Non-Gaussianity in multi-sound-speed disformally coupled inflation
  Most, if not all, scalar-tensor theories are equivalent to General Relativity
with a disformally coupled matter sector. In extra-dimensional theories such a
coupling can be understood as a result of induction of the metric on a brane
that matter is confined to. This article presents a first look at the
non-Gaussianities in disformally coupled inflation, a simple two-field model
that features a novel kinetic interaction. Cases with both canonical and
Dirac-Born-Infeld (DBI) kinetic terms are taken into account, the latter
motivated by the possible extra-dimensional origin of the disformality. The
computations are carried out for the equilateral configuration in the slow-roll
regime, wherein it is found that the non-Gaussianity is typically rather small
and negative. This is despite the fact that the new kinetic interaction causes
the perturbation modes to propagate with different sounds speeds, which may
both significantly deviate from unity during inflation.
