Subluminal Galilean Genesis

TL;DR

This work proposes a subluminal Galilean Genesis by breaking the full conformal symmetry down to Poincaré plus dilations with approximate Galilean UV invariance, yielding NEC-violating backgrounds whose perturbations propagate subluminally. The authors show that the resulting solution is stable and radiatively robust under dilation-protected operators, while preserving a scale-invariant spectrum for a spectator scalar σ due to dilation symmetry. They analyze how the NEC-violating phase can transition to a standard radiation-dominated epoch, emphasizing nontrivial junction conditions and energy transfer constraints, and discuss the role of higher Galileon terms and non-minimal gravity couplings, concluding that subluminality and NEC-violation remain compatible only within specific regimes and are not obviously extendable to a healthy Minkowski vacuum in the presence of gravity. The model yields distinctive signatures in higher-order correlators that could differentiate it from inflation, though key open questions about UV completion and NEC realization in a complete theory remain.

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.

Figures (1)

• Figure 1: Generic 1-PI loop diagram. The interaction vertices can be extracted from eq. \ref{['eq:alpha']}, upon expanding in $\pi$.