Subluminal Galilean Genesis

TL;DR

This paper presents an improved Galilean Genesis model that reduces superluminality issues by modifying symmetry assumptions, ensuring fluctuations remain subluminal and preserving scale-invariance of cosmological fluctuations.

Contribution

The authors introduce a version of the Galilean Genesis model with reduced symmetry, addressing superluminality and maintaining scale-invariance, with potential observational signatures in higher-order correlators.

Findings

Fluctuations are substantially subluminal in the new model.

Superluminality cannot be achieved through small perturbations.

Scale-invariance of cosmological fluctuations is preserved.

Abstract

We put forward an improved version of the Galilean Genesis model that addresses the problem of superluminality. We demote the full conformal group to Poincare symmetry plus dilations, supplemented with approximate galilean shift invariance in the UV and at small field values. In this way fluctuations around the NEC-violating cosmological background are made substantially subluminal, and superluminality cannot be reached by any small change of the solution, in contrast with the original model. Dilation invariance still protects the scale-invariance of correlation functions of a massless test scalar - which is the source of the observed cosmological fluctuations - but the explicit breaking of the conformal group can be potentially observed in higher-order correlators. We also highlight a subtlety in matching the NEC-violating phase with the standard cosmological evolution, and discuss the possible couplings of the Galileon to gravity.