TYPE Ia SUPERNOVAE AND THE HUBBLE CONSTANT

The focus of this review is the work that has been done during the 1990s on using Type Ia supernovae (SNe Ia) to measure the Hubble constant (H0). SNe Ia are well suited for measuring H0. A straightforward maximum-light color criterion can weed out the minority of observed events that are either intrinsically subluminous or substantially extinguished by dust, leaving a majority subsample that has observational absolute-magnitude dispersions of less than σobs (MB) ≃ σobs (MV) ≃ 0.3 mag. Correlations between absolute magnitude and one or more distance-independent SN Ia or parent-galaxy observables can be used to further standardize the absolute magnitudes to better than 0.2 mag. The absolute magnitudes can be calibrated in two independent ways: empirically, using Cepheid-based distances to parent galaxies of SNe Ia, and physically, by light curve and spectrum fitting. At present the empirical and physical calibrations are in agreement at MB ≃ MV ≃ −19.4 or −19.5. Various ways that have been used to match Cepheid-calibrated SNe Ia or physical models to SNe Ia that have been observed out in the Hubble flow have given values of H0 distributed throughout the range of 54–67 km s⁻¹ Mpc⁻¹. Astronomers who want a consensus value of H0 from SNe Ia with conservative errors could, for now, use 60 ± 10 km s⁻¹ Mpc⁻¹.