Warm Ekpyrosis

TL;DR

This paper introduces a new mechanism in ekpyrotic cosmology that generates a nearly scale-invariant spectrum of density perturbations through a coupling between the ekpyrotic field and ultra-relativistic matter, simplifying the model.

Contribution

It presents a novel coupling mechanism that produces scale-invariant perturbations without requiring a second scalar field or phase, reducing fine-tuning in ekpyrotic models.

Findings

Achieves nearly scale-invariant spectrum during ekpyrosis

Eliminates need for entropic field and conversion phase

Reduces constraints on the ekpyrotic equation of state

Abstract

We propose a mechanism to generate a nearly scale-invariant spectrum of adiabatic scalar perturbations about a stable, ekpyrotic background. The key ingredient is a coupling between a single ekpyrotic field and a perfect fluid of ultra-relativistic matter. This coupling introduces a friction term into the equation of motion for the field, opposing the Hubble anti-friction, which can be chosen such that an exactly scale-invariant (or nearly scale-invariant) spectrum of adiabatic density perturbations is continuously produced throughout the ekpyrotic phase. This mechanism eliminates the need for a second (entropic) scalar field and hence any need for introducing a second phase for converting entropic into curvature fluctuations. It also reduces the constraints on the equation of state during the ekpyrotic phase and, thereby, the need for parametric fine-tuning.