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