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35. The Hubble constant

We now discuss the impact of a Cepheid distance to the Fornax cluster in estimating the general expansion rate of the Universe. Below we present and discuss three independent estimates, where the analysis is based both on the new Fornax distance and the distances to other Key Project galaxies. At the end we intercompare the results for convergence and consistency. The first estimate is based solely on the Fornax cluster, its velocity and its Cepheid-based distance. This scrutinizes the flow sampled in one particular direction at a distance of ~ 20 Mpc. We then examine the inner volume of space, leading up to and including both the Virgo and Fornax clusters. This has the added advantage of averaging over different samples and a variety of directions, but it is still limited in volume (to an average distance of ~ 10 Mpc), and it is subject to the usual caveats concerning bulk flows and the adopted Virgocentric flow model (Table 4). The third estimate comes from using the Cepheid distance to Fornax to lock into secondary distance indicators, thereby allowing us to step out to cosmologically significant velocities (10,000 km/sec and beyond) corresponding to distances greater than 100 Mpc. Local flow uncertainties then are replaced by largescale flow uncertainties; while the systematically secure Cepheid distances are replaced by currently more controversial secondary distance indicators. This is done in order to increase volume and the sample. Averaging over the sky, and working at large redshifts, alleviates the flow problems. Examining consistency between independent the secondary distance estimates, and then averaging over their far-field estimates should provide a systematically secure value of H0 and, more importantly, a measure of its external error. Comparison of the three ``regional'' estimates (Fornax, local and far-field) then can be used to provide a check on the systematics resulting from the various assumptions made independently at each step.

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