Fluids, Superfluids and Supersolids: Dynamics and Cosmology of Self
  Gravitating Media
  We compute cosmological perturbations for a generic self-gravitating media
described by four derivatively- coupled scalar fields. Depending on the
internal symmetries of the action for the scalar fields, one can describe
perfect fluids, superfluids, solids and supersolids media. Symmetries dictate
both dynamical and thermodynamical properties of the media. Generically, scalar
perturbations include, besides the gravitational potential, an additional
non-adiabatic mode associated with the entropy per particle {\sigma}. While
perfect fluids and solids are adiabatic with {\sigma} constant in time,
superfluids and supersolids feature a non-trivial dynamics for {\sigma}.
Special classes of isentropic media with zero {\sigma} can also be found.
Tensor modes become massive for solids and supersolids. Such an effective
approach can be used to give a very general and symmetry driven modelling of
the dark sector.
