Gravitational Waves from Axion Monodromy

TL;DR

This paper explores how axion monodromy inflation can produce gravitational waves through a process similar to phase transitions, with potential signals detectable by future interferometers.

Contribution

It demonstrates that dynamical phase decomposition in axion monodromy inflation can generate observable gravitational waves, a novel mechanism linking inflationary dynamics to gravitational wave signals.

Findings

Gravitational waves can be produced during bubble collisions in axion monodromy.

The predicted gravitational wave frequency range spans from mHz to GHz.

The gravitational wave signal could be detectable by next-generation interferometers.

Abstract

Large field inflation is arguably the simplest and most natural variant of slow-roll inflation. Axion monodromy may be the most promising framework for realising this scenario. As one of its defining features, the long-range polynomial potential possesses short-range, instantonic modulations. These can give rise to a series of local minima in the post-inflationary region of the potential. We show that for certain parameter choices the inflaton populates more than one of these vacua inside a single Hubble patch. This corresponds to a dynamical phase decomposition, analogously to what happens in the course of thermal first-order phase transitions. In the subsequent process of bubble wall collisions, the lowest-lying axionic minimum eventually takes over all space. Our main result is that this violent process sources gravitational waves, very much like in the case of a first-order phase transition. We compute the energy density and peak frequency of the signal, which can lie anywhere in the mHz-GHz range, possibly within reach of next-generation interferometers. We also note that this "dynamical phase decomposition" phenomenon and its gravitational wave signal are more general and may apply to other inflationary or reheating scenarios with axions and modulated potentials.