Gauge Field Production in Axion Inflation: Consequences for Monodromy, non-Gaussianity in the CMB, and Gravitational Waves at Interferometers

TL;DR

This paper explores how axion-gauge field couplings during inflation influence cosmological signals, notably enhancing gravitational wave production at interferometer scales, with implications for observations and string theory models.

Contribution

It provides a unified analysis of axion-gauge interactions' effects on inflationary perturbations, including backreaction effects and gravitational wave signals, extending previous results.

Findings

Backreaction prolongs inflation by about 10 e-foldings.

Enhanced gravitational wave signals could be observed by LIGO/VIRGO.

String theory models can produce significant axion-gauge couplings for phenomenology.

Abstract

Models of inflation based on axions, which owe their popularity to the robustness against UV corrections, have also a very distinct class of signatures. The relevant interactions of the axion are a non-perturbative oscillating contribution to the potential and a shift-symmetric coupling to gauge fields. We review how these couplings affect the cosmological perturbations via a unified study based on the in-in formalism. We then note that, when the inflaton coupling to gauge fields is high enough to lead to interesting observational results, the backreaction of the produced gauge quanta on the inflaton dynamics becomes relevant during the final stage of inflation, and prolongs its duration by about 10 e-foldings. We extend existing results on gravity wave production in these models to account for this late inflationary phase. The strong backreaction phase results in an enhancement of the gravity wave signal at the interferometer scales. As a consequence, the signal is potentially observable at Advanced LIGO/VIRGO for the most natural duration of inflation in such models. Finally, we explicitly compute the axion couplings to gauge fields in string theory construction of axion monodromy inflation and identify cases where they can trigger interesting phenomenological effects.