Behavior of perturbations on spherically symmetric backgrounds in multi-Galileon theory

TL;DR

This paper analyzes perturbations in multi-Galileon theories around spherically symmetric backgrounds, revealing inherent issues with superluminal propagation and strong coupling that challenge the model's viability.

Contribution

It demonstrates that under stability and screening conditions, multi-Galileon theories cannot avoid superluminal signals and strong coupling problems.

Findings

Solutions exhibit superluminal propagation under stability.

Strongly coupled fluctuations cannot be eliminated.

Theoretical issues persist despite screening mechanisms.

Abstract

We consider multi-Galileon theory, the most general Galilean invariant theory with $N$ scalar fields linearly coupled to the trace of the stress-energy tensor. We study the behavior of perturbations on a static spherically symmetric background with a massive point source, and show that, under the assumptions of stability and successful Vainshtein screening, solutions cannot be found that are free of both superluminal propagation and slowly moving, strongly coupled fluctuations. The latter imply that the theoretical and phenomenological issues related to a very low strong-interaction scale cannot be avoided in this model.