Growth of cosmological structures in an interacting vacuum scenario

The widely accepted standard model of cosmology consists of a cosmological constant, Λ, responsible for the present expansion of the Universe, and cold dark matter, (CDM), accountable for structure formation. This famous model, also known as ΛCDM, suffers from both theoretical and observational problems, which motivate the study of alternative models. In this thesis I present an alternative scenario: the interacting vacuum. In this class of models, unlike in ΛCDM, vacuum energy, a dark energy with w = −1 equation of state, and CDM are allowed to exchange energy and momentum. I focus, in particular, on the geodesic CDM case, where only a pure energy exchange is allowed. Within this subclass, three relevant models are considered: the linear vacuum model, whose energy exchange is proportional to the vacuum energy density, the generalised Chaplygin gas (GCG) interacting model, obtained decomposing the original GCG into the two interacting dark components, and the Shan-Chen (SC) interacting model obtained as well from a decomposition of the original SC model. The aim of this elaborate is, using the covariant gauge-invariant (CGI) theory of cosmological perturbations, to show, for the aforementioned models, how the vacuum-CDM interaction affects the growth of cosmic structures. It is found that a growing/decaying vacuum energy always leads, for fixed initial conditions, to a less/more decreasing growth rate f w.r.t. the non-interacting limit, namely the ΛCDM model.