Sometimes science is in slow mode.
Back in 2016, researchers observing the cluster MACS J1039 noted that its gravity was magnifying and twisting the light of much more distant systems, and, in those arcs, the shine of a supernova could just be seen.
This effect is called a gravitational lens. Light is affected by gravity just like everything else, and high mass objects – like MACS J1039 – can bend the path of light that was planning to go to one part of the universe so that it instead reaches us. This can actually focus a lot of extra light our way and make objects that are otherwise invisible bright enough to be seen.
The path this bent light takes is longer than it would be if the light was traveling straight toward us. Just how much longer is a matter of geometry. Imagine placing mirrors in different places that all allow you to see what is behind a blind corner. All those mirrors may be showing you the same thing, but because they are in different places, the light has to travel different distances to reach you.
On the scales of parking garages and roadways, these differences don’t really matter, but because light takes time to travel, on galaxy cluster scales, these differences can cause different versions of the same distant object to appear at different times. If the difference in geometry causes the light to travel a few extra days, we’ll see events like a supernova on repeat with the timing of two versions appearing the extra few days apart when they are brightest. With massive clusters, however, the difference can be years.
In MACS J1039, the three images of the background galaxy that showed us a supernova in 2016 are all separated by no more than weeks in the distance their light has to travel, and researchers saw the supernova at a variety of stages of brightening and fading. There are two other versions of that galaxy gravitationally lensed into visibility in this field, however, and they both require light to travel a much longer path to reach us. According to new calculations published in Nature Astronomy by Steven Rodney and his collaborators, those two additional versions of the background galaxy will have the supernova around 2037 and 2042, give or take a couple of years.
The “give or take a couple of years” comes from our lack of understanding of dark matter and how it does and does not cluster in galaxy clusters. Dark matter is a somewhat infuriating form of matter that interacts with gravity – because everything interacts with gravity – but doesn’t interact at all with light except through that gravity. Dark matter doesn’t give light off, it doesn’t reflect it; it just bends light via the gravitational pull of its own mass.
The research team behind this new work has run a variety of models to estimate how dark matter is clustered around the luminous stuff we see in this galaxy cluster, and based on those models, they have calculated how long it will take light to get sharply bent through the denser regions. The path the light takes isn’t a straight line, and in fact, it may be more of a ping-pong-ball path than even a simple curve. We don’t know, yet.
When the light from the supernova does finish its crazy detour through galaxy cluster MACS J1039, we will finally have a chance to sort which models are closest to reality when the light from that supernova once again makes it to our telescopes.
The scientists involved do express concern that while the 2037-ish arrival of supernova light should be visible, the 2042 arrival may be too faint because not enough light will be bent our way. I’m hoping that by 2042 things will be advanced enough that we’ll get all the data we need to put some serious constraints on our understanding of dark matter.
Unrelated to the geometry of space and time, this story also reminds us that the universe is big and that it is old. The supernova light that we’re seeing on repeat was initially fired into space ten billion years ago, and it originates from a star that was formed, lived, and exploded within the first four billion years of our universe. We are seeing into our universe’s distant past, and it’s helping us understand something fundamental about our universe.
“A gravitationally lensed supernova with an observable two-decade time delay,” Steven A. Rodney et al., 2021 September 13, Nature Astronomy