Type Ia Supernovae, Evolution, and the Cosmological Constant

We explore the possible role of evolution in the analysis of data on SNe Ia at cosmological distances. First, using a variety of simple sleuthing techniques, we nd evidence that the properties of the high and low redshift SNe Ia observed so far dier from one another. Next, we examine the eects of including simple phenomenological models for evolution in the analysis. The result is that cosmological models and evolution are highly degenerate with one another, so that the incorporation of even very simple models for evolution makes it virtually impossible to pin down the values of M and , the density parameters for nonrelativistic matter and for the cosmological constant, respectively. We evaluate the Bayes factor for models with evolution versus models without evolution, which, if one has no prior predilection for or against evolution, is the odds ratio for these two classes of models. The resulting values are always of order one, in spite of the fact that the models that incl...