The Hubble Constant: A Summary of the Hubble Space Telescope Program for the Luminosity Calibration of Type Ia Supernovae by Means of Cepheids

This is the final summary paper of our 15 yr program using the Hubble Space Telescope (HST) to determine the Hubble constant from the Cepheid-calibrated luminosity of Type Ia supernovae. Several recent developments have made it necessary to put the summary on H₀ on a broader basis than we originally thought (see the four papers cited in the text). The new Cepheid distances of the subset of 10 galaxies, which were hosts of normal SNe Ia, give weighted mean luminosities in B, V, and I at maximum light of -19.49, -19.46, and -19.22, respectively. These calibrate the adopted SNe Ia Hubble diagram from Paper III to give H₀ = 62.3 ± 1.3(random) ± 5.0(systematic) in units of km s^-1 Mpc^-1. This is a global value because it uses the Hubble diagram between redshift limits of 3000 and 20,000 km s^-1, beyond the effects of any local random and streaming motions. Local values of H₀ between 4.4 and 30 Mpc from Cepheids, SNe Ia, 21 cm line widths, and the tip of the red giant branch (TRGB) all agree within 5% of our global value. This agreement of H₀ on all scales from ~4 to 200 Mpc finds its most obvious explanation in the smoothing effect of vacuum energy on the otherwise lumpy gravitational field due to the nonuniform distribution of the local galaxies. The present value of H₀ is consistent with physical methods relying on the time delay of gravitational lenses and the Sunyaev-Zeldovich effect, and it is not in contradiction with the CMB data. Our value of H₀ is 14% smaller than the value of H₀ found by Freedman et al. because our independent Cepheid distances to the six SNe Ia-calibrating galaxies used in that analysis average 0.35 mag larger than those used earlier.