Note on the production of scale-invariant entropy perturbation in the Ekpyrotic universe

TL;DR

This paper proposes a stable mechanism for generating scale-invariant entropy perturbations in the Ekpyrotic universe using non-minimal couplings, addressing instability issues in traditional models.

Contribution

It introduces a non-minimally coupled scalar field as a stable entropy mode, enabling scale-invariance without tachyonic instability in Ekpyrotic models.

Findings

Stable scale-invariant entropy perturbations achieved

Non-minimal couplings stabilize the entropy field

Requires additional mechanisms for curvature perturbation conversion

Abstract

In the standard entropic mechanism adopted in the simple Ekpyrotic models to generate the nearly scale-invariant and Gaussian primordial perturbation, the entropy direction is tachyonically unstable. In this paper, we consider the stable production of the scale-invariant entropy perturbation in the Ekpyrotic universe via non-minimal couplings. In this model the non-minimally coupled massless scalar field serves as a spectator and is stabilized by the introduced non-minimal couplings. It always corresponds to the entropy field during the contraction and with appropriate couplings can obtain a scale-invariant spectrum. This scenario requires additional mechanisms such as curvaton or modulated preheating to convert the entropy perturbation to the curvature perturbation after the bounce.