Baryon Acoustic Oscillations
In the first million years after the Big Bang, the cosmic plasma rings with sound waves excited by the initial inflationary(?) perturbations. The radiation pressure from the cosmic microwave background keeps the ionized gas from clustering; this pressure leads to relativistic sound waves that propagate until the Universe becomes neutral at redshift 1000. These sound waves lead to the dramatic acoustic oscillations seen in cosmic microwave background anisotropy data, but they also leave a faint imprint in the clustering of galaxies and matter today. The signature is a small excess in number of pairs of galaxies separated by 150 Mpc (500 million light-years). These features are often referred to as the baryon acoustic oscillations (BAO).