EXTRAGALACTIC DISTANCE SCALES: it H 0 FROM HUBBLE (EDWIN) TO HUBBLE (HUBBLE TELESCOPE)

In the first fifty years after Edwin Hubble announced a linear relationship between distances and redshifts of external galaxies, the accepted value of his constant dropped by (or the Universe expanded and aged by) a factor of 5 to 10. More recently, different groups, often using nearly the same data, have passionately defended distance scales that differ by about a factor of two. The sections of this review explore (1) the history of extragalactic distance scales, (2) the relationships between the Hubble constant, H ₀, and other cosmological parameters, (3) types of distance indicators, (4) ways of measuring distances in practice, (5) values of H ₀ reported recently on the basis of these methods, (6) the continuing discrepancies between the 'long' and 'short' distance scales, and (7) prospects for future convergence on a single, global value of H, so that we can all get back to doing other things. The units of the Hubble constant are km s^-1 Mpc^-1 (or reciprocal time), and no one now strongly favors any value outside the range 40–90 km s^-1 Mpc^-1 (time scales of 11–25 Gyr).