Aspects of Zel'dovich approximation

A generalized version of the Zel'dovich approximation which is applicable in both the radiation-dominated and the matter-dominated epochs is presented. This approximation allows one to follow the growth of inhomogeneities from the time the mode enters the Hubble radius until it turns around. Comparison of the results with the standard spherical model shows that the analytic approximation is quite good even in the nonlinear regime. Detailed application to cold dark matter models and seeded models are given