Acoustic oscillations in the baryons before recombination appear as baryon acoustic oscillations (BAOs) or wiggles in the matter power spectrum Pm that are out of phase with the CMB acousitic oscillations in P&gamma.
Unlike the CMB acoustic oscillations, baryon oscillations reflect the velocity of the fluid at recombination rather than the density. The velocity at release from photon drag kinematically form baryon overdensities.
With the acoustic standard ruler calibrated by the CMB, baryon oscillation observations measure the distance to the redshift that they are measured, here z=0 and hence the Hubble constant. At higher redshift, BAO probe the evolution in the expansion and cosmic acceleration.
Transfer Function Codes: The form of the BAOs can be studied in linear theory by examining the matter transfer function