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 sky. If the source is variable, it can be used to constrain the cosmic
expansion rate and dark energy models. Achieving these cosmological goals
requires many lensed transients with precise time delay measurements. Lensed
supernovae (SN) are attractive for this purpose because they have relatively
simple photometric behavior, 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, AT2016jka ("SN Requiem"). It
appeared in an evolved galaxy at $z=1.95$, gravitationally lensed by a
foreground galaxy cluster. It is likely 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 $<$200 days. We predict
a fourth image will appear close to the cluster core in the year 2037$\pm$2.
Observation of the fourth image could provide a time delay precision of
$\approx$7 days, $<1\%$ of the extraordinary 20 year baseline. The SN
classification and the predicted reappearance time could be improved with
further lens modelling and a comprehensive analysis of systematic
uncertainties.
