Precision primordial 4He measurement from the CMB -: art. no. 103503

Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) are two major pillars of cosmology. Standard BBN accurately predicts the primordial light element abundances (He-4, D, He-3 and Li-7), depending on one parameter, the baryon density. Light element observations are used as a baryometer. The CMB anisotropies also contain information about the content of the Universe which allows an important consistency check on the big bang model. In addition CMB observations now have sufficient accuracy to not only determine the total baryon density, but also resolve its principal constituents H and He-4. We present a global analysis of all recent CMB data, with special emphasis on the concordance with BBN theory and light element observations. We find Omega(B)h(2)=0.0250(-0.0026)(+0.0019) and Y-p=0.250(-0.014)(+0.010) (fraction of baryon mass as He-4) using CMB data alone, in agreement with He-4 abundance observations. The determination of Y-p allows us to constrain the relativistic degrees of freedom during BBN, measured through the effective number of light neutrino species, N-nu,N-eff=3.02(-0.79)(+0.85), in accord with the standard model of particle physics. With this concordance established we show that the inclusion of standard, N(nu,eff)equivalent to3, BBN theory priors significantly reduces the volume of parameter space. In this case, we find Omega(B)h(2)=0.0245(-0.0028)(+0.0015) and Y-p=0.2493(-0.0010)(+0.0007). We also find that the inclusion of deuterium abundance observations reduces the Y-p and Omega(B)h(2) ranges by a factor of similar to2. Further light element observations and CMB anisotropy experiments will refine this concordance and sharpen BBN and the CMB as tools for precision cosmology.