[Previous] | [Session 116] | [Next]
D. Branch (U. of Oklahoma)
Observations of high--redshift Type Ia supernovae (SNe Ia) have indicated that the cosmic expansion is accelerating under the influence of some sort of dark energy. Making further observations of high--redshift SNe Ia with SNAP, to extend and more accurately determine the distance--redshift relation, is the most promising way to probe the nature of the dark energy. It will be necessary to make sure that SN Ia evolution --- a systematic variation in the properties of SNe Ia as a function of redshift --- does not cause the wrong distance--redshift relation to be inferred from the data. Fortunately, a complete understanding of the origin and physics of SNe Ia, however desirable, will not be required for taking SN Ia evolution into account. SN Ia evolution differs in an important respect from the kind of cosmic evolution that presents such a challenge to some other approaches to observational cosmology. Number counts, for example, evolve with cosmic time in a way that cannot be determined by making observations of the local universe. The properties of a SN Ia, on the other hand, depend on the properties of its progenitor star, not directly on the age of the universe. It follows that SN Ia evolution is expected to be limited to a modest amount of progenitor population drift. Because the local universe produces SNe Ia from stellar populations that have a range of properties such as age and metallicity, the population drift can be controlled empirically. Strategies for doing this will be briefly discussed.