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 Uni verse on the primordial cloud collapse. We study the evolution of a cl oud with a density perturbation, delta, from the recombination era to the present. As an example, we study an initial perturbation delta(i) = 10(-6) for a cloud of mass 10(4), 10(5) and 10(6) M(circle dot). In particular, we study the influence of the physical processes on the ev olution of the peculiar velocity factor, f=d ln delta/d ln 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 th at the physical mechanisms (photon drag, photon cooling, recombination , etc.) amplify perturbations, and the cosmological constant is partic ularly important in a Universe dominated by it with a small matter den sity.