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Galaxy cluster
The red circles on this image from the Hubble Space Telescope indicate the three spots where flashes from Supernova Refsdal showed up at different times. The middle circle indicates the spot where it was last observed on Dec. 11. (Credit: NASA / ESA / GLASS / Frontier Fields / CLASH)

Astronomers traced one of the weirder twists in relativity to determine when they’d see an “instant replay” of a distant supernova, and now the Hubble Space Telescope has shown that their prediction was right. They say it marks the first time a supernova observation was predicted in advance.

The confirmation comes in the form of Hubble’s observations of the galaxy cluster MACS J1149.5+2223 over the course of more than a year. When scientists studied pictures taken in November 2014, they identified a supernova flash that had been split into four separate images, due to the cluster’s gravitational lensing effect.

Here’s how a gravitational lens works: The cluster’s galaxies have so much mass that they warp spacetime significantly. As a result, the light beams coming from bright objects behind them are bent, as if the beams were passing through a funhouse lens. The phenomenon works in accordance with the principles of Albert Einstein’s theory of general relativity. In his honor, the characteristic four-point pattern that astronomers saw is known as an “Einstein Cross.”

“While studying the supernova, we realized that the galaxy in which it exploded is already known to be a galaxy that is being lensed by the cluster,” University of South Carolina astronomer Steve Rodney, one of the authors of a research paper about the observations, said in a Hubble news release. “The supernova’s host galaxy appears to us in at least three distinct images caused by the warping mass of the galaxy cluster.”

That means the light emanating from the exploding galaxy was taking three different routes on its way to Earth, each traveling a different distance from there to here. Rodney and his colleagues modeled the mass distribution of the galaxies in the cluster, including the mysterious stuff known as dark matter. Then they worked out the timing for the supernova flash for each of the routes through spacetime. They determined that one of the flashes had already occurred, back in 1998, but was not observed by any telescope. The Einstein Cross was the middle flash. And the third flash was yet to come.

“We used seven different models of the cluster to calculate when and where the supernova was going to appear in the future. It was a huge effort from the community to gather the necessary input data using Hubble, VLT-MUSE, and Keck and to construct the lens models,” said UCLA astronomer Tommaso Treu, another author of the study. “And remarkably, all seven models predicted approximately the same time frame for when the new image of the exploding star would appear.”

Since the end of October, Hubble has been checking MACS J1149.5+2223 periodically to check whether the predictions were correct, and the third flash was observed at last on Dec. 11. The preliminary findings suggest that four of the seven models matched the observed data particularly well.

The study authors said the results served as “a remarkable and powerful validation of the model predictions specifically and of general relativity indirectly.” They also provided a textbook example of the scientific method, in which observations give rise to hypotheses that are tested by making further observations. Now the modeling methods will be going through more tests in the Hubble team’s Frontier Fields program, which focuses on the power of gravitational lensing.

The latest research paper about the observations – “Deja Vu All Over Again: The Reappearance of Supernova Refsdal” – has been submitted to Astrophysical Journal Letters. Berkeley astronomer Patrick Kelly is the lead author. Check out the European Space Agency’s Hubble site and the European Southern Observatory for more about Supernova Refsdal, which is named after a Norwegian pioneer in the study of gravitational lenses.

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