Gravitational Waves from Axion Monodromy
  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.
