Bouncing cosmology from nonlinear dark energy with two cosmological constants

TL;DR

This paper investigates non-singular bouncing and cyclic cosmologies within a dark energy model with a quadratic equation of state, highlighting conditions for bounces and accelerated expansion, and addressing the cosmological constant problem.

Contribution

It introduces a class of closed FLRW models with nonlinear dark energy that always admit a bounce and explores their dynamical behavior and implications for cosmic evolution.

Findings

Bouncing models are always present in closed cosmologies with nonlinear dark energy.

A subset of models exhibits early- and late-time acceleration connected by deceleration.

Imposing bounds on matter and radiation densities influences the occurrence of bounces.

Abstract

We explore the dynamics of FLRW cosmologies which consist of dark matter, radiation and dark energy with a quadratic equation of state. Standard cosmological singularities arise due to energy conditions which are violated by dark energy, therefore we focus our analysis on non-singular bouncing and cyclic cosmologies, in particular focusing on the possibility of closed models always having a bounce for any initial conditions. We analyse the range of dynamical behaviour admitted by the system, and find a class of closed models that admit a non-singular bounce, with early- and late-time accelerated expansion connected by a decelerating phase. In all cases, we find the bouncing models are only relevant when dark matter and radiation appear at a certain energy scale, and so require a period such as reheating. We then investigate imposing an upper bound on the dark matter and radiation, such that their energy densities cannot become infinite. We find that bounces are always the general closed model, and a class of models exist with early- and late-time acceleration, connected by a decelerating phase. We also consider parameter values for the dark energy component, such that the discrepancy between the observed value of $\Lambda$ and the theoretical estimates of the contributions to the effective cosmological constant expected from quantum field theory would be explained. However, we find that the class of models left does not allow for an early- and late-time accelerated expansion, connected by a decelerating period where large-scale structure could form. Nonetheless, our qualitative analysis serves as a basis for the construction of more realistic models with realistic quantitative behaviour.