Baryon Acoustic Oscillations (BAOs)

The Universe is about 13.7 billion years old. Between two very particular ages of the very young universe (when it was only a few hundreds of thousand years old) called inflation and recombination, the Universe is filled with an ionized plasma, hot and dense, in which photons and baryons are strongly bound, "coupled". Under the effect of scattering between the photons and the charged particles of the plasma, the photons are ''trapped'' in the plasma. During this time, the interplay between the plasma pressure and the radiation pressure results in ''sound waves'': spherical perturbations of the density (and pressure) propagating rapidely around each initial overdensity, similar to the waves that form on the surface of a lake when we throw a stone in it.

As the Universe expands, the baryonic matter cools down, eventually allowing the nuclei and electrons to bind into stable, neutral atoms at a time called recombination (approximately 370000 years after the Big Bang). The photons suddenly liberated from the matter. The Universe becomes ''transparent'' and the baryonic shells propagating in the form of sound waves freeze, leaving an imprint in the distribution of matter.

It is quite an amazing fact that this signature, known as Baryon Acoustic Oscillations (BAOs), of a phenomenon that happened more the 13 billion years ago, is still visible in the sky and measurable. In particular, astronomers are able to recognise this signature in the large-scale distribution of galaxies. The distance traveled by the over-density shell, provides a feature of known physical size and as such, it acts as a standard ruler

Standard rullers are very important to astronomers, because they give us a way to track the expansion of the universe, and to learn more about the mysterious energy that drives this expansion, called Dark Energy

Weinberg et al. (2013) give a complete overview of the history of the detection of BAOs. The first prediction of the BAOs effect, in the Cosmic Microwave Background and the late-time matter power spectrum dates back to the late 1960s (Sakharov 1966), at a time when the pure baryons cosmologies predicted a very strong effect. The BAOs effect was identified as a standard ruler in the 1990s (Kamionkowski et al. 1994). Early results form the 2dF Galaxy Redshift Survey (2dFGRS) (Percival et al. 2001) gave the first hints for the BAO feature in the data, followed by convincing detections in the SDSS Data Release 3 and final 2dFGRS samples (Eisenstein et al. 2005; Cole et al. 2005).