The CMB is already one of the pillars of the Big Bang model. However it may also become our most powerful tool to distinguish contending models and to determine their cosmological parameters. To realize this goal, more than 20 observational groups and two new satellites are gearing up to make precise measurements of the CMB at small angular scales. In such a situation it is important to keep track of what the CMB data can already say about cosmological parameters.

Current CMB data can already be used to constrain cosmological parameters. The results are model dependent. We have obtained contraints on Hubble's constant h and the density of the Universe Ωo in the context of open and critical density CDM models with Λ = 0. In critical density models we obtain h = 0.30+0.18−0.07. This low value is inconsistent with direct measurements of h but fully consistent with four other cosmological measurements: Big Bang nucleosynthesis, cluster baryonic fraction, age constraints from globular clusters and limits on the shape parameter Γ of matter power spectra (in Ωo = 1 models). If Ωo is left as a free parameter the constraints on h are less restrictive: h = 0.40+0.57−0.14. This is fully consistent with local h measurements and the four other cosmological measurements mentioned above. The best-fit density is Ωo = 0.85 and we set an upper limit of Ωo > 0.4 at ∼ 95% CL.

At this conference Ostriker has claimed that open-CDM models with Ωo ≈ 0.3 and h ≈ 0.70 are compatible with all current data. However our new CMB data analysis rules this model out at more than ∼ 4σ.