Cluster abundances are oddly insensitive to canonical early dark energy .
Early dark energy with sound speed equal to the speed of light can not 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 .