Evolution of coupled scalar perturbations through smooth reheating. II. Thermal fluctuation regime

TL;DR

This paper investigates how thermal fluctuations influence the evolution of curvature perturbations during the reheating phase after inflation, providing numerical results without relying on slow-roll assumptions.

Contribution

It introduces a linear fluctuation-dissipation framework to analyze thermal effects on curvature perturbations, extending previous gauge-invariant evolution equations.

Findings

Thermally modified power spectrum depends on freeze-out parameters.

Numerical power spectrum obtained without slow-roll approximations.

Supports the embedding of warm inflation within the Standard Model.

Abstract

Curvature perturbations with short wavelengths exit the Hubble horizon when the universe may contain a thermal plasma in addition to an inflaton field that drives its expansion. We solve the corresponding fluctuation-dissipation dynamics at linear order, building upon a previously established set of gauge-invariant evolution equations. The properties of the noise autocorrelator are constrained via a matching of equilibrium correlators to quantum-statistical physics deep inside the Hubble horizon. The curvature power spectrum is determined numerically, without slow-roll approximations or assumptions about the equilibration of the inflaton field. As applications, we scrutinize two issues from recent literature: the model dependence of the thermally modified power spectrum as a function of freeze-out parameters, and the viability of embedding warm inflation within the Standard Model (we offer support for the latter proposal). The role of pre-horizon-exit acoustic oscillations is illustrated.