Quantum signatures from the Early Universe

Inflation is the standard paradigm to understand how the various structures we see in the Universe formed starting from tiny quantum fluctuations of one (or more) scalar fields driving a period of accelerated expansion at the very initial phases of the Universe. An interesting issue is how such quantum fluctuations became classical perturbations sourcing the seeds for structure formation, given that the structures we see in the universe are classical objects (from Cosmic Microwave Background temperature anisotropies to the large-scale distribution of galaxies). In recent years there have been new progresses on this topic, in particular on the possibility that this “quantum to classical transition” could take place through decoherence effects, induced by a suitable environment interacting with the quantum fluctuations’ system (which is, in this case, an open quantum system). This thesis aims first at a review of this subject, focusing in particular on the open quantum system approach, used by the papers in the most recent years, and highlighting the differences with respect to the closed quantum system approach to the problem, adopted by previous papers. Then, our original analysis will investigate the possibility of considering observational signatures of this quantum to classical transition. We will consider the case in which decoherence effects are induced by non-linear gravitational couplings, in the form predicted by General Relativity in single-field model of inflation, between the different types of cosmological perturbations originating from inflation, namely scalar (curvature perturbations) and tensor (primordial gravitational waves) perturbations. More in detail, we will analyze the corrections from a subhorizon tensorial perturbations environment to the power spectrum of primordial scalar (curvature) perturbations, which will be encoded in a blue correction to the scalar index. This is the main result of this thesis and it is very straightforwardly extendable to other cases to be considered in future works.