Description
We study a class of models where inflation is driven by a self-gravitating
adiabatic medium.
Such class of media admits an effective description in terms of four
scalar fields related to the Goldstone bosons for the spontaneous
breaking of spacetime translations. Depending on the symmetry of the
effective action of the medium, one can have perfect barotropic or
non-barotropic fluids and solids.
In particular, we reconsider and extend the previous results on fluid
and solid inflation, studying in detail the effect of entropic
perturbation, which can be present even if the medium is adiabatic, and
the violation of the Weinberg theorem which manifests with the growing
of curvature perturbation ${\cal R }$ at superhorizon scales and its
offset with the curvature of uniform density hypersurfaces $\zeta$. Such
offset makes essential the study of reheating showing that the $\zeta$
is typically continuous during such a phase.
The presence of entropic perturbation in a adiabatic medium can boost
the ratio of scalar to tensor ratio, making such class of models rather
interesting from a phenomenological point of view.
