  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.

High Energy Physics - Theory;Cosmology and Nongalactic Astrophysics;General Relativity and Quantum Cosmology