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4.4. The Baryon Density At 10 Gyr

Although the majority of baryons in the recent/present universe are dark, it is still possible to constrain the baryon density indirectly using observational data (see, e.g. Steigman, Hata & Felten 1999, Steigman, Walker & Zentner 2000; Steigman 2001). The magnitude-redshift relation determined by observations of type Ia supernovae (SNIa) constrain the relation between the present matter density (OmegaM) and that in a cosmological constant (OmegaLambda). The allowed region in the OmegaLambda - OmegaM plane derived from the observations of Perlmutter et al. (1997), Schmidt et al. (1998), and Perlmutter et al. (1999) are shown in Figure 11.

Figure 11

Figure 11. The 68% (solid) and 95% (dotted) contours in the OmegaLambda - OmegaM plane consistent with the SNIa data (see the text). Geometrically flat models lie along the line labelled k = 0.

If, in addition, it is assumed that the universe is flat (kappa = 0; an assumption supported by the CMB data), a reasonably accurate determination of OmegaM results: OmegaM(SNIa; Flat) = 0.28+0.08-0.07 (Steigman, Walker & Zentner 2000; Steigman 2001). But, how to go from the matter density to the baryon density? For this we utilize rich clusters of galaxies, the largest collapsed objects, which provide an ideal probe of the baryon fraction in the present universe fB. X-ray observations of the hot gas in clusters, when corrected for the baryons in stars (albeit not for any dark cluster baryons), can be used to estimate fB. Using the Grego et al. (2001) observations of the Sunyaev-Zeldovich effect in clusters, Steigman, Kneller & Zentner (2002) estimate fB and derive a present-universe (t0 approx 10 Gyr; z ltapprox 1) baryon density: eta10 = 5.1+1.8-1.4 (OmegaB h2 = 0.019+0.007-0.005).

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