Parametric Resonance of Entropy Perturbations in Massless Preheating

TL;DR

This paper demonstrates that entropy perturbations in a massless preheating model can undergo parametric resonance, leading to exponential growth of curvature fluctuations, which may cause phenomenological issues.

Contribution

It confirms the existence of parametric instability of entropy modes in massless preheating using a covariant perturbative approach, resolving prior conflicting results.

Findings

Entropy modes exhibit parametric resonance leading to exponential growth.

Back-reaction effects can suppress curvature fluctuations but may not do so early enough.

Results align with previous studies, clarifying the controversy.

Abstract

Here, we revisit the question of possible preheating of entropy modes in a two field model with a massless inflaton coupled to a matter scalar field. Using a perturbative approximation to the covariant method we demonstrate that there is indeed a parametric instability of the entropy mode which then at second order leads to exponential growth of the curvature fluctuation on super-Hubble scale. Back-reaction effects shut off the induced curvature fluctuations, but possibly not early enough to prevent phenomenological problems. This confirms previous results obtained using different methods and resolves a controversy in the literature.