Superluminality in dilatationally invariant generalized Galileon theories

TL;DR

This paper investigates the stability and superluminal propagation of perturbations in dilatationally-invariant Galileon theories, revealing that while Minkowski backgrounds can be stable and subluminal, FLRW backgrounds inherently exhibit superluminal propagation.

Contribution

It demonstrates that generalized Galileon models can be stable but still exhibit superluminal signals in cosmological backgrounds, highlighting a fundamental limitation of these theories.

Findings

Minkowski backgrounds can be stable and subluminal.

FLRW backgrounds inevitably show superluminal propagation.

Superluminality is a common issue in generalized Galileon models.

Abstract

We consider small perturbations about homogeneous backgrounds in dilatationally-invariant Galileon models. The issues we address are stability (absence of ghosts and gradient instabilities) and superluminality. We show that in Minkowski background, it is possible to construct the Lagrangian in such a way that any homogeneous Galileon background solution is stable and small perturbations about it are subluminal. On the other hand, in the case of FLRW backgrounds, for any Lagrangian functions there exist homogeneous background solutions to the Galileon equation of motion and time-dependence of the scale factor, such that the stability conditions are satisfied, but the Galileon perturbations propagate with superluminal speed. Thus, a popular class of the generalized Galileon models is plagued by superluminality.