Baryon Acoustic Oscillations

Observations of baryon acoustic oscillations (BAO) in the clustering of galaxies offer an attractive method for setting cosmological constraints. The physics of the BAO production is well understood, and it is hard to imagine that galaxy formation processes will significantly distort this large-scale signal. The large volume of the Universe and redshift range probed by proposed SKA surveys will enable BAO to be observed at a sequence of redshifts out to z ∼ 2. Using the sound horizon scale as a cosmological ruler will test the geometry of the Universe in the dark-energy dominated regime. The BAO themselves will constrain the cosmological matter density through the physics of their production. In this conference proceedings I review the basics of BAO physics, and the use of these features to constrain cosmological models for our Universe. 1. The physics of BAO production In the standard model for cosmological structure growth, a period of rapid inflation in the scale of the Universe caused the seed perturbations for cosmological structure to grow from Gaussian-distributed quantum fluctuations. The clustering of these perturbations depends on the form of the inflationary potential, although most models predict that the power spectrum of fluctuations is close to P (k) ∝ k, the Harrison-Zeldovich form (Harrison 1970; Zeldovich 1972). Following this period of inflation, the baryonic matter is ionised and coupled to the radiation through Thompson scattering. This hot plasma is initially capable of supporting sound waves but, as the Universe cools, the baryonic matter undergoes recombination and atomic elements are formed. After the epoch of recombination, sound waves are no longer supported, and the radiation decouples from the baryons eventually becoming the CMB. As the pressure changes, the densities of the constituents change following their different equations-of-state. The evolution of the Universe is initially dominated by the radiation, then the matter and finally dark-energy takes over. The changes that occur as the Universe evolves imprint characteristic scales on the distribution of matter that depend on the average matter density and the fraction of matter that is baryonic (Silk 1968; Peebles & Yu 1970; Sunyaev & Zel’dovich 1970; Bond & Efstathiou 1984; Holtzman 1989). The growth of perturbations is governed by the Jeans scale: perturbations smaller than the Jeans scale do not collapse due to pressure support, while larger perturbations are free to grow through gravity. In the radiation dominated era, the dark matter has negligible density compared to the photon-baryon fluid, and the causally connected perturbations in this fluid are stabilised by the high radiation pressure. The horizon scale of the universe at the epoch when this stabilising pressure fails marks a change in the perturbation evolution. Both the matter density and baryon fraction are imprinted in the overall shape of the matter power spectrum by this mechanism. In addition to changes in the overall shape of the power spectrum, oscillations arise because sound waves in the coupled baryon-photon plasma will lead to the expansion of the baryonic material in a spherical shell around a small perturbation, reaching a radius rS(z∗), the comoving sound horizon size at recombination, before sound waves are no longer supported. At the high redshifts of interest the vacuum energy can be neglected, and (Hu & Sugiyama 1995)