Fine-tuning challenges for the matter bounce scenario

Abstract

The author points out fine-tuning issues in bouncing cosmologies, which are seemingly unavoidable for a nearly scale-invariant spectrum as observed in the Cosmic Microwave Background (CMB). To suppress destabilizing anisotropies in the contracting phase, an exponential fine-tuned ``ekpyrosis'' (the anisotropy suppressing phase) and a coupling of the corresponding degrees of freedom to those of the subsequent matter-like contracting phase is necessary.