I’ve been a bit pressed for time recently (to put it mildly) so am a bit late catching up on a wonderful observation (by Kelly et al.) reported in last week’s issue of Science. Here’s the abstract:
In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster’s gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.
And here’s a nice picture of the system which I ripped of from a nice report in Physics World:
Multiple images of background objects caused by gravitational lensing have been observed before, but the key thing about this particular “Einstein Cross” is that the background object is a type of exploding star called a supernova. That means that the light it emits will decay over time. That light reaches us via four different paths around the intervening galaxy cluster so monitoring the different evolution in the four images will yield direct measurements of the physical scale of the cluster and hopefully answer a host of interesting cosmological questions.
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In the Dark 