H_0: The Incredible Shrinking Constant, 1925-1975

The story of the Hubble constant logically begins just wehere the Curtis-Shapley debate on the distance scale of the universe ended, with Hubble's discovery of Cepheid variables in several nebulae that we now recognize as galaxies within the Local Group, which settled the issue of the existence of external galaxies. Hubble's own values of H was in the range of 500-550 km/sec/Mpc. The "best buy" value shrank in several large steps beginning in 1952, each being predicated on the recognition of some fundamental mistake in the previous distance scale calibrations. But it shrank more for some workers than for others, and by 1973 there was a clear polarization between a "long" recognition that general relativity permits, indeed nearly requires, an expanding universe; the gradual elimination of alternative explanations of redshift-distance relations; and the repelling of a late assault in the form of steady state cosmology, within whose framework H_0 is a well-defined, never-varying number of only moderate importance. (SECTION: A Debate on The Scale of the Universe)