#### March 14, 14:15 in A315

#### Domenico Sapone (IFT/UAM Madrid, Spain)

#### Fingerprinting Dark Energy III: distinctive marks of viscosity

The characterization of dark energy is one of the primary goals in
cosmology especially in view of future experiments that are planned to reach an high sensitivity on
cosmological parameters. It is known that if we move away from the 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 (apart from the equation of state parameter): the sound speed and the anisotropic stress.
Here we solve analytically the equations of perturbations in the dark energy sector finding simple and accurate approximated solutions.
We also find that the evolution of the density perturbations are governed by an {\em 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.