Karl Schwarzschild Lecture The Ups and Downs of the Hubble Constant

A brief history of the determination of the Hubble constant H0 is given. Early attempts following Lemaître (1927) gave much too high values due to errors of the magnitude scale, Malmquist bias and calibration problems. By 1962 most authors agreed that 75 ∼< H0 ∼< 130. After 1975 a dichotomy arose with values near 100 and others around 55. The former came from apparent-magnitude-limited samples and were affected by Malmquist bias. New distance indicators were introduced; they were sometimes claimed to yield high values of H0, but the most recent data lead to H0 in the 60’s, yet with remaining difficulties as to the zero-point of the respective distance indicators. SNe Ia with their large range and very small luminosity dispersion (avoiding Malmquist bias) offer a unique opportunity to determine the large-scale value of H0. Their maximum luminosity can be well calibrated from 10 SNe Ia in local parent galaxies whose Cepheids have been observed with HST. An unforeseen difficulty – affecting all Cepheid distances – is that their P-L relation varies from galaxy to galaxy, pre-sumably in function of metallicity. A proposed solution is summarized here. The conclusion is that H0 = 62.3 ± 1.3 (random) ±5.3 (systematic) on all scales. The expansion age becomes then (with Ωm = 0.3, ΩΛ = 0.7) 15.1 Gyr.