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In standard Big Bang cosmology, the universe expands uniformly; and locally, according to the Hubble law, v = H0 d, where v is the recession velocity of a galaxy at a distance d, and H0 is the Hubble constant, the expansion rate at the current epoch. More than seven decades have now passed since Hubble (1929) initially published the correlation between the distances to galaxies and their recession velocities, thereby providing evidence for the expansion of the universe. But pinning down an accurate value for the Hubble constant has proved extremely challenging. There are many reasons for this difficulty, but primary among them is the basic difficulty of establishing accurate distances over cosmologically significant scales.

The Hubble constant enters in a practical way into numerous cosmological and astrophysical calculations. H0-1 sets the age of the universe, t0, and the size of the observable universe, Robs = ct0, given a knowledge of the total energy density of the universe. The square of the Hubble constant relates the total energy density of the universe to its geometry (Kolb & Turner 1990; Peacock 1999). In addition, the Hubble constant defines the critical density of the universe, rhocrit = 3 H2 / 8 pi G. The critical density further specifies the epoch in the universe at which the density of matter and radiation were equal, so that the growth of structure in the universe is also dependent on the expansion rate. The determination of many physical properties of galaxies and quasars (e.g., mass, luminosity, energy density) all require knowledge of the Hubble constant, as does the proportion of primordial light elements (H, D, 3He, 4He and Li) synthesized in the first few minutes after the Big Bang.

Measuring an accurate value of H0 was one of the motivating reasons for building the NASA/ESA Hubble Space Telescope (HST). Thus, in the mid-1980's, measurement of H0 with the goal of 10% accuracy was designated as one of three "Key Projects" of the HST, and teams from the astronomical community were encouraged to propose to undertake these initiatives (16) A team headed by the late Dr. Marc Aaronson began preparing our proposal in 1984; following peer review (subsequent to the Challenger explosion in 1986), our group was awarded the Key Project on the Extragalactic Distance Scale in 1986. Very sadly, Marc met a tragic and untimely death in 1987. We began our initial observations of the closest galaxies in our sample in 1991, shortly after the launch of HST, but most of the project was carried out after the refurbishment mission (in December 1993) when a new camera with optics that corrected for the spherical aberration of the primary mirror was installed.

The overall goal of the H0 Key Project (hereafter, Key Project) was to measure H0 based on a Cepheid calibration of a number of independent, secondary distance determination methods. Given the history of systematic errors dominating the accuracy of distance measurements, the approach we adopted was to avoid relying on a single method alone, and instead to average over the systematics by calibrating and using a number of different methods. Determining H0 accurately requires the measurement of distances far enough away that both the small and large-scale motions of galaxies become small compared to the overall Hubble expansion. To extend the distance scale beyond the range of the Cepheids, a number of methods that provide relative distances were chosen. We have used the HST Cepheid distances to provide an absolute distance scale for these otherwise independent methods, including the Type Ia supernovae, the Tully-Fisher relation, the fundamental plane for elliptical galaxies, surface-brightness fluctuations, and Type II supernovae.

The previous 29 papers in this series have provided the distances to individual galaxies based on the discovery and measurement of Cepheids, discussed the calibration of the data, presented interim results on the Hubble constant, and provided the calibration of secondary methods, and their individual determinations of the Hubble constant. A recent paper by Mould et al. (2000a) combines the results for secondary methods (Gibson et al. 2000; Ferrarese et al. 2000a; Kelson et al. 2000; Sakai et al. 2000) with a weighting scheme based on numerical simulations of the uncertainties. In this paper, we present the final, combined results of the Key Project. This analysis benefits from significant recent refinements and improvements to the Cepheid period-luminosity relation, as well as the HST WFPC2 photometric scale, and puts all of the data for the Key Project and other efforts onto a new common zero point. Establishing plausible limits for the Hubble constant requires a careful investigation of systematic errors. We explicitly note where current limits in accuracy have been reached. We intend this paper to provide an assessment of the status of the global value of H0.

In this paper, we summarize our method and determination of Cepheid distances in Section 2 and Section 3. In Section 4 and Section 5, we apply a correction for the nearby flow field and compare the value of H0 obtained locally with that determined at greater distances. Secondary methods, and the determination of H0 on large scales are discussed in Section 6 and Section 7. The remaining sources of uncertainty in the extragalactic distance scale and determination of H0 are discussed in Section 8. In Section 9 we compare our results to methods that can be applied directly at high redshifts, specifically the Sunyaev-Zel'dovich and gravitational lensing techniques. In Section 10, we give the implications of these results for cosmology.

16 The other two Key Projects selected were Quasar Absorption Lines, and the Medium-Deep Survey. Back.

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