Combined constraints on deviations of dark energy from an ideal fluid from Euclid and Planck

Understand

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.