Physical Review Letters
Quintessence, a time-varying energy component that may account for the accelerated expansion of the universe, can be characterized by its equation of state and sound speed. In this paper, we show that if the quintessence density is at least 1% of the critical density at the surface of last scattering the cosmic microwave background anisotropy can distinguish between models whose sound speed is near the speed of light versus near zero, which could be useful in distinguishing competing candidates for dark energy.
Erickson, Joel K.; Caldwell, R. R.; Steinhardt, Paul J.; Armendariz-Picon, C.; and Mukhanov, V., "Measuring the Speed of Sound of Quintessence" (2002). Open Dartmouth: Faculty Open Access Scholarship. 2911.