  The gravitationally lensed Supernova Refsdal appeared in multiple images,
produced through gravitational lensing by a massive foreground galaxy cluster.
After the supernova appeared in 2014, lens models of the galaxy cluster
predicted an additional image of the supernova would appear in 2015, which was
subsequently observed. We use the time delays between the images to perform a
blinded measurement of the expansion rate of the Universe, quantified by the
Hubble constant (H0). Using eight cluster lens models, we infer H0 = 64.8
+4.4-4.3 km / s / Mpc, where Mpc is the megaparsec. Using the two models most
consistent with the observations, we find H0 = 66.6 +4.1-3.3 km / s / Mpc. The
observations are best reproduced by models that assign dark-matter halos to
individual galaxies and the overall cluster.
