TYPE Ia SUPERNOVAE AND THE HUBBLE CONSTANT

▪ Abstract 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 (H 0 ). SNe Ia are well suited for measuring H 0 . 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 σ obs (M B ) ≃ σ obs (M V ) ≃ 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 M B ≃ M V ≃ −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 H 0 distributed throughout the range of 54–67 km s −1 Mpc −1 . Astronomers who want a consensus value of H 0 from SNe Ia with conservative errors could, for now, use 60 ± 10 km s −1 Mpc −1 .