Modulated Perturbations from Instant Preheating after new Ekpyrosis

TL;DR

We address generating scale-invariant curvature perturbations after ekpyrosis by transferring the spectrum of an isocurvature field $ξ$ to radiation through modulated instant preheating. The decay timing of a preheat field $χ$ is controlled by a ξ-dependent coupling $h(ξ)$, yielding an extra curvature power $P_{ abla ζ+} = \left( \frac{3}{2} \frac{1}{h} \frac{\partial h}{\partial ξ} \right)^2 P_ξ$, while the total spectrum can be dominated by the isocurvature contribution. The mechanism is insensitive to bounce details and does not generically generate non-Gaussianities in the simplest exponential $h(ξ)$ case, offering a robust alternative to curvaton-based approaches and appealing for both ekpyrotic and inflationary contexts.

Abstract

We present a mechanism to transfer the spectrum of perturbations in a scalar isocurvature field $ξ$ onto the matter content in the radiation era via modulated, instant preheating after ekpyrosis. In this setup, $ξ$ determines the coupling constant relevant for the decay of a preheat matter field into Fermions. The resulting power spectrum is scale invariant if $ξ$ 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 $ξ$ 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. We estimate non-Gaussianities and find no additional contributions in the most simple realizations, in contrast to models using the curvaton mechanism.