Stochastic Gravitational Waves from Post-inflationary Structure Formation

TL;DR

This paper investigates how early matter-dominated phases after inflation lead to the formation of inflaton halos, which generate a stochastic gravitational wave background detectable in the present universe.

Contribution

It extends N-body simulations and develops semi-analytical models to predict gravitational wave spectra from post-inflationary structure formation.

Findings

Gravitational waves from inflaton halos match semi-analytical predictions.

The spectrum can extend to scenarios with low-temperature thermalization.

Potential detectability of the gravitational wave background in future experiments.

Abstract

Following inflation, the Universe may pass through an early matter-dominated phase supported by the oscillating inflaton condensate. Initially small fluctuations in the condensate grow gravitationally on subhorizon scales and can collapse to form nonlinear ``inflaton halos''. Their formation and subsequent tidal interactions will source gravitational waves, resulting in a stochastic background in the present Universe. We extend N-body simulations that model the growth and interaction of collapsed structures to compute the resulting gravitational wave emission. The spectrum of this radiation is well-matched by semi-analytical estimates based on the collapse of inflaton halos and their tidal evolution. We use this semi-analytic formalism to infer the spectrum for scenarios where the early matter-dominated phase gives way to a thermalized universe at temperatures as low as $100\,\mathrm{MeV}$ and we discuss the possible experimental opportunities created by this signal in inflationary models in which thermalization takes place long after inflation has completed.