Can dark energy viscosity be detected with the Euclid survey?

TL;DR

This paper investigates whether the Euclid survey can detect dark energy viscosity by analyzing its impact on cosmological perturbations, weak lensing, and galaxy power spectra, and forecasts the survey's sensitivity to these properties.

Contribution

It provides an analytical study of how dark energy viscosity influences observable cosmological signals and forecasts Euclid's ability to constrain viscosity and sound speed parameters.

Findings

Euclid can constrain models with very small viscosity and sound speed.

Constraints on models with high sound speed require additional observables.

Euclid alone has limited sensitivity to dark energy viscosity.

Abstract

Recent work has demonstrated that it is important to constrain the dynamics of cosmological perturbations, in addition to the evolution of the background, if we want to distinguish among different models of the dark sector. Especially the anisotropic stress of the (possibly effective) dark energy fluid has been shown to be an important discriminator between modified gravity and dark energy models. In this paper we use approximate analytical solutions of the perturbation equations in the presence of viscosity to study how the anisotropic stress affects the weak lensing and galaxy power spectrum. We then forecast how sensitive the photometric and spectroscopic Euclid surveys will be to both the speed of sound and the viscosity of our effective dark energy fluid when using weak lensing tomography and the galaxy power spectrum. We find that Euclid alone can only constrain models with very small speed of sound and viscosity, while it will need the help of other observables in order to give interesting constraints on models with a sound speed close to one. This conclusion is also supported by the expected Bayes factor between models.