Astronomers have seen the same supernova 3 times with Gravitational Lensing.

It is hard for humans to wrap their heads around the fact that there are galaxies so far away that the light coming from them can be warped in a way that they actually experience a type of time delay. But this is exactly what happens with extreme gravitational lens shapes, like those that give us beautiful images of Einstein's rings. In fact, the time dilation around some of these galaxies can be so extreme that the light from a single event, such as a supernova, can actually show up on Earth at dramatically different times. That's exactly what a team led by Dr. Steven Rodney of the University of South Carolina and Dr. Gabriel Brammer of the University of Copenhagen discovered. Except that three copies of this supernova have already appeared – and the team thinks it will reappear again, in 20 years.

Finding such a supernova is important not only for its mental bending qualities but also for resolving a major debate in the cosmological community. The rate of expansion of the universe has surpassed the predicted rate when calculated from the background radiation of cosmic microwaves. More often than not, this cosmological puzzle is solved by invoking “dark energy”, a dark force that is supposedly responsible for the increase in the acceleration rate. But scientists do not really know what black energy is, and to discover it they need a better model of the physics of the early universe.

One way to get this best model is to find an event that is actively deformed by the gravitational lens. Importantly – the same event must show up at two separate, distinct times to provide input to a calculation about the ratio of the distance between the galaxy doing the lensing and the background galaxy that was the source of the event.

This ratio is a significant component of the calculation of some of the variables associated with dark energy. And the supernova candidate Drs. Rodney and Brammer found are among the best defined so far. This is only the third example of a magnified lentil supernova. Quasars have also been caught with their own time delays, but the variable nature of quasars themselves makes them less than ideal for the kind of angular distance calculations needed by cosmologists.

The new supernova, named AT2016jka, was extracted from data gathered by Hubble in 2016. Located in “the most spectacular galaxy targeted by REQUIEM [the Hubble observation program that captured the data],” it is in the galaxy called MRG-M0138.

MRG-M0138 is “quadrupled”, which means that four copies of the galaxy can be seen scattered around a cluster of galaxies closer to our own galaxy, called MAC J0138.02155. So when the team was surveying data in the region in July 2019, they noted the three-point sources of light that were present in data from July 2016 were no longer there. Data from July 2016 likely captured one supernova lensed in three different ways.

However, the fourth intended objective is not reflected in the Hubble data. Using their lens model for the system, the team determined that the fourth image is expected to appear around 2037, give or take a couple of years. With such a long reference time between occurrences of the same event, this supernova would provide valuable data for the debate about time dilatation in gravitational lens events.

Unfortunately, it also means that scientists must wait almost 20 years to get hold of this data. It also means that they have to keep a watchful eye on that part of the sky in the 2-year window the calculations predict the fourth image of the supernova would appear in. It might not be a bad idea to keep an eye on it the rest of the time as well in case it appears earlier than expected.

If all goes well, that final piece of data as to the exact date of the peak brightness of the supernova will be well monitored by a new fleet of cosmological instruments. Telescopes such as Vera Rubin and Nancy Grace promise to watch hundreds of these elongated supernovae which can provide even more data to further constrain dark energy. Hopefully, they’ll be able to catch the final gasp of the supernova in MRG-M0138 as well, to cap off some great detective work and prove how incredible gravitational time dilation truly is.