Stochastic gravitational waves from postinflationary structure formation

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 semianalytical estimates based on the collapse of inflaton halos and their tidal evolution. We use this semianalytic 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 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.