A Quantum Tale of Primordial Perturbations: from Inflation to Reheating

In this thesis, we discuss two main problems in the history of primordial perturbations where the quantum aspects are particularly relevant: the quantum-to- classical transition of primordial perturbations during inflation and the nonlinear evolution of the perturbations at horizon reentry during a condensate dominated reheating phase. We investigate the quantum-to-classical transition of the superhorizon curvature perturbations by considering them as an open quantum system, interacting gravitationally with a time-dependent environment of short wavelength tensor modes in a single-field inflation model. For our analysis, we recast the gravitational interactions predicted by General Relativity in single-field inflation into derivativeless and derivative terms, where only the latter were previously considered in the literature. For the first time, we show that derivativeless interactions, also when considered alone, can induce decoherence in our framework in at least 15 efolds after horizon crossing. When including also the derivative interactions, instead, significant non-Markovian effects emerge and delay decoherence to 17 efolds after horizon crossing. This result shows, for the first time, the importance of the interplay between different interactions in such calculations. At the same time, we introduce a method to modify the quantum master equation to account for a time-dependent environment. Finally, we show that the resummation properties of the quantum master equation apply succesfully also to the gravitational interactions during inflation for the power spectrum, and we extend this resummation framework, for the first time, to the bispectrum. We then follow the evolution of the primordial perturbations after the end of inflation, during the early matter domination before perturbative reheating. A very long early matter domination can be considered, lasting up to 50 e-folds, with the only requirement that it should end before BBN, at a temperature Trh > O(1)MeV. During such a long early matter domination, perturbations can reenter the horizon, grow linearly and then form structures such as halos. Inside these halos, by gravitational relaxation, inflaton stars can nucleate. These stars are very compact, so, after growth in mass, they can collapse into primordial black holes. In some regions of the parameter space this mechanism produces black holes of mass 10^10g − 10^15g, which are tightly constrained by BBN and CMB data. These constraints translate directly into the reheating parameter space: for intermediate inflationary energy scales, the constraints on reheating temperature are raised up to Trh = 10GeV for some values of the inflaton masses. This thesis therefore provides a concrete demonstration of how research in early universe cosmology can benefit from tools and physical insights taken from quantum and condensed matter physics.