The Dark Energy Survey Supernova Program: Slow supernovae show cosmological time dilation out to $z \sim 1$

TL;DR

This study uses 1504 supernova light curves from the Dark Energy Survey to precisely confirm that supernova durations are stretched by a factor of (1+z), supporting the expansion of the universe and ruling out non-expanding models.

Contribution

It provides the most precise measurement to date of cosmological time dilation using supernova light curves across a wide redshift range.

Findings

Supernova light curve widths scale with (1+z) as expected.

Measured time dilation parameter b is approximately 1.

Results strongly support the expanding universe model.

Abstract

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 parameterising 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. Firstly, we find that a power of $b \approx 1$ minimises 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 $b=1.003\pm0.005$ (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.