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

Abstract

Direct observations of the supposedly universal primordial deuterium abundance imply a relatively large baryon density $Ω_B= (0.019-0.030)h^{-2}$ (95% C.L.). On the other hand, concordance between the previously accepted $^4 He$ and $^7 Li$ abundances and standard Big Bang Nucleosynthesis requires the thrice smaller value $Ω_B=(0.005-0.010)h^{-2}$ (95% C.L.). For each $Ω_B$, 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 $Ω_{cl}$ in clusters of galaxies. The higher-$Ω_B$ values are consistent with clusters being a fair sample of the Universe, and then imply $Ω_{m} =(0.3-0.9)$, a medium or critical density Universe. Said otherwise, the observed limits $f_B > 0.1, ~ Ω_m > 0.3$ imply $Ω_B > 0.03$. If the newer$^4 He$ abundance observations are accepted, this is consistent with standard BBN.