Analytical Study of Mode Coupling in Hybrid Inflation

TL;DR

This paper analytically examines the coupling of fluctuation modes during reheating in hybrid inflation, revealing that short wavelength back-reaction is insufficient to suppress long wavelength instabilities, potentially affecting observable curvature perturbations.

Contribution

It provides an analytical assessment of mode coupling during reheating in hybrid inflation, highlighting the limitations of back-reaction in controlling entropy mode instabilities.

Findings

Back-reaction of short wavelength modes is too weak to truncate long wavelength instabilities.

Long wavelength entropy modes can grow significantly during reheating.

Potential impact on observable curvature perturbations in hybrid inflation models.

Abstract

We provide an analytical study of the coupling of short and long wavelength fluctuation modes during the initial phase of reheating in two field models like hybrid inflation. In these models, there is - at linear order in perturbation theory - an instability in the entropy modes of cosmological perturbations which, if not cut off, could lead to curvature fluctuations which exceed the current observational values. Here, we demonstrate that the back-reaction of short wavelength fluctuations is too weak to lead to a truncation of the instability for the long wavelength modes on time scales comparable to the typical instability time scale of the long wavelength entropy modes. Hence, unless there are other mechanisms which truncate the instability, then in models such as hybrid inflation the curvature perturbations produced during reheating on scales of current observational interest may be very important.