Type Ia Supernovae as Extragalactic Distance Indicators

Because Type Ia supernovae (SNe Ia) are not perfect standard candles, it is important to be able to use distance–independent observables (DIOs) to define subsets of SNe Ia that are " nearly standard candles " or to correct SN Ia absolute magnitudes to make them nearly homogeneous (" standardized candles "). This is not crucial for the measurement of H 0 , but it is for the measurement of q 0 and of parent–galaxy peculiar velocities. We discuss the use of various photometric and spectroscopic SN Ia DIOs, and a parent–galaxy DIO, for this purpose. We also discuss the status of the absolute–magnitude calibration of SNe Ia. We find that SNe Ia, whether calibrated by means of (1) Cepheids in their parent galaxies, (2) fitting their optical–ultraviolet spectra with detailed non-LTE model atmosphere calculations, or (3) by considering that the light curve is powered by the decay of radioactive 56 Ni, firmly indicate that the value of H 0 is low, less than or about 60 km s −1 Mpc −1. Some issues regarding the determination of q 0 by means of SNe Ia are discussed briefly. Finally, we conjecture that even if q 0 =0.5, there probably is no cosmic age problem.