When Matter Matters

TL;DR

This paper proves that in the Subluminal Galilean Genesis scenario, Galileon perturbations are at most luminal in a matter-free universe, but can become superluminal when matter is included, impacting UV completion considerations.

Contribution

It provides a comprehensive proof that Galileon perturbations are luminal in matter-free cases and shows superluminal propagation occurs when matter is present, with implications for model UV completion.

Findings

Perturbations are at most luminal without matter.

Superluminal propagation occurs with matter inclusion.

Numerical simulations support the analytic proof.

Abstract

We study a recently proposed scenario for the early universe: Subluminal Galilean Genesis. We prove that without any other matter present in the spatially flat Friedmann universe, the perturbations of the Galileon scalar field propagate with a speed at most equal to the speed of light. This proof applies to all cosmological solutions -- to the whole phase space. However, in a more realistic situation, when one includes any matter which is not directly coupled to the Galileon, there always exists a region of phase space where these perturbations propagate superluminally, indeed with arbitrarily high speed. We illustrate our analytic proof with numerical computations. We discuss the implications of this result for the possible UV completion of the model.