Instability of the cosmological DBI-Galileon in the non-relativistic limit

TL;DR

This paper investigates the non-relativistic limit of the DBI-Galileon model, revealing that it reduces to a shift-symmetric Horndeski theory but suffers from fundamental ghost instabilities in perturbations, challenging its viability in late-time cosmology.

Contribution

It demonstrates that the non-relativistic limit of the DBI-Galileon model is equivalent to a shift-symmetric Horndeski theory and identifies inherent ghost instabilities in perturbations.

Findings

The non-relativistic limit expands around General Relativity.

Scalar and tensor perturbations contain ghost degrees of freedom.

Instability persists for all parameter combinations studied.

Abstract

The DBI-Galileon model is a scalar-tensor theory of gravity which is defined as the most general theory of the dynamics of a 4D brane embedded in a 5D bulk. It is of particular interest as it introduces only a few free parameters, all with a clear physical meaning, such as the brane tension which is related to the cosmological constant. From the tight constraints on the gravitational waves speed, we are naturally led to consider the non-relativistic limit of the model where the kinetic energy of the brane is small compared to its tension, that we study in the context of late-time cosmology. We show that the DBI-Galileon in the non-relativistic limit is an expansion around General Relativity (GR) which can be expressed as a shift-symmetric Horndeski theory. We develop the description of this theory at the background and perturbation level. However, by studying the scalar and tensor perturbations around a flat FLRW background, we find that they contain a ghost degree of freedom leading to a fatal instability of the vacuum for every combination of the free parameters. In order to avoid it in the more general cases of shift-symmetric Horndeski theories, we emphasize which of the Horndeski terms are in competition to produce this instability.