Cluster Probes of Dark Energy Clustering

TL;DR

This paper investigates how different properties of early dark energy, especially sound speed, affect cluster abundances and their potential to distinguish dark energy models when combined with CMB data.

Contribution

It introduces the role of sound speed in early dark energy models and demonstrates how combining cluster surveys with CMB data can constrain dark energy properties.

Findings

Cold early dark energy with low sound speed can be detected via cluster abundances.

Combining cluster data with CMB spectra constrains early dark energy fraction to 0.3%.

Projected constraints can distinguish sound speeds of 0.1 and 1 at 99% confidence.

Abstract

Cluster abundances are oddly insensitive to canonical early dark energy. Early dark energy with sound speed equal to the speed of light cannot be distinguished from a quintessence model with the equivalent expansion history for $z<2$ but negligible early dark energy density, despite the different early growth rate. However, cold early dark energy, with a sound speed much smaller than the speed of light, can give a detectable signature. Combining cluster abundances with cosmic microwave background power spectra can determine the early dark energy fraction to 0.3 % and distinguish a true sound speed of 0.1 from 1 at 99 % confidence. We project constraints on early dark energy from the Euclid cluster survey, as well as the Dark Energy Survey, using both current and projected Planck CMB data, and assess the impact of cluster mass systematics. We also quantify the importance of dark energy perturbations, and the role of sound speed during a crossing of $w=-1$.