Type Ia Supernovae and their implications for cosmology

Models for Type la Supernovae (SNe la) are reviewed. It is shown that there are strong reasons to believe that most SNe la represent thermonuclear disruptions of C-O white dwarfs, when these white dwarfs reach the Chandrasekhar limit and ignite carbon at their centers. Different progenitor scenarios are reviewed critically and the strengths and weaknesses of each scenario are presented in detail. It is argued that theoretical considerations currently favor single-degenerate models, in which the white dwarf accretes from a subgiant or giant companion. However, it is still possible that more than one progenitor class contributes to the observed sample. The relation of the different models to the use of SNe la for the determination of cosmological parameters is discussed. It is shown that while the observed diversity of SNe la may argue for the existence of different progenitor classes, this does not affect the interpretation of an accelerating expansion of the universe. Crucial observational tests of the conclusions are suggested.