Naturally inflating on steep potentials through electromagnetic dissipation

TL;DR

This work shows that axion-like inflatons can drive inflation on steep potentials if they couple to gauge fields via a Chern-Simons-type term, with dissipative gauge-field production slowing the inflaton and sustaining slow-roll. A key result is that the gauge-field backreaction yields a quasi-scale-invariant curvature spectrum, but the simplest realizations produce amplitudes incompatible with observations unless a large number of gauge fields (or other suppression mechanisms) reduces the signal. The authors provide analytic estimates for the gauge-field production, backreaction, and perturbations, and outline two UV-complete avenues (two-axion models and extra dimensions) to realize large couplings. The mechanism opens a UV-friendly path for natural inflation, with testable implications for parity-violating gravitational waves and potential non-Gaussian signatures.

Abstract

In models of natural inflation, the inflaton is an axion-like particle. Unfortunately, axion potentials in UV-complete theories appear to be too steep to drive inflation. We show that, even for a steep potential, natural inflation can occur if the coupling between axion and gauge fields is taken into account. Due to this coupling, quanta of the gauge field are produced by the rolling of the axion. If the coupling is large enough, such a dissipative effect slows down the axion, leading to inflation even for a steep potential. The spectrum of perturbations is quasi-scale invariant, but in the simplest construction its amplitude is larger than $10^{-5}$. We discuss a possible way out of this problem.