Combined constraints on deviations of dark energy from an ideal fluid
  from Euclid and Planck
  Cosmological fluids are commonly assumed to be distributed in a spatially
homogeneous way, while their internal properties are described by a perfect
fluid. As such, they influence the Hubble-expansion through their respective
densities and equation of state parameters. The subject of this paper is an
investigation of the fluid-mechanical properties of a dark energy fluid, which
is characterised by its sound speed and its viscosity apart from its equation
of state. In particular, we compute the predicted spectra for the integrated
Sachs-Wolfe effect for our generalised fluid, and compare them with the
corresponding predictions for weak gravitational lensing and galaxy clustering,
which had been computed in previous work. We perform statistical forecasts and
show that the integrated Sachs-Wolfe signal obtained by cross correlating
Euclid galaxies with Planck temperatures, when joined to galaxy clustering and
weak lensing observations, yields a percent sensitivity on the dark energy
sound speed and viscosity. We prove that the iSW effect provides strong
degeneracy breaking for low sound speeds and large differences between the
sound speed and viscosity parameters.
