Entropic mechanisms with generalized scalar fields in the Ekpyrotic universe

TL;DR

This paper investigates the stability of entropic mechanisms with generalized scalar fields in the Ekpyrotic universe, demonstrating that non-minimal couplings can stabilize models and produce observationally consistent primordial perturbations.

Contribution

It extends previous entropic models to include non-standard kinetic terms and shows that non-minimal couplings can ensure stability and observational viability.

Findings

Minimal coupling models are unstable with generalized kinetic terms.

Non-minimal coupling models can be stable and generate scale-invariant perturbations.

Stable models align with current cosmological observations.

Abstract

For the Ekpyrotic universe, the entropic mechanisms with minimal couplings, which have been used to generate nearly scale invariant primordial perturbations, was proved to be unstable. To overcome this difficulty, some non-minimal coupling entropic models were proposed. In this paper we extend these studies to the cases where all the scalar fields have non-standard kinetic terms. We first prove that in these general cases, without non-minimal couplings the entropic models are still unstable. The condition for the stability conflicts with the requirement for achieving scale invariant perturbations. Then we study the non-minimal coupling models and show that at least for some simple cases these models are stable and able to generate the primordial perturbations consistent with current observations.