On the validity of the perturbative description of axions during inflation

TL;DR

This paper examines the perturbative limits of axion-gauge field interactions during inflation, revealing tensions with recent tensor mode generation proposals and deriving new constraints on gauge couplings, especially for non-Abelian fields.

Contribution

It provides a self-consistent perturbative analysis of axions coupled to gauge fields during inflation, highlighting constraints on models generating primordial tensor modes.

Findings

Perturbation theory constrains axion-gauge field couplings during inflation.

Recent tensor mode generation proposals are in tension with perturbative consistency.

New bounds on gauge couplings for non-Abelian gauge fields in inflation models.

Abstract

Axions play a central role in many realizations of large field models of inflation and in recent alternative mechanisms for generating primordial tensor modes in small field models. If these axions couple to gauge fields, the coupling produces a tachyonic instability that leads to an exponential enhancement of the gauge fields, which in turn can decay into observable scalar or tensor curvature perturbations. Thus, a fully self-consistent treatment of axions during inflation is important, and in this work we discuss the perturbative constraints on axions coupled to gauge fields. We show how the recent proposal of generating tensor modes through these alternative mechanisms is in tension with perturbation theory in the in-in formalism. Interestingly, we point out that the constraints are parametrically weaker than one would estimate based on naive power counting of propagators of the gauge field. In the case of non-Abelian gauge fields, we derive new constraints on the size of the gauge coupling, which apply also in certain models of natural large field inflation, such as alignment mechanisms.