Gravitational wave background from non-Abelian reheating after axion-like inflation

TL;DR

This paper investigates how non-Abelian gauge fields during axion-like inflation can produce a gravitational wave background through a self-amplifying reheating process, with potential observational signatures.

Contribution

It introduces a lower bound on gravitational wave production from non-Abelian reheating, highlighting differences from Abelian models and providing a minimal parameter framework.

Findings

Moderate gravitational wave energy density produced

Non-Abelian models may avoid overproduction issues

Hydrodynamic fluctuations contribute to GW signals

Abstract

A pseudoscalar inflaton $\varphi$, coupled to the topological charge density $F\tilde{F}$ of a non-Abelian sector, can decay to gauge bosons ($\varphi\to g g $), which may thermalize rapidly. The friction felt by $\varphi$ is then increased by non-Abelian "strong sphalerons", leading to a self-amplifying process that can efficiently heat up the medium. We determine a lower bound for the gravitational wave production rate from such a process, originating via hydrodynamic fluctuations and particle collisions, in terms of a minimal number of parameters. Only a moderate fraction of energy density is converted to gravitational waves, suggesting that non-Abelian models may avoid the overproduction observed in some Abelian cases.