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5.1. The Case for Omega < 1

Everyone agrees that the ``simplest'' universe would be a flat Einstein-de Sitter model. But we shall hear several claims during the present meeting that this model is now hard to reconcile with the data. Several lines of evidence suggest that gravitating CDM contributes substantially less than OmegaCDM = 1. The main lines of evidence are

(i) The baryon fraction in clusters is 0.15-0.2, On the other hand, the baryon contribution to omega is now pinned down by deuterium measurements to be around Omegab = 0.015 h2, where h is the Hubble constant in units of 100 km/sec/Mpc. If clusters are a fair sample of the universe, then this is incompatible with a dark matter density high enough to make Omega = 1.

(ii) The presence of clusters of galaxies with z = 1 is hard to reconcile with the rapid recent growth of structure that would be expected if OmegaCDM were unity.

(iii) The Supernova Hubble diagram (even though the case for actual acceleration may not be compelling) seems hard to reconcile with the large deceleration implied by an Einstein-de Sitter model.

(iv) The inferred ages of the oldest stars are only barely consistent with an Einstein-de Sitter model, for the favoured choices of Hubble constant.