Fine-tuning challenges for the matter bounce scenario

TL;DR

This paper examines the challenges of implementing a matter bounce cosmology, highlighting the fine-tuning issues needed to suppress anisotropy and ensure a stable matter-like contraction phase for generating scale-invariant perturbations.

Contribution

The study identifies the necessity of exponential fine-tuning to suppress anisotropy, questioning the viability of the matter bounce scenario without additional mechanisms.

Findings

Anisotropy growth threatens matter bounce stability.

Suppressing anisotropy requires exponential fine-tuning.

Matter-like phase may never initiate without suppression.

Abstract

A bouncing universe with a long period of contraction during which the average density is pressureless (the same equation of state as matter) as cosmologically observable scales exit the Hubble horizon has been proposed as an explanation for producing a nearly scale-invariant spectrum of adiabatic scalar perturbations. A well-known problem with this scenario is that, unless suppressed, the energy density associated with anisotropy grows faster than that of the pressureless matter, so the matter-like phase is unstable. Previous models introduce an ekpyrotic phase after the matter-like phase to prevent the anisotropy from generating chaotic mixmaster behavior. In this work, though, we point out that, unless the anisotropy is suppressed first, the matter-like phase will never start and that suppressing the anisotropy requires extraordinary, exponential fine-tuning.