Gravitational Waves from a Gauge Field Non-minimally Coupled to Gravity

TL;DR

This paper explores how a non-minimal coupling between a gauge field and gravity during inflation amplifies gravitational waves, potentially detectable by future space-based detectors, and causes multiple peaks in the GW spectrum.

Contribution

It introduces the impact of the $RFF$ coupling on gauge field amplification and GW production, revealing enhanced amplitudes and spectral features.

Findings

Coupling $RFF$ enhances gauge field mode amplitude.

Multiple peaks appear in the GW energy spectrum.

Produced GWs could be observed by future space-based detectors.

Abstract

An axion-like spectator during inflation can trigger a tachyonic instability which amplifies the modes of one of the helicities of the gauge field, resulting in the production of parity-violating gravitational waves (GWs). In this paper we investigate the impact of the coupling $RFF$ of the gauge field to gravity on the production of GWs. We find that such a coupling introduces a multiplicative factor to the tachyonic mass, which effectively enhances the amplitude of the gauge field modes. Produced GWs are expected to be observed by future space-based GW detectors. Additionally, we find that the strong backreaction due to particle production leads to multiple peaks in the energy spectrum of GWs.