Growth of density perturbations with a cosmological constant

We investigate the influence both of the cosmological constant and of the physical processes that take place during the evolution of the Universe on the primordial cloud collapse. We study the evolution of a cloud with a density perturbation, δ, from the recombination era to the present. As an example, we study an initial perturbation δi = 10−6 for a cloud of mass 104,105 and 106 M⊙. In particular, we study the influence of the physical processes on the evolution of the peculiar velocity factor, f= d In δ/d In a, where ‘a’ is the scalefactor of the Universe. We compare our results with the analytic expression for f obtained by Lahav et al. Our results show that the physical mechanisms (photon drag, photon cooling, recombination, etc.) amplify perturbations, and the cosmological constant is particularly important in a Universe dominated by it with a small matter density.