One-loop infrared rescattering by enhanced scalar fluctuations during inflation

TL;DR

This paper demonstrates that excited field perturbations during inflation cause a resonant IR cascade, significantly modifying the curvature perturbation spectrum and impacting models related to primordial black holes and gravitational waves.

Contribution

It provides the first analytic one-loop results for IR power enhancement due to resonant cascades during inflation, applicable to various excited states.

Findings

Resonant IR cascade significantly alters the curvature power spectrum.

The effect impacts phenomenology of primordial black holes and gravitational waves.

Analytic results applicable to multiple excited state scenarios.

Abstract

We show that, whenever the perturbations of some field are excited during inflation by a physical process on sub-horizon scales, they unavoidably generate, even through gravitational interactions alone, a significant resonant IR cascade of power down to scales that are of the order of the horizon at that time (we denote these scales as near IR). We provide general analytic one-loop results for the enhancement of the IR power of the curvature perturbation generated by this effect, highlighting the role played by the resonance. We then study a number of examples in which the excited state is: (i) an isocurvature field, (ii) the curvature perturbation itself, (iii) a mixture of curvature and isocurvature fluctuations driven to an excited state by their coupled dynamics. In the cases shown, the cascade significantly modifies the near IR part of the power spectrum of the curvature perturbation with respect to the linear theory, indicating that this effect can impact the phenomenology associated with a variety of mechanisms considered in the literature, notably concerning primordial black holes and gravitational waves.