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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.
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