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 z 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 Δ tₑₘ, and parameterising the observed duration as Δ tobs=Δ tₑₘ(1+z)ᵇ, we fit for the form of time dilation using two methods. Firstly, we find that a power of b ≈ 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±0.005 (stat) ±\,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.