Primordial Gravitational Waves from Axion-Gauge Fields Dynamics

TL;DR

This paper proposes a model where axion and SU(2) gauge fields during inflation generate a detectable, chiral gravitational wave signal that can be distinguished from standard vacuum fluctuations, relaxing constraints on inflationary energy scales.

Contribution

It introduces a spectator axion-gauge field sector that produces observable gravitational waves with unique properties, expanding the mechanisms for gravitational wave generation during inflation.

Findings

Spectator fields can produce large, chiral gravitational waves.

The gravitational wave signal can be detectable even with low inflationary Hubble rate.

The model relaxes the usual relation between tensor-to-scalar ratio and inflation energy scale.

Abstract

Inspired by the chromo-natural inflation model of Adshead&Wyman, we reshape its scalar content to relax the tension with current observational bounds. Besides an inflaton, the setup includes a spectator sector in which an axion and SU(2) gauge fields are coupled via a Chern-Simons-type term. The result is a viable theory endowed with an alternative production mechanism for gravitational waves during inflation. The gravitational wave signal sourced by the spectator fields can be much larger than the contribution from standard vacuum fluctuations, it is distinguishable from the latter on the basis of its chirality and, depending on the theory parameters values, also its tilt. This production process breaks the well-known relation between the tensor-to-scalar ratio and the energy scale of inflation. As a result, even if the Hubble rate is itself too small for the vacuum to generate a tensor amplitude detectable by upcoming experiments, this model still supports observable gravitational waves.