Constraints on Flat Cosmologies with Tracking Quintessence from Cosmic Microwave Background Observations

We constrain cosmological parameters in flat cosmologies with tracking dark energy (or quintessence) using the existing data on cosmic microwave background (CMB) anisotropies. We perform a maximum likelihood analysis using combined data from COBE-DMR, BOOMERanG, DASI and MAXIMA, obtaining estimates for the dark energy density ΩQ and equation of state wQ, the physical baryon density Ωbh2, the scalar perturbation spectral index nS, the ratio R between the tensor and scalar perturbation amplitude (or the tensor spectral index nT). Dark energy is found to be the dominant cosmological component ΩQ=0.71+0.05-0.04, with an equation of state wQ=-0.82+0.14-0.11 (68% C.L.). Our best fit value of the physical baryon density is in good agreement with the primordial nucleosynthesis bound. We find no significant evidence for deviations from scale invariance, although a scalar spectral index slightly smaller than unity is marginally preferred. Finally, we find that the contribution of cosmological gravitational waves is negligible. These results confirm that quintessence is slightly preferred with respect to ordinary cosmological constant by the present CMB data.