A gravitationally lensed supernova with an observable two-decade time delay

When the light from a distant object passes very near to a foreground galaxy or cluster, gravitational lensing can cause it to appear as multiple images on the sky1. If the source is variable, it can be used to constrain the cosmic expansion rate2 and dark energy models3. Achieving these cosmological goals requires many lensed transients with precise time-delay measurements4. Lensed supernovae are attractive for this purpose because they have relatively simple photometric behaviour, with well-understood light curve shapes and colours—in contrast to the stochastic variation of quasars. Here we report the discovery of a multiply imaged supernova, AT 2016jka (‘SN Requiem’). It appeared in an evolved galaxy at redshift 1.95, gravitationally lensed by a foreground galaxy cluster5. It is probably a type Ia supernova—the explosion of a low-mass stellar remnant, whose light curve can be used to measure cosmic distances. In archival Hubble Space Telescope imaging, three lensed images of the supernova are detected with relative time delays of <200 d. We predict that a fourth image will appear close to the cluster core in the year 2037 ± 2. Observation of the fourth image could provide a time-delay precision of ~7 d, <1% of the extraordinary 20 yr baseline. The supernova classification and the predicted reappearance time could be improved with further lens modelling and a comprehensive analysis of systematic uncertainties. The discovery of a multiply imaged, probably of type Ia, supernova in a galaxy at redshift 1.95 enables a time-delay measurement with an uncertainty of <1%. The prediction that a new image will appear in the year 2037 ± 2 allows the use of this system as a cosmological probe.