For astronomers, the stars have aligned perfectly.
An optics trick predicted by Einstein’s theory of general relativity allowed Patrick Kelly of the University of California, Berkeley and his colleagues to witness the same supernova explosion four separate times at different locations in the night sky.
Supernova Refsdal, named after a Norwegian astrophysicist, erupted nine billions year ago—so far away that even the Hubble Space Telescope wasn’t able to detect it without a special cosmic maneuver, in which light rays from a star are bent and magnified by the gravity of an intervening galaxy cluster.
Albert Einstein suggested that matter and energy warp spacetime, producing the phenomenon we call gravity. When light nears massive objects (which warp spacetime more than smaller ones), it gets bent around those objects. This effect is called gravitational lensing, and astronomers routinely use it like a powerful telescope to study fuzzy, distant objects that are otherwise obscured by large celestial bodies—like galaxies or galaxy clusters. What enabled Kelly to see multiple images of Supernova Refsdal is a specific kind of gravitational lensing, contingent on the position of the stars with respect to its intervening galaxy cluster.
Here’s Dennis Overbye, writing for The New York Times:
How this cosmic telescope works depends on how the stars are aligned. If a star and its intervening lens are slightly out of line, the distant light can appear as arcs. If they are exactly lined up, the more distant star can appear as a halo known as an Einstein ring, or as evenly separated images—the Einstein Cross.
A classic example of an Einstein Cross produced by gravitational lensing is a quasar in the constellation Pegasus that sits directly behind Huchra’s lens, the galaxy that bends the quasar’s light, producing four images of it from our vantage point on Earth.
In the case of Supernova Refsdal, multiple images of the same supernova appear to us on Earth, billions of years after the event itself actually took place. And since each ray of light traveled a different path to get here, each of the four images represents a different moment of the supernova event. Dr. Kelly and his team, who discovered this unprecedented lensing effect by accident, published the results yesterday in the journal Science.
Here’s Overbye again:
The light from this spiral has been bent and magnified both by the gravity of the intervening cluster, which is five billion light-years distant, and by one very massive galaxy in the cluster.
Kelly believes that the supernova has appeared in the sky before, in 1964 and 1995. Based on their calculations, the Einstein Cross could manifest again in another part of the sky at some point in the next ten years. These time differences may be able to help scientists better understand dark matter, as well as nail down how quickly the universe is expanding.
Photo Credits: NASA / ESA , NASA