Four versions of the same supernova explosion have been captured because a large galaxy between us and the event is distorting the path on which the light travels to reach us. The event not only makes visible a supernovae more distant than we normally see but provides the opportunity astronomers have been dreaming of to test three of the biggest questions in cosmology. Even more opportunities should arise in future.

One of the key predictions of General Relativity is that mass bends spacetime, and therefore light. Einstein predicted that very massive objects could focus light in a manner analogous with glass lenses, an effect finally observed in 1979.

Depending on the locations of the relevant objects we often see multiple images of the same distant quasar or galaxy. Since this light follows different paths to reach us the distance traveled on each will not be identical, so we are seeing some slightly delayed relative to the others. This makes little difference for an object whose brightness barely varies.

However, in 1964 Sjur Refsdal pointed out that different images of the same supernova would capture different moments in the explosion's evolution, and might be used to test the rate at which the universe is expanding. Great efforts have been made to find such an example of such a valuable case. Dr Patrick Kelly of the University of California, Berkeley was looking for distant galaxies and came across the sight of four images of a nine billion year old supernova around a galaxy in the MACS J1149.6+2223 cluster.

“It really threw me for a loop when I spotted the four images surrounding the galaxy - it was a complete surprise,” Kelly says. The discovery has been announced in Science and named SN Refsdal.
 

^{Credit: Australian National University. Co-author Dr Brad Tucker explains some of the many exciting features of the new discovery.}

Refsdal was photographed from the November 3-20 last year, long enough to see one of the images, dubbed S3, brighten substantially, while the others showed no detectable change.

Co-author Dr Brad Tucker of the Australian National University says, “It’s perfectly set up, you couldn’t have designed a better experiment. You can test some of the biggest questions about Einstein’s theory of relativity all at once - it kills three birds with one stone.”

The 20-30 times magnification provided by the intervening galaxy provides a rare opportunity to such a distant supernova. Since SN Refsdale is a Type 1a supernova it provides a particularly valuable “standard candle” we can use to measure the expansion of the universe in a previously inaccessible era.

The images formed by distant quasars have allowed us to map the mass distribution within lensing galaxies, providing one of our major sources of knowledge on dark matter distribution and the strength of the gravitational constant. SN Refsdale will be an even better resource for both, Tucker says, because using its light at different points in its evolution will be like “having multiple quasars to do the same test.”

The lensing galaxy is just one in a cluster, and the authors calculate that its neighbors should also lens the same event, with the light arriving at different times. They believe one such appearance took place in 1964, just as Refsdal was proposing his theory, but no sufficiently powerful telescopes were looking in the right place to record it. More importantly, they predict future such events in 1-4 years time, which will provide us with an opportunity to not only witness this explosion on other occasions, but to see how the different galaxies affect how it looks.