|Baryon acoustic oscillations
In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h^-1 Mpc (h being the Hubble constant in units of 100 km s^-1 Mpc^-1). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like Type Ia supernovae, BAOs provide a standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how dark energy has affected the expansion of the Universe.