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