Fingerprinting Dark Energy III: distinctive marks of viscosity
  The characterisation of dark energy is one of the primary goals in cosmology
especially now that many new experiments are being planned with the aim of
reaching a high sensitivity on cosmological parameters. It is known that if we
move away from the simple cosmological constant model then we need to consider
perturbations in the dark energy fluid. This means that dark energy has two
extra degrees of freedom: the sound speed $\cs$ and the anisotropic stress
$\sigma$. If dark energy is inhomogenous at the scales of interest then the
gravitational potentials are modified and the evolution of the dark matter
perturbations is also directly affected. In this paper we add an anisotropic
component to the dark energy perturbations. Following the idea introduced in
\cite{Sapone:2009mb}, we solve analytically the equations of perturbations in
the dark sector, finding simple and accurate approximated solutions. We also
find that the evolution of the density perturbations is governed by an
effective sound speed which depends on both the sound speed and the anisotropic
stress parameter. We then use these solutions to look at the impact of the dark
energy perturbations on the matter power spectrum and on the Integrated
Sachs-Wolfe effect in the Cosmic Microwave Background.
