Evolution of Cosmological Perturbations during Reheating

TL;DR

This paper studies how scalar perturbations evolve during the reheating phase after inflation, showing that the Bardeen parameter remains conserved, supporting standard assumptions about perturbation amplitudes.

Contribution

It demonstrates that entropy perturbations are negligible during reheating, validating the conservation of the Bardeen parameter and the conventional inflationary perturbation transfer.

Findings

Entropy perturbation generated by energy transfer is negligible.

The Bardeen parameter remains conserved during reheating.

Supports the standard inflation-to-reheating perturbation relation.

Abstract

The behavior of scalar perturbations on superhorizon scales during the reheating stage is investigated by replacing the rapidly oscillating inflaton field by a perfect fluid obtained by spacetime averaging and the WKB approximation. The influence of the energy transfer from the inflaton to radiation on the evolution of the Bardeen parameter is examined for realistic reheating processes. It is shown that the entropy perturbation generated by the energy transfer is negligibly small, and therefore the Bardeen parameter is conserved in a good accuracy during reheating. This justifies the conventional prescription relating the amplitudes of quantum fluctuations during inflation and those of adiabatic perturbations at horizon crossing in the post-Friedmann stage.