Covariant evolution of perturbations during reheating in two-field inflation

TL;DR

This paper introduces a covariant framework for analyzing how reheating affects primordial perturbations in two-field inflation models, extending the system to include radiation as an effective scalar field and examining two specific scenarios.

Contribution

It develops a covariant method to incorporate radiation into two-field inflation models, enabling detailed study of reheating effects on perturbations with novel geometrical interpretation.

Findings

Reheating modifies the evolution of adiabatic and isocurvature perturbations.

Coupling to radiation influences non-Gaussianity in inflation models.

Ultra-light field scenario maintains isocurvature interactions throughout inflation.

Abstract

We develop a covariant method for studying the effects of a reheating phase on the primordial adiabatic and isocurvature perturbations in two-field models of inflation. To model the decay of the scalar fields into radiation at the end of inflation, we introduce a prescription in which radiation is treated as an additional effective scalar field, requiring us to extend the two-field setup into a three-field system. In this prescription, the coupling between radiation and the scalars can be interpreted covariantly in terms geometrical quantities that parametrize the evolution of a background trajectory in a three-field space. In order to obtain concrete results, we consider two scenarios characterized for having unsuppressed isocurvature fluctuations at the end of inflation: (1) canonical two-field inflation with the product exponential potential, which sources a large negative amount of non-gaussianity and, (2) two-field inflation with an ultra-light field, a model in which the isocurvature mode becomes approximately massless, and its interaction with the curvature perturbation persists during the entire period of inflation. In both cases we discuss how their predictions are modified by the coupling of the scalar fields to the radiation fluid.