Isocurvature perturbations in the Ekpyrotic universe

TL;DR

This paper demonstrates that the Ekpyrotic universe naturally predicts isocurvature perturbations due to brane interactions, which can be correlated with adiabatic modes and may seed curvature perturbations.

Contribution

It explicitly shows how to separate adiabatic and isocurvature modes in the Ekpyrotic scenario and discusses their potential correlations and implications.

Findings

Isocurvature perturbations are a generic prediction of the Ekpyrotic universe.

Adiabatic and isocurvature modes can be explicitly separated in field space.

Curvature perturbations may be entirely seeded by isocurvature perturbations.

Abstract

The Ekpyrotic scenario assumes that our visible Universe is a boundary brane in a five-dimensional bulk and that the hot Big Bang occurs when a nearly supersymmetric five-brane travelling along the fifth dimension collides with our visible brane. We show that the generation of isocurvature perturbations is a generic prediction of the Ekpyrotic Universe. This is due to the interactions in the kinetic terms between the brane modulus parametrizing the position of the five-brane in the bulk and the dilaton and volume moduli. We show how to separate explicitly the adiabatic and isorcuvature modes by performing a rotation in field space. Our results indicate that adiabatic and isocurvature pertubations might be cross-correlated and that curvature perturbations might be entirely seeded by isocurvature perturbations.