First successful prediction of supernova thanks to gravitational lensing

December 19, 2015

A year after first appearing in a formation known as Einstein cross, Refsdal supernova reappeared just as predicted, made possible by gravitational lensing.

Refsdal supernova. Four red circles are Nov 2014 images. A bit to the left is its Dec 2015 reappearance. Far top-left circle is where Refsdal supernova would have appeared had it been observed.

First appearance of Refsdal supernova

In November 2014, the light from supernova that occurred 9.3 billion years ago in a distant galaxy had reached Hubble Space Telescope in four separate routes, creating four different images of the same event. They didn’t appear at the same time, but during the time period of few weeks.

Since supernova, a star ending its life in enormous explosion, is a rare event, astronomers didn’t think that four different stars exploded in a single galaxy in a short period of time. It was actually the first time multiple images of the same supernova were observed thanks to a gravitational lensing by a galaxy cluster 5 billion light-years away.

What is gravitational lensing?

Gravitational lensing is bending and magnification of light from a distant object by another object, usually a galaxy cluster, that’s closer to us than the distant object being observed. 

According to Einstein’s general theory of relativity, large objects bend space and time around them. As light travels near such a massive object, in this case the galaxy cluster that’s closer to us, it changes its path and slows down for a while. Light is carried by particles called photons, and as a source, in this case supernova, emits photons, some of them travel closer to the massive object hence changing their path and slowing down more, while some travel farther from the object and slow down and bend their path to lesser extent.

Refsdal supernova is visible at three locations at three different times thanks to gravitational lensing

Bending of light through gravitational lenses can cause the light from a single source to appear as four separate images in our telescopes, magnified and appearing at different times. This formation is known as Einstein cross, first observed as four images of the same quasar in a cross-like formation around a galaxy in the foreground.

Hubble Space Telescope uses gravitational lensing often, with the help of massive galaxy clusters that magnify the light of more distant galaxies behind them, which would otherwise be undetectable.

Predicting the December 2015 reappearance with theoretical models

Ever since first observed in November 2014, Refsdal supernova, named in honor of Norwegian astronomer Sjur Refsdal, has been focus of seven different models that tried to predict when another image of it will appear again. Interestingly, all of them predicted a similar time frame and the supernova reappeared on December 11, 2015, as expected.

“It felt almost reassuring that the theory of relativity and the models based on it worked so well,” said Patrick Kelly, of the University of California, Berkeley, USA who is the lead author of a paper “Hubble has showcased the modern scientific method at its best. Testing predictions through observations provides powerful means of improving our understanding of the cosmos.”

The detection of Refsdal supernova’s reappearance will help refinement of said models in order to better understand how mass is distributed within a galaxy cluster, including the distribution of dark matter.

According the models, Refsdal supernova appeared first in 1998, but sadly, nobody was looking.

Image credits: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI)

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