Sound in the early universe

The standard cosmological model is based on the approximation that the universe is homogeneous and isotropic. The overwhelming success that this model has enjoyed is based on its ability to predict the primordial abundance of elements and the isotropy of the microwave background. However, the universe itself is not homogeneous. Any realistic model must include the perturbations from homogeneity that evolved into galaxies and galaxy clusters. Perturbations on these scales behaved like sound waves at sufficiently early times when the gas in the universe was almost completely ionized. In this talk I will review the work done on the linear and nonlinear evolution of these sound waves. The isotropy of the microwave background places firm limits on the amplitude of the sound waves at the epoch of recombination. As a result we are able to rule out the hypothesis that the universe consists entirely of radiation and ordinary matter.