No Warm Inflation From a Vanilla Axion

TL;DR

This paper demonstrates that standard axion properties prevent the realization of strong warm inflation due to thermal friction constraints, leaving only weak warm axion inflation feasible with potential super-Planckian issues.

Contribution

It provides a theoretical analysis showing that discrete shift symmetry of axions excludes strong warm inflation, highlighting limitations and possible alternatives.

Findings

Strong warm inflation is incompatible with axion shift symmetry.

Weak warm axion inflation remains possible but involves super-Planckian decay constants.

Non-minimal and axion-like models may offer alternative solutions.

Abstract

At finite temperature, the coupling of an axion to non-Abelian gauge fields causes dissipation due to sphaleron heating. This mechanism has been considered as ideal for realizing warm inflation since it can lead to large thermal friction while preserving the flatness of the potential. We show, however, that requiring standard properties of an axion -- in particular a discrete shift symmetry -- excludes the strong regime of warm slow-roll inflation, in which thermal friction dominates. The present argument, which does not rely on any phenomenological input, leaves room for the weak regime of warm axion inflation, but in this case a super-Planckian decay constant represents a well-known issue. Finally, we discuss non-minimal and axion-like models as way out.