Statistical characteristics of Large Scale Structure

We investigate the mass functions of different elements of the Large Scale Structure – walls, pancakes, filaments and clouds – and the impact of transverse motions – expansion and/or compression – on their statistical characteristics. Using the Zel'dovich theory of gravitational instability we show that the mass functions of all structure elements are approximately the same and the mass of all elements is found to be concentrated near the corresponding mean mass. At high redshifts, both the mass function and the mean mass of formed elements depend upon the small scale part of the initial power spectrum and, in particular, upon the mass of dominant fraction of dark matter (DM) particles. These results generalize the Press-Schechter approach and are used to obtain independent estimates of probable redshifts of the reionization and reheating periods of the Universe. We show that the transverse motions do not significantly change the redshift evolution of the observed mass function and the mean linear number density of low mass pancakes related to absorption lines in the spectra of the farthest quasars. We apply this approach to the observed Lyman-α clouds and obtain direct estimates of the variance of initial density perturbations and the shape of initial power spectrum on small scale. In turn, these estimates restrict the mass of dominant fraction of DM particles. Key words. cosmology: large-scale structure of the Universe — dark matter – galaxies: formation.