Baryon Acoustic Oscillations

Think back to the density fluctuations in the CMB. These are overdensities in dark matter and in the coupled baryon/photon fluid. In a given overdensity, the pressure is provided by the photons scattering off electrons, and since its an overdensity, it is also an over pressurized region -- it will set up an outwardly propogating wave in the photon/baryon fluid.

When recombination hits, the photons diffuse out and leave a ring of baryonic overdensity standing in place.

Animation of evolving density profiles

This leads to a charateristic physical scale at recombination, which in comoving coordinates is about 150 Mpc in radius. These baryonic overdensities should lead to an excess in galaxy formation on these physical scales.

Since there will be many overlapping rings, the spatial signature will not be obvious on the sky, but should show up in the galaxy 2pt correlation function on these scales.

Cosmological constraints:

BAO amplitude: 

In early universe, fluctuations are suppressed on small scales during the radiation era. If Omega_M is high, the radiation era ended earlier, so there is more small scale power remaining, which means relatively less large scale power (in the 2pt CF signature). Strong sensitivity to Omega_M.

BAO length scale:

The physical scale of the ring depends on the expansion history and sound speed in early universe. Weaker sensitivity to Omega_M.

First measurement in SDSS data (Eisenstein+ 05)
OmegaM - OmegaL constraints (Kowalski+ 08)