An 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 proemial risetime determined by Goldhaber (1998a,b) for high-redshift SNe Ia from the Supernova Cosmology Project. The statistical likelihood that the two samples have different fiducial risetimes is 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. If the observed evolution affects the peak luminosities of SNe Ia, it may provide a simpler explanation than a cosmological constant for the observed faintness of high-redshift SNe Ia. subject headings: supernovae: general−cosmology: observations