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3. Hubble Parameter H0

The Hubble parameter H0 ident 100 h km s-1 Mpc-1 remains uncertain, although no longer by the traditional factor of two. The range of h determinations has been shrinking with time (Kennicutt, Freedman, & Mould 1995). De Vaucouleurs long contended that h approx 1. Sandage has long contended that h approx 0.5, although a recent reanalysis of the Type Ia supernovae (SNe Ia) data coauthored by Sandage and Tammann concludes that the latest data are consistent with h = 0.6 ± 0.04 (Saha et al. 1999).

The Hubble parameter has been measured in two basic ways: (1) Measuring the distance to some nearby galaxies, typically by measuring the periods and luminosities of Cepheid variables in them; and then using these ``calibrator galaxies'' to set the zero point in any of the several methods of measuring the relative distances to galaxies. (2) Using fundamental physics to measure the distance to some distant object(s) directly, thereby avoiding at least some of the uncertainties of the cosmic distance ladder (Rowan-Robinson 1985). The difficulty with method (1) was that there was only a handful of calibrator galaxies close enough for Cepheids to be resolved in them. However, the HST Key Project on the Extragalactic Distance Scale has significantly increased the set of calibrator galaxies. The difficulty with method (2) is that in every case studied so far, some aspect of the observed system or the underlying physics remains somewhat uncertain. It is nevertheless remarkable that the results of several different methods of type (2) are rather similar, and indeed not very far from those of method (1). This gives reason to hope for convergence.