ASPECTS OF ZELDOVICH APPROXIMATION

A generalized version of the Zel'dovich approximation which is applica ble in both the radiation-dominated and the matter-dominated epochs is presented. This approximation allows one to follow the growth of inho mogeneities from the time the mode enters the Hubble radius until it t urns around. Comparison of the results with the standard spherical mod el shows that the analytic approximation is quite good even in the non linear regime. Detailed application to cold dark matter models and see ded models are given.