Axion inflation in the regime of homogeneous backreaction

TL;DR

This paper studies the gravitational wave background generated during axion inflation with gauge field coupling, focusing on the homogeneous backreaction regime and its implications for detectability and inflaton dynamics.

Contribution

It provides a perturbative analysis of homogeneous backreaction in axion inflation, demonstrating scenarios with detectable gravitational waves and controlled inflaton gradient energy.

Findings

Detectable SGWB can occur with subdominant inflaton gradient energy.

Homogeneous backreaction can produce oscillations in inflaton velocity.

Rapid gradient energy increase may invalidate homogeneous approximation.

Abstract

We investigate the Stochastic Gravitational Wave Background (SGWB) produced in models of axion inflation coupled to gauge fields. Achieving a detectable signal at Pulsar Timing Array, astrometry, or interferometer frequencies requires a sufficiently strong amplification of the gauge fields, at a level that induces significant backreaction on the inflaton background dynamics. Numerical studies based on the approximation of homogeneous backreaction (i.e., neglecting inhomogeneities of the inflaton field) exhibit oscillations in the inflaton velocity, with corresponding peaks in the SGWB spectrum. The most recent lattice simulations have questioned the validity of this regime, showing examples characterized by a rapid increase in the inflaton gradient energy and the breakdown of homogeneous backreaction. We compute this energy density perturbatively within the assumption of homogeneous backreaction, obtaining examples with a detectable SGWB and with an inflaton gradient energy that remains subdominant to the inflaton zero-mode kinetic energy throughout inflation.