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

Direct observations of the supposedly universal primordial deuterium abundance imply a relatively large baryon density Ω B = (0.019-0.030) h -2 = (0.029-0.10) (95% confidence limit, hereafter CL) for Hubble constant H 0 ≡ 100 h = (70 ± 15) km -1 s -1 Mpc -1 . From new observations of both the X-ray bremsstrahlung and the Sunyaev-Zeldovich effect, we obtain the baryon fraction in rich clusters, f B = (0.13 ± 0.03)(62/ H 0 ) 1.4 . Together with the observed baryon density this implies that rich clusters are a fair sample of the universe (baryon enhancement ϒ ~ 1) and that the cosmic total mass density Ω m = (0.3-0.9) (95% CL). This conclusion agrees with dynamical measures of Ω m on large scales. Conversely, this dynamically measured Ω m > 0.3 (2.4 σ) and the rich cluster limit f B > 0.1 imply Ω B > 0.025 for h < 0.7, ϒ ~ 1. This relatively large baryon density is consistent with standard big bang nucleosynthesis and some recent 4 He abundance observations.