A gauge-invariant approach to interactions in the dark sector
  We outline a gauge-invariant framework to calculate cosmological
perturbations in dark energy models consisting of a scalar field interacting
with dark matter via energy and momentum exchanges. Focusing on three
well-known models of quintessence and three common types of dark-sector
interactions, we calculate the matter and dark energy power spectra as well as
the Integrated Sachs-Wolfe (ISW) effect in these models. We show how the
presence of dark-sector interactions can produce a large-scale enhancement in
the matter power spectrum and a boost in the low multipoles of the cosmic
microwave background anisotropies. Nevertheless, we find these enhancements to
be much more subtle than those found by previous authors who model dark energy
using simple ansatz for the equation of state. We also address issues of
instabilities and emphasise the importance of momentum exchanges in the dark
sector.
