Annu. Rev. Astron. Astrophys. 1998. 36: 17-55 Copyright © 1998 by Annual Reviews. All rights reserved

1. INTRODUCTION

One way to illustrate the rapid progress in using Type Ia supernovae (SNe Ia) for cosmology is to compare the situation now with that of six years ago when the article "Type Ia Supernovae as Standard Candles" (Branch & Tammann 1992) appeared in this series of reviews. At that time, optimism was expressed about checking that light curves of SNe Ia are time-dilated, as they should be if the universe really is expanding; about using SNe Ia to measure galaxy peculiar velocities; and about using them to determine the cosmic deceleration - but hardly any significant results on these matters were at hand. Now, thanks to the exertions of supernova (SN) observers, the time dilation has been established (Goldhaber et al 1997, Leibundgut et al 1996, Riess et al 1997b); peculiar motions are beginning to be estimated (Riess et al 1995b, 1997a, Hamuy et al 1996b); and above all, estimates of the matter-density parameter _m and the cosmological-constant contribution are beginning to be made (Perlmutter et al 1997a, 1998, Garnavich et al 1998). All of this work entails using SNe Ia as precise indicators of relative distances in a purely empirical way. Dramatic progress is being made on these matters, but at this time a review article would be premature.

A competitive measurement of the Hubble constant (H₀), on the other hand, requires absolute distances but less precision. Traditionally, those who have used SNe Ia to estimate H₀ have obtained values that have been, in the context of the longstanding distance-scale controversy, low values. For example, before Cepheid variables in any SN Ia parent galaxy had been discovered Branch & Tammann (1992) offered H₀ = 57 ± 7 km s^-1 Mpc^-1. (The units of H₀ will not be repeated.) Within the last six years, so much has been said and done about obtaining the value of H₀ from SNe Ia that this alone is the topic of this review.

Most of the literature citations are from the 1990s. Many earlier ones that are now mainly of historical interest can be found in the review by Branch & Tammann (1992). For a recent comprehensive collection of articles on practically all aspects of SN Ia research, see Thermonuclear Supernovae, edited by Ruiz-Lapuente et al (1997b), and for a recent collection of articles on various ways to estimate H₀, see The Extragalactic Distance Scale, edited by Livio et al (1997).

Sections 2 and 3 focus on empirical matters, with the former devoted to the observational properties of SNe Ia and the latter to the determination of H₀ by means of Cepheid-based calibrations of SN Ia absolute magnitudes. Then the physical properties of SNe Ia are discussed in Section 4, and the determination of H₀ by physical methods is the subject of Section 5. Section 6 states the conclusion.