The calibration of the extragalactic distance scale: Methods and problems

Measuring distances in the expanding universe by the classical method of 'standard candles' creates a calibration problem: what is the power of the standard candles, and how standard are they? Hubble's law has recession velocity proportional to distance from the observer, v = H₀d; but what is the value of H₀, the Hubble constant? Ten years after the launch of the Hubble Space Telescope (HST), which has made a major impact on this problem, we review progress. The HST Key Project on the extragalactic distance scale has provided a number of calibration points, galaxies at characteristic distances of 10 Mpc. HST has resolved the primary standard candles, Cepheid variable stars, beyond the reach of ground based telescopes. These galaxy 'survey markers' in turn calibrate a number of more penetrating distance indicators which reach out an order of magnitude further, sufficiently far that noise in the Hubble flow δv << v in a frame at rest with respect to the microwave background radiation. Supernovae, galaxy scaling relations, and the properties of their stellar populations are among these secondary distance indicators. The Key Project finds that the mean over four different measurement techniques is H₀ = 68 ± 7 km s^-1 Mpc^-1. The use of different measurement techniques identifies the degree of systematic error in the calibration, although some uncertainties, for example the distance of the Large Magellanic Cloud, affect all these techniques systematically. More accurate measurements are possible, by this methodology and by stand-alone experiments, through interferometry, extended and improved HST instruments, and through observations at a variety of regions in the electromagnetic spectrum.