Combining the baryon budget with CMBR measurements

Measurements of the Cosmic Microwave Background Radiation (CMBR) provide a powerful tool for measuring the primary cosmological parameters. However, there is a large degree of parameter degeneracy in simultaneous measurements of the matter density, Ωm, and the Hubble parameter, H0. In the present paper we use the presently available CMBR data together with measurements of the cosmological baryon-tophoton ratio, η, from Big Bang nucleosynthesis, and the relative mass fraction of baryons in clusters to break the parameter degeneracy in measuring Ωm and H0. We find that present data is inconsistent with the standard Ω = 1, matter dominated model. Our analysis favours a medium density universe with a rather low Hubble parameter. This is compatible with new measurements of type Ia supernovae, and the joint estimate of the two parameters is Ωm = 0.45 +0.07 −0.07 andH0 = 39 +14 −13 km s Mpc. We stress that the upper bound on the Hubble parameter is likely to be much more uncertain than indicated here, because of the limited number of free parameters in our analysis.