CfA discoveries from recent journals

Perhaps the most astonishing and revolutionary discovery in cosmology was that galaxies are moving away from us. Hubble's 1929 paper provides the underpinning of the big bang picture of creation in which the universe is expanding, and has been for 13.8 billion years. Astronomers since then have been steadily working to refine this general picture, and in 1998, two teams (one led by CfA scientists) further astonished the world with their results showing that the universe would expand forever -- and not only that: it is accelerating outward. They used supernovae to probe the distant cosmos. These discoveries have led to more sophisticated questions, with a primary task today being to understand in detail the expansion history of the universe, that is, how the rate of expansion of the universe evolved from the time of the big bang to the way it is today. The answers to this question directly address the properties of the acceleration mechanism, the nature of dark matter, the evolution of galaxies in early times, and more.

Precision measurements of the cosmic distance scale are crucial for probing this behavior, and one particularly powerful method uses what are called baryon acoustic oscillations (BAO). Baryons refer to ordinary matter, and acoustic oscillations are sound waves. Sound waves caused by density fluctuations were bouncing through the cosmos during its first 400,000 years. Then, once ionized atoms became neutral, radiation no longer interacted strongly with matter and the cosmic microwave background was released. The intensity maps of the background radiation contain a record of these sound waves – the BAO. Astronomers calculate that at the time the cosmic background was produced, sound waves (traveling at the speed of sound) could have spread across a distance of about 500 million light-years, leaving in their wake a coherent record in the matter distribution that eventually condensed into galaxies and clusters of galaxies. Because the scale of this acoustic distortion is so large, many times the size of galaxy clusters, the BAO signature was only modestly altered subsequently as the universe evolved; simulations and theory suggest deviations are below 1%. The robustness of the scale of this distinctive clustering signature allows it to be used as a standard ruler to measure the cosmic distance scale, and indeed the imprint of the BAOs has been detected in a variety of observations of the structure of the nearby universe.

CfA astronomers Daniel Eisenstein and Cameron McBride were among a large team of scientists probing BAOs by using the clustering of galaxies as seen at a time when the universe was about 8.2 billion years old. They examined 264,283 galaxies of this general epoch observed by the Sloan Digital Sky Survey, and measured from their spatial distribution the signature of the acoustic waves left behind to a precision of better than 10%. Their conclusions about the big bang are consistent overall with the picture of cosmic evolution that has emerged from many other lines of evidence (but add some tantalizing hints of mystery: their measurement of the current rate of expansion as 67.5 +- 1.7 km/second/megaparsec is actually a tad smaller than the currently favored value). The amazing power of the technique is that it gives a snapshot of the universe at this era, and that it relies on completely different data from those used by other studies that rely on supernova or cosmic background radiation.

Reference(s): 

"The Clustering of Galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Measuring DA and H at z = 0.57 from the Baryon Acoustic Peak in the Data Release 9 Spectroscopic Galaxy Sample," Lauren Anderson et al., MNRAS 439, 83, 2014.