Generation of gravitational waves during early structure formation between cosmic inflation and reheating

TL;DR

This paper investigates the generation of gravitational waves during the early universe's structure formation phase between inflation and reheating, highlighting potential detectable signals from non-linear cosmic structures.

Contribution

It provides the first estimates of gravitational wave signals from early structure formation during the pre-reheating era, including both linear and non-linear effects.

Findings

Potentially large gravitational wave signals detectable by future observatories

Growth of density perturbations leads to non-linear structure formation

Various effects like halo collapse and turbulence contribute to GW emission

Abstract

In the pre-reheating era, following cosmic inflation and preceding radiation domination, the energy density may be dominated by an oscillating massive scalar condensate, such as is the case for quadratic chaotic inflation. We have found in a previous paper that during this period, a wide range of sub-Hubble scale perturbations are subject to a preheating instability, leading to the growth of density perturbations ultimately collapsing to form non-linear structures. We compute here the gravitational wave signal due to these structures in the linear limit and present estimates for emission in the non-linear limit due to various effects: the collapse of halos, the tidal interactions, the evaporation during the conversion of the inflaton condensate into radiation and finally the ensuing turbulent cascades. The gravitational wave signal could be rather large and potentially testable by future detectors.