Isocurvature perturbations in the Ekpyrotic universe

TL;DR

The paper shows that isocurvature perturbations arise generically in the Ekpyrotic Universe due to kinetic couplings between the brane modulus $\varphi$ and the dilaton $\chi$, and it develops a two-field framework with a field-space rotation to separate adiabatic and entropy modes. It derives coupled equations for the gravitational potential $\Phi$ and the entropy perturbation $\delta s$, revealing that $\delta s$ can source curvature perturbations on super-Hubble scales and that adiabatic/isocurvature modes can be cross-correlated. Depending on the dynamical regime, especially the relative motion of $\chi$ and $\varphi$, the model can yield a scale-invariant curvature spectrum seeded by entropy perturbations and predict nontrivial pre-bounce correlations. The results imply that the observable perturbation spectrum in the Ekpyrotic scenario is sensitive to the pre-bounce field dynamics and reheating details, offering a mechanism distinct from inflation for generating cosmological 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.