Unbraiding the Bounce: Superluminality around the Corner

TL;DR

This paper analyzes a cosmological bounce model, revealing issues with sound speed divergence, and uncovers stable superluminal solutions that challenge the model's UV-completion prospects.

Contribution

It reconstructs the Lagrangian for the bounce model, identifies superluminal solutions, and discusses their implications for UV-completion.

Findings

Bounce solution starts with divergent sound speed

Existence of stable superluminal solutions

Superluminal states connect to the original bounce solution

Abstract

We study a particular realization of the cosmological bounce scenario proposed recently by Ijjas and Steinhardt. First, we find that their bouncing solution starts from a divergent sound speed and ends with its vanishing. Thus, the solution connects two strongly coupled configurations. These pathologies are separated from the bouncing regime by only a few Planck times. We then reveal the exact structure of the Lagrangian, which reproduces this bouncing solution. This reconstruction allowed us to consider other cosmological solutions of the theory and analyze the phase space. In particular, we find other bouncing solutions and solutions with superluminal sound speed. These stable superluminal states can be continuously transformed into the solution constructed by Ijjas and Steinhardt. We discuss the consequences of this feature for a possible UV-completion.