Modulated perturbations from instant preheating after new ekpyrosis

I present a mechanism to transfer the spectrum of perturbations in a scalar isocurvature field xi onto the matter content in the radiation era via modulated, instant preheating after ekpyrosis. In this setup, xi determines the coupling constant relevant for the decay of a preheat matter field into fermions. The resulting power spectrum is scale invariant if xi remains close to a scaling solution in new ekpyrotic models of the Universe; by construction the spectrum is independent of the detailed physics near the bounce. The process differs from the curvaton mechanism, which has been used recently to revive the ekpyrotic scenario, in that no peculiar behavior of xi shortly before or during the bounce is needed. In addition, a concrete and efficient realization of reheating after ekpyrosis is provided; this mechanism is not tied to ekpyrotic models, but could equally well be used in other setups, for instance inflationary ones. I estimate non-Gaussianities and find no additional contributions in the most simple realizations, in contrast to models using the curvaton mechanism.