Rolling axions during inflation: perturbativity and signatures

TL;DR

This paper investigates the amplification of gauge fields by rolling axions during inflation, analyzing perturbativity limits and their implications for observable primordial gravitational waves and black holes.

Contribution

It extends previous perturbativity constraints to scenarios where a pseudo-scalar field influences gauge fields during inflation, clarifying the validity of resulting signals at various scales.

Findings

Perturbativity limits are satisfied for CMB-scale signals.

Applications at smaller scales approach the perturbativity boundary.

Results are reliable up to order-one corrections for small-scale phenomena.

Abstract

The motion of a pseudo-scalar field $X$ during inflation naturally induces a significant amplification of the gauge fields to which it is coupled. The amplified gauge fields can source characteristic scalar and tensor primordial perturbations. Several phenomenological implications have been discussed in the cases in which (i) $X$ is the inflation, and (ii) $X$ is a field different from the inflation, that experiences a temporary speed up during inflation. In this second case, visible sourced gravitational waves (GW) can be produced at the CMB scales without affecting the scalar perturbations, even if the scale of inflation is several orders of magnitude below what is required to produce a visible vacuum GW signal. Perturbativity considerations can be used to limit the regime in which these results are under perturbative control. We revised limits recently claimed for the case (i), and we extend these considerations to the case (ii). We show that, in both cases, these limits are satisfied by the applications that generate signals at CMB scales. Applications that generate gravitational waves and primordial black holes at much smaller scales are at the limit of the validity of this perturbativity analysis, so we expect those results to be valid up to possibly order one corrections.