Gravitational waves from axion monodromy

Abstract

Large field ination is arguably the simplest and most natural variant of slowroll ination. Axion monodromy may be the most promising framework for realising this scenario. As one of its defining features, the long-range polynomial potential possesses shortrange, instantonic modulations. These can give rise to a series of local minima in the postin ationary region of the potential. We show that for certain parameter choices the inaton 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 firstorder 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 inationary or reheating scenarios with axions and modulated potentials.