Modulated perturbations from instant preheating after new ekpyrosis

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.

Ekpyrotic perturbations with small non-Gaussian corrections

“…Unless the fast-roll parameter ϵ during the ekpyrotic phase is very close to 3, this implies that (just as for the standard entropic mechanism) the potential has to reach the grand unified scale V ek-end ≈ ð10 −2 M Pl Þ 4 in order for the curvature perturbations to have an amplitude in agreement with the observed value of about 2 × 10 −9 [1]. Another possibility, of a somewhat different character, is that conversion can occur at the bounce itself via the process of modulated (p)reheating [40]. The idea here is that at the bounce massive matter particles can be copiously produced, with their subsequent decay into ordinary fermions being modulated by a coupling function hðδsÞ.…”

“…The idea here is that at the bounce massive matter particles can be copiously produced, with their subsequent decay into ordinary fermions being modulated by a coupling function hðδsÞ. As shown in [40], the amplitude of the resulting perturbations is proportional to h ;s =h, and thus all predictions depend on the ability to derive the precise form of hðδsÞ in a realistic setting. This conversion model has the property that no bending of the trajectory need to occur before the bounce.…”

Ekpyrotic and cyclic cosmology

“…There are many ways in which such a bending of the background trajectory can occur. We will now discuss the various possibilities considered in the literature, namely conversion after the ekpyrotic phase during kinetic energy domination [61], conversion during the ekpyrotic phase [57,56] or during the transition to a ghost condensate phase [22,30], and conversion after the big bang by modulated preheating [9].…”

“…Finally, it has been proposed [9] that, instead of converting entropy perturbations into curvature perturbations before the big crunch/big bang transition, the conversion could occur during the phase shortly following the bang through modulated reheating. The concept is that massive matter fields are produced copiously at the brane collision and dominate the energy density immediately after the bang.…”

Tilted ekpyrosis

“…The corresponding change in the local equation of state, controlled by the local value of the field, leads to a density perturbation, / [18]. Other mechanisms have also been proposed which could convert the isocurvature field fluctuations to density perturbations including a kinetic conversion due to an abrupt change in the field trajectory after the ekpyrotic phase [11] or a curvaton-type conversion due to modulated reheating in an expanding phase following the bounce [19]. In any case any linear process preserves the scale dependence of the power spectrum and we have n ¼ n .…”