The Dark Energy Survey Supernova Program: slow supernovae show cosmological time dilation out to z ~ 1.

We present a precise measurement of cosmological time dilation using the light curves of 1504 Type Ia supernovae from the Dark Energy Survey spanning a redshift range |$0.1\lesssim z\lesssim 1.2$|⁠. We find that the width of supernova light curves is proportional to |$(1+z)$|⁠ , as expected for time dilation due to the expansion of the Universe. Assuming Type Ia supernovae light curves are emitted with a consistent duration |$\Delta t_{\rm em}$|⁠ , and parametrizing the observed duration as |$\Delta t_{\rm obs}=\Delta t_{\rm em}(1+z)^b$|⁠ , we fit for the form of time dilation using two methods. First, we find that a power of |$b \approx 1$| minimizes the flux scatter in stacked subsamples of light curves across different redshifts. Secondly, we fit each target supernova to a stacked light curve (stacking all supernovae with observed bandpasses matching that of the target light curve) and find |$b=1.003\pm 0.005$| (stat) |$\pm \, 0.010$| (sys). Thanks to the large number of supernovae and large redshift-range of the sample, this analysis gives the most precise measurement of cosmological time dilation to date, ruling out any non-time-dilating cosmological models at very high significance. [ABSTRACT FROM AUTHOR]