The scalar bi-spectrum during preheating in single field inflationary models

TL;DR

This paper investigates the impact of preheating on the scalar bi-spectrum in single field inflation models, finding that super-Hubble scale curvature perturbations and the bi-spectrum remain unaffected during preheating.

Contribution

The study analytically and numerically shows that preheating does not significantly alter the scalar bi-spectrum in single field inflationary models.

Findings

Super-Hubble curvature perturbations remain constant during preheating.

The scalar bi-spectrum contributions are negligible during preheating.

Certain terms in the third order action cancel out, reducing bi-spectrum impact.

Abstract

In single field inflationary models, preheating refers to the phase that immediately follows inflation, but precedes the epoch of reheating. During this phase, the inflaton typically oscillates at the bottom of its potential and gradually transfers its energy to radiation. At the same time, the amplitude of the fields coupled to the inflaton may undergo parametric resonance and, as a consequence, explosive particle production can take place. A priori, these phenomena could lead to an amplification of the super-Hubble scale curvature perturbations which, in turn, would modify the standard inflationary predictions. However, remarkably, it has been shown that, although the Mukhanov-Sasaki variable does undergo narrow parametric instability during preheating, the amplitude of the corresponding super-Hubble curvature perturbations remain constant. Therefore, in single field models, metric preheating does not affect the power spectrum of the large scale perturbations. In this article, we investigate the corresponding effect on the scalar bi-spectrum. Using the Maldacena's formalism, we analytically show that, for modes of cosmological interest, the contributions to the scalar bi-spectrum as the curvature perturbations evolve on super-Hubble scales during preheating is completely negligible. Specifically, we illustrate that, certain terms in the third order action governing the curvature perturbations which may naively be expected to contribute significantly are exactly canceled by other contributions to the bi-spectrum. We corroborate selected analytical results by numerical investigations. We conclude with a brief discussion of the results we have obtained.