Fredagskollokvium: Observing lensed supernovae: The Refsdal legacy

Half a century ago, long before the phenomenon of gravitational
lensing was even observed (except around the Sun), Sjur Refsdal wrote
a couple of seminal works on the subject. Amongst other things, he
proposed how the mesaurement of the time delay between images of a
Type Ia supernova, being lensed by a galaxy, could be used to probe
not only the mass of the lensing galaxy, but also the expansion
velocity and the size of our Universe. More than 50 years later, I
can finally report on the first discovery of a lensed Type Ia
supernova with multiple (4!) images; iPTF16geu.