Scalar Perturbations in Nonsingular Universes from Interacting Vacuum

TL;DR

This paper investigates the stability of scalar perturbations in nonsingular cosmological models derived from an interacting vacuum, revealing potential instabilities near the bounce due to diverging vacuum perturbations.

Contribution

It introduces a detailed analysis of scalar perturbations in nonsingular universes with interacting vacuum, highlighting instability issues at the bounce.

Findings

Vacuum perturbation diverges near the bounce

Nonsingular models can have physically interesting phase space orbits

Energy transfer choices are ill-defined at the bounce

Abstract

In this paper we examine the stability of scalar perturbations in nonsingular models which emerge from an interacting vacuum component. The analysis developed in this paper relies on two phenomenological choices for the energy exchange between a nonrelativistic fluid and a vacuum component. In both scenarios it can be shown that closed models may furnish nonsingular orbits of physical interest in phase space once a decelerated past era is connected to a graceful exit to late-time acceleration. Regarding such configurations as background spacetimes we introduce scalar perturbations in order to examine the stability of these models in a high energy domain. We explicitly show that the vacuum perturbation is not an independent variable and diverges as dynamics approaches the bounce. This feature assigns a rather unstable signature to the dynamics making the choices for the energy transfer ill defined at least for nonsingular configurations at the bounce scale.