With what we currently know about astronomy, predicting when a star will end in a dramatic supernova is nearly impossible. But there are exceptions, among them, the so-called Supernova Requiem, which astronomers predict we will see go boom in 2037. How can they make such a prediction? Well, the trick is that they have seen it explode three times already.
This might sound like some sci-fi time-traveling effect but it is actually very real. The mechanism behind it is called gravitational lensing. A massive object, in this case a galaxy cluster, can warp space-time so much that it acts as a magnifying zoom lens in front of whatever is behind it. So the light of a background object is warped, magnified, and split into multiple copies, arriving at different times.
In the case of Supernova Requiem, the lensing object is galaxy cluster MACS J0138.0-215. The Hubble Space Telescope spotted and captured three separate images of the supernova in 2016, each one a different image of the supernova's light at a different time during the explosion. The supernova had faded by 2019. Now, in a new paper in Nature Astronomy, astronomers predict the future appearance of a fourth image expected around 2037.
This is not the first lensed supernova caught repeating, but it is still very exciting. Astronomers find these rare occasions very useful to study stars as well as studying gravity and the mysterious components of the universe like dark matter and dark energy.
"This new discovery is the third example of a multiply imaged supernova for which we can actually measure the delay in arrival times," lead researcher Steve Rodney of the University of South Carolina said in a statement. "It is the most distant of the three, and the predicted delay is extraordinarily long. We will be able to come back and see the final arrival, which we predict will be in 2037, plus or minus a couple of years."
The reason why the last image of the supernova will take such a long time to appear is that it is the closest to the cluster center. That’s where there the gravitational well created by the matter is at its deepest.
"This is the last one to arrive because it's like the train that has to go deep down into a valley and climb back out again. That's the slowest kind of trip for light," Rodney explained.
The cosmological applications of such observations are very exciting. To precisely work out when the fourth image will appear, models of the distribution of dark matter in the cluster are necessary. Dark matter is the invisible matter that permeates the cosmos. The time delays of the light tell us something about the accelerated expansion of the universe, which is believed to be caused by the equally mysterious dark energy.
"These long time delays are particularly valuable because you can get a good, precise measurement of that time delay if you are just patient and wait years, in this case more than a decade, for the final image to return," Rodney said. "It is a completely independent path to calculate the universe's expansion rate. The real value in the future will be using a larger sample of these to improve the precision."