Constraining Dark Energy Anisotropic Stress

TL;DR

This paper explores how current and future cosmological observations can constrain the imperfect properties of dark energy, such as viscosity and sound speed, but finds these parameters are difficult to measure precisely.

Contribution

It introduces a general parameterization of dark energy including viscosity and sound speed and assesses the observational prospects for constraining these properties.

Findings

Current data poorly constrain viscosity and sound speed.

Future CMB data will likely not significantly improve constraints.

A perfect fluid model may suffice for observational descriptions of dark energy.

Abstract

We investigate the possibility of using cosmological observations to probe and constrain an imperfect dark energy fluid. We consider a general parameterization of the dark energy component accounting for an equation of state, speed of sound and viscosity. We use present and future data from the cosmic microwave background radiation (CMB), large scale structures and supernovae type Ia. We find that both the speed of sound and viscosity parameters are difficult to nail down with the present cosmological data. Also, we argue that it will be hard to improve the constraints significantly with future CMB data sets. The implication is that a perfect fluid description might ultimately turn out to be a phenomenologically sufficient description of all the observational consequences of dark energy. The fundamental lesson is however that even then one cannot exclude, by appealing to observational evidence alone, the possibility of imperfectness in dark energy.