Inhomogeneous and interacting vacuum energy
  Vacuum energy is a simple model for dark energy driving an accelerated
expansion of the universe. If the vacuum energy is inhomogeneous in spacetime
then it must be interacting. We present the general equations for a
spacetime-dependent vacuum energy in cosmology, including inhomogeneous
perturbations. We show how any dark energy cosmology can be described by an
interacting vacuum+matter. Different models for the interaction can lead to
different behaviour (e.g., sound speed for dark energy perturbations) and hence
could be distinguished by cosmological observations. As an example we present
the cosmic microwave microwave background anisotropies and the matter power
spectrum for two different versions of a generalised Chaplygin gas cosmology.
