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

TL;DR

This paper investigates the properties of dark energy as a fluid with sound speed and viscosity, using combined Euclid and Planck data to constrain deviations from an ideal fluid and improve understanding of dark energy's nature.

Contribution

It introduces a generalized dark energy fluid model including sound speed and viscosity, and demonstrates how combined cosmological observations can tightly constrain these properties.

Findings

Percent-level sensitivity on dark energy sound speed and viscosity.

ISW effect helps break degeneracies between sound speed and viscosity.

Combined data improves constraints on dark energy fluid properties.

Abstract

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.