2007ApJ...667L..37H

Recent studies indicate that Type Ia supernovae (SNe Ia) consist of two groups–a ``prompt'' component whose rates are proportional to the host galaxy star formation rate, whose members have broader light curves and are intrinsically more luminous, and a ``delayed'' component whose members take several Gyr to explode, have narrower light curves, and are intrinsically fainter. As cosmic star formation density increases with redshift, the prompt component should begin to dominate. We use a two-component model to predict that the average light curve width should increase by 6% from z=0 to 1.5. Using data from various searches, we find an 8.1%±2.7% increase in average light curve width for non-subluminous SNe Ia from z=0.03 to 1.12, corresponding to an increase in the average intrinsic luminosity of 12%. To test whether there is any bias after supernovae are corrected for light curve shape we use published data to mimic the effect of population evolution and find no significant difference in the measured dark energy equation of state parameter, w. However, future measurements of time-variable w will require standardization of SN Ia magnitudes to 2% up to z=1.7, and it is not yet possible to assess whether light curve shape correction works at this level of precision. Another concern at z=1.5 is the expected order-of-magnitude increase in the number of SNe Ia that cannot be calibrated by current methods.