  We discuss the general framework for a perfect continuum medium in cosmology
and show that an interesting generalization of the fluids normally used is for
the medium to have rigidity and, hence, be analogous to an elastic solid. Such
models can provide perfect, adiabatic fluids which are stable even when the
pressure is negative, if the rigidity is sufficiently large, making them
natural candidates to describe the dark energy. In fact, if the medium is
adiabatic and isotropic, they provide the most general description of
linearized perturbations. We derive the equations of motion and wave
propagation speeds in the isotropic case. We point out that anisotropic models
can also be incorporated within the same formalism and that they are classified
by the standard Bravais Lattices. We identify the adiabatic and isocurvature
modes allowed in both the scalar and vector sectors and discuss the predictions
they make for CMB and matter power spectra. We comment on the relationship
between these models and other fluid-based approaches to dark energy, and
discuss a possible microphysical manifestation of this class of models as a
continuum description of defect-dominated scenarios.
