Fine-tuning challenges for the matter bounce scenario

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.

arXiv comment6 pages, 3 figures

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• Viable tensor-to-scalar ratio in a symmetric matter bounce
• Microphysical manifestations of viscosity and consequences for anisotropies in the very early universe
• Quantum Cosmic No-Hair Theorem and Inflation
• Primordial features from ekpyrotic bounces
• Massive gravity and the suppression of anisotropies and gravitational waves in a matter-dominated contracting universe
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• Cuscuton Bounce Beyond the Linear Regime: Bispectrum and Strong Coupling Constraints
• Crossing the Big Bang singularity
• Cosmology in a Globally U(1) Symmetric Scalar-Tensor Gravity