Cosmological Constraints from Archeops

We analyze the cosmological constraints that Archeops (Benoît et al. \cite{archeops_cl}) places on adiabatic cold dark matter models with passive power-law initial fluctuations. Because its angular power spectrum has small bins in ℓ and large ℓ coverage down to COBE scales, Archeops provides a precise determination of the first acoustic peak in terms of position at multipole lpeak=220+/- 6, height and width. An analysis of Archeops data in combination with other CMB datasets constrains the baryon content of the Universe, Omegabh 2 = 0.022+0.003-0.004, compatible with Big-Bang nucleosynthesis and with a similar accuracy. Using cosmological priors obtained from recent non-CMB data leads to yet tighter constraints on the total density, e.g. Omegatot =1.00+0.03-0.02 using the HST determination of the Hubble constant. An excellent absolute calibration consistency is found between Archeops and other CMB experiments, as well as with the previously quoted best fit model. The spectral index n is measured to be 1.04+0.10-0.12 when the optical depth to reionization, tau , is allowed to vary as a free parameter, and 0.96+0.03-0.04 when tau is fixed to zero, both in good agreement with inflation.