We present cosmological parameter constraints on flat cosmologies dominated by dark energy using various cosmological data including the recent Archeops angular power spectrum measurements .
A likelihood analysis of the existing Cosmic Microwave Background data shows that the presence of dark energy is not requested , in the absence of further prior .
This comes from the fact that there exist degeneracies among the various cosmological parameters constrained by the Cosmic Microwave Background .
We found that there is a degeneracy in a combination of the Hubble parameter H _ { 0 } and of the dark energy equation of state parameter w _ { \scriptscriptstyle Q } , but that w _ { \scriptscriptstyle Q } is not correlated with the primordial index n of scalar fluctuations and the baryon content \Omega _ { b } h ^ { 2 } .
Preferred primordial index is n = 0.95 \pm 0.05 ( 68 \% ) and baryon content \Omega _ { b } h ^ { 2 } = 0.021 \pm 0.003 .
Adding constraint on the amplitude of matter fluctuations on small scales , \sigma _ { 8 } , obtained from clusters abundance or weak lensing data may allow to break the degenaracies , although present-day systematics uncertainties do not allow firm conclusions yet .
The further addition of the Hubble Space Telescope measurements of the local distance scale and of the high redshift supernovae data allows to obtain tight constraints .
When these constraints are combined together we find that the amount of dark energy is 0.7 ^ { +0.10 } _ { -0.07 } ( 95 \% C.L . )
and that its equation of state is very close to those of the vacuum : w _ { \scriptscriptstyle Q } < -0.75 ( > 95 \% C.L . ) .
In no case do we find that quintessence is prefered over the classical cosmological constant , although robust data on \sigma _ { 8 } might rapidly bring light on this important issue .