Measuring the Speed of Dark: Detecting Dark Energy Perturbations

TL;DR

This paper explores how the microphysics of dark energy, especially its sound speed, influences cosmic structures and how current and future data can constrain these properties, revealing potential early dark energy effects.

Contribution

It introduces methods to probe dark energy microphysics via perturbation sound speed and analyzes current data constraints, highlighting prospects for future measurements.

Findings

Current data cannot tightly constrain dark energy sound speed.

Best fit models suggest possible early dark energy and non-quintessence degrees of freedom.

Future observations could detect spatial and temporal variations in dark energy.

Abstract

The nature of dark energy can be probed not only through its equation of state, but also through its microphysics, characterized by the sound speed of perturbations to the dark energy density and pressure. As the sound speed drops below the speed of light, dark energy inhomogeneities increase, affecting both CMB and matter power spectra. We show that current data can put no significant constraints on the value of the sound speed when dark energy is purely a recent phenomenon, but can begin to show more interesting results for early dark energy models. For example, the best fit model for current data has a slight preference for dynamics (w(a)\ne-1), degrees of freedom distinct from quintessence (c_s\ne1), and early presence of dark energy (Omega_ de(a<<1)\ne0). Future data may open a new window on dark energy by measuring its spatial as well as time variation.