The Shaw Prize in Astronomy for 2014 is awarded to Daniel Eisenstein of Harvard University, Shaun Cole of Durham University and John A Peacock of the University of Edinburgh. The Laureates are being honoured for measurements of features in the large-scale structure of galaxies used to constrain the cosmological model, including baryon acoustic oscillations (BAO) and redshift-space distortions.

The early universe was hot and dense. Baryonic matter was highly ionized and consisted of bare nuclei and free electrons, a state known as plasma. Then as now, photons were far more numerous than either nuclei or electrons. As a consequence of photon-electron scattering, the plasma was tightly coupled to the photons. This situation pertained during the first 400,000 years after the big bang. At that point the temperature had dropped to 3000 kelvin and the electrons and nuclei combined into neutral atoms which decoupled from the photons.

Baryon acoustic waves were excited by density perturbations that arose shortly after the big bang and later seeded the formation of galaxies. They are essentially sound waves that propagated through the primordial plasma at about half the speed of light prior to decoupling. After decoupling, these waves ceased propagating, thereby imprinting a well-defined length scale of about 450 million light years on matter density correlations at the current epoch.

The BAO scale provides a “standard ruler” with many applications in cosmology. It was first observed in temperature fluctuations of the cosmic microwave background on an angular scale of one degree. In 2005, two papers reported its detection in the spatial correlation of galaxies. Oriented perpendicular to the line of sight, the angle subtended by the ruler calibrates the relation between distance and redshift. Oriented parallel, it determines the Hubble parameter as a function of redshift. By now the latter has confirmed the astonishing discovery, made by using type Ia supernovae as “standard candles”, that the expansion of the universe recently transitioned from deceleration to acceleration.

Next is a remark about redshift-space distortions and their synergy with the BAO. Density inhomogeneities and large-scale flows go hand in hand.  Accelerations due to density inhomogeneities drive the flows and the flows enhance density inhomogeneities. Early measurements of redshift space distortions constrained the mean density of the Universe. Large-scale flows of galaxies also distort the BAO “standard ruler”. This effect is substantially mitigated by simulating the flows based on the gravitational field deduced from observations of the large-scale clustering of galaxies. In essence, this procedure straightens out the ruler. It is referred to as reconstruction.

Finally, a word about the relation of the current Prize to previous ones in 2004, 2006, and 2010. The 2004 Prize was based, in part, on the prediction of baryon acoustic oscillations. In 2006, the Prize recognized the remarkable discovery that the expansion of the universe was accelerating. Most recently, the Prize of 2010 honoured the WMAP satellite’s precise determination of fundamental cosmological parameters from observations of the microwave background.

Astronomy Selection Committee
The Shaw Prize

27 May 2014  Hong Kong