Backreaction of a massless minimally coupled scalar field from inflationary quantum fluctuations

Abstract

In this thesis we study a massless, minimally coupled scalar field in a FLRW spacetime with piecewise constant deceleration parameter. We consider the global Bunch-Davies vacuum of this theory during inflation era and initially match, using the sudden matching approximation, the mode functions of the field on a geometry where the energy density is dominated by radiation. We proceed to construct the one-loop expectation value of the energy momentum tensor in the Bunch-Davies vacuum. We find a logarithmic enhancement of the energy density due to the scalar field over the background energy density far away from the matching, which is though not enough to make the contribution of the former comparable to the latter until the transition to matter era. We then match on a geometry where matter dominates, and discover that the energy density due to the scalar field is insignificant, compared to the energy density of the classical background in late times. Thus, its backreaction onto the geometry is negligible. The same conclusions also apply for gravitational waves coming from inflationary fluctuations. The results of this thesis will be submitted for publication.