Structure formation in alternatives to CDM

Abstract

The standard CDM is currently the most accepted theory of the Universe. The model is characterised by a nearly-scale invariant primordial curvature power spectrum from inflation, cold and non-interacting dark matter particles and a cosmological constant .
Despite the success of CDM, the nature of the dark sectors is still unknown. Moreover, the model faces some small-scale observational controversies that are still not resolved. In this thesis, we investigate structure formation in alternative scenarios to CDM using high-resolution cosmological simulations. In the first part, we focus on non-cold dark matter (e.g. warm dark matter) and non-standard inflation (e.g. thermal inflation) models, that display damped matter density fluctuations on small scales. We investigate first the effects of adding thermal velocities to the gravitationally-induced velocities in simulations of warm dark matter. Considering different non-standard linear power spectra, we then assess which features survive the non-linear evolution and leave interesting signatures in non-linear power spectra and halo statistics. Exploiting these results, we present a new smooth- space filter, to use in the Press-Schechter approach, to model the dark matter halo mass function, which overcomes shortcomings of other filters used in the literature. In the second part, we quantify the effects of modified gravity on neutral hydrogen abundances and 21-cm power spectra, finding that the HI clustering is a powerful test of gravity at redshifts .