A Preliminary Indication of Evolution of Type Ia Supernovae from their Risetimes

We have compared the risetime for samples of nearby and high-redshift type Ia supernovae (SNe Ia). The fiducial risetime of the nearby SNe Ia is 2.5±0.4 days longer than the preliminary measurement of the risetime by Goldhaber (1998a,b) and Groom (1998) for high-redshift SNe Ia from the Supernova Cosmology Project. If further analysis of the high-redshift data does not lead to a significant change in the value or uncertainty in the risetime, then the statistical likelihood that the two samples have different fiducial risetimes remains high (5.8σ) and indicates possible evolution between the samples of SNe Ia. We consider the likely effects of several sources of systematic error, but none of these resolves the difference in the risetimes. Currently, we cannot directly determine the impact of the apparent evolution on previous determinations of cosmological parameters. subject headings: supernovae: general−cosmology: observations