Inflation with multiple sound speeds: a model of multiple DBI type
  actions and non-Gaussianities
  In this letter we study adiabatic and isocurvature perturbations in the frame
of inflation with multiple sound speeds involved. We suggest this scenario can
be realized by a number of generalized scalar fields with arbitrary kinetic
forms. These scalars have their own sound speeds respectively, so the
propagations of field fluctuations are individual. Specifically, we study a
model constructed by two DBI type actions. We find that the critical length
scale for the freezing of perturbations corresponds to the maximum sound
horizon. Moreover, if the mass term of one field is much lighter than that of
the other, the entropy perturbation could be quite large and so may give rise
to a growth outside sound horizon. At cubic order, we find that the
non-Gaussianity of local type is possibly large when entropy perturbations are
able to convert into curvature perturbations. We also calculate the
non-Gaussianity of equilateral type approximately.
