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1.3.2 Hubble Parameter H0

The Hubble parameter H0 ident 100h km s-1 Mpc-1 remains uncertain, although by less than the traditional factor of two. de Vaucouleurs long contended that h approx 1. Sandage has long contended that h approx 0.5, and he and Tammann still conclude that the latest data are consistent with h = 0.55 ± 0.05 (Sandage 1995; Sandage & Tammann 1995, 1997; Tammann & Federspiel 1996). A majority of observers currently favor a value intermediate between these two extremes, and the range of recent determinations has been shrinking (Kennicutt, Freedman, & Mould 1995; Tammann et al. 1996; Freedman 1997).

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 success of the Hubble Space Telescope (HST) Cepheid measurement of the distance to M100 (Freedman et al. 1994, Ferrarese et al. 1996) shows that the HST Key Project on the Extragalactic Distance Scale can significantly increase the set of calibrator galaxies - in fact, it already has done so. Adaptive optics from the ground may also be able to contribute to this effort, although the first published result of this approach (Pierce et al. 1994) is not entirely convincing. 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.