UC Berkeley

We present a self-consistent comparison of the rise times for low- and high-redshift Type Ia supernovae. Following previous studies, the early light curve is modeled using a t2 law, which is then mated with a modified Leibundgut template light curve. The best-fit t2 law is determined for ensemble samples of low- and high-redshift supernovae by fitting simultaneously for all light-curve parameters for all supernovae in each sample. Our method fully accounts for the nonnegligible covariance among the light-curve fitting parameters, which previous analyses have neglected. Contrary to recent results by Riess et al., we find fair to good agreement between the rise times of the low- and high-redshift Type Ia supernovae. The uncertainty in the rise time of the high-redshift Type Ia supernovae is presently quite large (roughly ±1.2 days statistical), making any search for evidence of evolution based on a comparison of rise times premature. Furthermore, systematic effects on rise-time determinations from the high-redshift observations, due to the form of the late-time light curve and the manner in which the light curves of these supernovae were sampled, can bias the high-redshift rise-time determinations by up to +3.6-1.9 days under extreme situations. The peak brightnesses - used for cosmology - do not suffer any significant bias, nor any significant increase in uncertainty.