Parameterizing dark sector perturbations via equations of state
  The evolution of perturbations is a crucial part of the phenomenology of the
dark sector cosmology. We advocate parameterizing these perturbations using
equations of state for the entropy perturbation and the anisotropic stress. For
small perturbations, these equations of state will be linear in the density,
velocity and metric perturbations, and in principle these can be related back
to the field content of the underlying model allowing for confrontation with
observations. We illustrate our point by constructing gauge invariant entropy
perturbations for theories where the dark sector Lagrangian is a general
function of a scalar field, its first and second derivatives, and the metric
and its first derivative, ${\cal L}={\cal
L}(\phi,\partial_\mu\phi,\partial_\mu\partial_\nu\phi,g_{\mu\nu},\partial_{\alpha}g_{\mu\nu})$.
As an example, we show how to apply this approach to the case of models of
Kinetic Gravity Braiding.
