Baryonic Mass Fraction in Rich Clusters and the Total Mass Density in the Cosmos

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Abstract
Direct observations of the supposedly universal primordial deuterium abundance imply a relatively large baryon density Omega_B= (0.019-0.030) h**(-2) (95% C.L.). On the other hand, concordance between the accepted 4 He and 7 Li abundances and standard Big Bang Nucleosynthesis requires the thrice smaller value Omega_B=(0.005-0.010) h**(-2) (95% C.L.). This lower-Omega_B may already be excluded by present baryon inventories showing Omega_B geq (0.015) (95% C.L.) for h=0.7 For each Omega_B, we use X-ray and Sunyaev-Zeldovich observations of the baryon fraction f_B in rich clusters of galaxies, in order to obtain limits on the total mass density Omega_cl in clusters of galaxies. The higher-Omega_B values are consistent with clusters being a fair sample of the Universe, and then imply Omega_m =(0.3-0.9), a medium or critical density Universe. The lower-Omega_B value would require unreasonable baryon enhancement in clusters. Said otherwise, the observed limits f_B > 0.12, Omega_m > 0.3 imply Omega_B > 0.04, which is inconsistent with standard BBN, unless the 4 He abundance has been seriously underestimated or the tau-neutrino is massive and unstable.

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