4.2. SBBN Baryon Density - The Baryon Density At 20 Minutes
The universal abundance of baryons which follows from SBBN and our adopted primordial D-abundance is: 10 = 5.6+0.6-1.2 (B h2 = 0.020+0.002-0.004). For the HST Key Project recommended value for H0 (h = 0.72 ± 0.08; Freedman et al. 2001), the fraction of the present universe critical density contributed by baryons is small, B 0.04. In Figure 10 is shown a comparison among the various determinations of the present mass/energy density (as a fraction of the critical density), baryonic as well as non-baryonic. It is clear from Figure 10 that the present universe (z 1) baryon density inferred from SBBN far exceeds that inferred from emission/absorption observations (Persic & Salucci 1992, Fukugita, Hogan & Peebles 1998). The gap between the upper bound to luminous baryons and the BBN band is the "dark baryon problem": at present, most of the baryons in the universe are dark. Evidence that although dark, the baryons are, indeed, present comes from the absorption observed in the Ly forest at redshifts z 2 - 3 (see, e.g. Weinberg et al. 1997). The gap between the BBN band and the band labelled by M is the "dark matter problem": the mass density inferred from the structure and movements of the galaxies and galaxy clusters far exceeds the SBBN baryon contribution. Most of the mass in the universe must be nonbaryonic. Finally, the gap from the top of the M band to = 1 is the "dark energy problem".
Figure 10. The various contributions to the present universal mass/energy density, as a fraction of the critical density (), as a function of the Hubble parameter (H0). The curve labelled Luminous Baryons is an estimate of the upper bound to those baryons seen at present (z 1) either in emission or absorption (see the text). The band labelled BBN represents the D-predicted SBBN baryon density. The band labelled by "M" (M = 0.3 ± 0.1) is an estimate of the current mass density in nonrelativistic particles ("Dark Matter").