Superluminality in the Bi- and Multi- Galileon

TL;DR

This paper demonstrates that superluminal fluctuations are an inherent feature of Bi- and Multi-Galileon theories under trivial asymptotic conditions, challenging previous claims of their absence.

Contribution

It shows that superluminal modes are unavoidable in these models, even without the Cubic Galileon term, and discusses potential ways to avoid them by altering asymptotic conditions.

Findings

Superluminal fluctuations are common in Multi-Galileon theories with Cubic Galileon.

Superluminal modes exist around point sources even without the Cubic Galileon.

Modifying asymptotic conditions may influence superluminality presence.

Abstract

We re-explore the Bi- and Multi-Galileon models with trivial asymptotic conditions at infinity and show that propagation of superluminal fluctuations is a common and unavoidable feature of these theories, unlike previously claimed in the literature. We show that all Multi-Galileon theories containing a Cubic Galileon term exhibit superluminalities at large distances from a point source, and that even if the Cubic Galileon is not present one can always find sensible matter distributions in which there are superluminal modes at large distances. In the Bi-Galileon case we explicitly show that there are always superluminal modes around a point source even if the Cubic Galileon is not present. Finally, we briefly comment on the possibility of avoiding superluminalities by modifying the asymptotic conditions at infinity.