Constraints on the Hubble constant from supernova Refsdal's reappearance

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 that 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_-4.3^+4.4 kilometers per second per megaparsec. Using the two models most consistent with the observations, we find H₀=66.6_-3.3^+4.1 kilometers per second per megaparsec. The observations are best reproduced by models that assign dark-matter halos to individual galaxies and the overall cluster.