Annu. Rev. Astron. Astrophys. 1992. 30: 359-89 Copyright © 1992 by Annual Reviews. All rights reserved

4. PROSPECTS FOR COSMOLOGY

With an absolute-magnitude scatter of no more than 0.25 mag, SNe Ia can be expected to play an increasingly important role in cosmology. A shadow may be cast upon them because of the need to apply an extinction correction that is unusual and not understood (Section 2.5), but this is not a fundamental problem because their average extinction is apparently quite modest, and even without extinction corrections their scatter is less than 0.3 mag. Moreover, at large distances the highly extinguished SNe Ia will be discriminated against observationally. The selection against underluminous supernovae is easy to see in the Asiago Supernova Catalogue (Barbon et al 1989); almost all classified supernova well outside the local supercluster are SNe Ia. For this reason, the contamination of type Ia samples by types II, Ib, and Ic can be controlled. SNe Ib, SNe Ic, and almost all SNe II are considerably less luminous than SNe Ia; SNe Ib and SNe Ic are considerably redder; and of course all are distinguishable spectroscopically. Likewise, unusual supernovae such as SN 1885A, SN 1991bg, and SN 1986G tend to be underluminous and can be controlled. Overluminous supernovae such as SN 1991T (if in or behind the Virgo cluster) may pose more of a problem, because even if rare their frequency will be artificially increased in flux-limited samples. Spectroscopic information may be needed to weed them out.

One way to protect remote SN Ia samples against interlopers and extinction may be to concentrate on elliptical galaxies. So far, all classified supernovae in elliptical galaxies except SN 1991bg (and possibly, but not probably, SN 1939B; Section 2.3) have been ordinary SNe Ia. The disadvantage that elliptical galaxies are slow supernova producers (Tammann 1991a) may be offset by the fact that supernova searches at high redshift will be conducted in very rich clusters of galaxies where the fraction of galaxies that are elliptical is exceptionally high.

The future role of SNe Ia in cosmology will be illustrated here by four examples.