Evolution of small-scale cosmological baryon perturbations and matter transfer functions

The evolution of small-scale cosmological perturbations is carefully reexamined. Through interactions with photons via electrons, baryon perturbations show interesting behavior on some physical scales. Characteristic features of the evolution of baryon density fluctuations are discussed. In CDM models, a power-law growing phase of the small-scale baryon density fluctuations is found, characterized by a terminal velocity after the diffusion (Silk) damping and before the decoupling epoch. A transfer function for total matter-density fluctuations is then studied by taking those physical processes into account. We present an analytic transfer function as a function of redshift z that is applicable for the entire range up to a solar-mass scale in the high-z universe, and that is also suitable for high baryon fraction models.