Gravitational Waves from Gauge Quanta Produced during Inflation

TL;DR

This paper studies how a fast-rolling axion during inflation can produce gauge quanta that generate gravitational waves, with the amplitude depending on the axion decay constant and including effects of backreaction.

Contribution

It provides a detailed analysis of gravitational wave production from gauge quanta in a spectator axion model, incorporating strong backreaction effects and deriving bounds on the GW spectrum.

Findings

Decreasing the axion decay constant enhances GW production.

The GW amplitude is constrained to be below 10^{-10} in the nHz to mHz range.

A lower bound on the axion decay constant is established based on initial conditions.

Abstract

A fast-rolling axion can transfer its kinetic energy to a gauge field via the Chern-Simons coupling, leading to copious production of gauge quanta, which can act as a source of gravitational waves (GWs) with potentially observable amplitudes. In this work, we investigate GW production in a spectator axion model when strong backreaction is taken into account. We find that decreasing the decay constant of the axion enhances GW production. Since the initial value of the axion is larger than its quantum fluctuations, such a condition imposes a lower bound on the axion dacay constant, which sets an upper bound on the amplitude of the energy spectrum of GWs. As a result, the amplitude of the predicted GW energy spectrum is lower than $10^{-10}$ in the nHz to mHz frequency range.