Supernovae have long been the subject of intense study by astronomers and astrophysicists. From seemingly birthing the elements of the universe to becoming what are essentially fellow remnants of end-of-life stars to black holes, their existence in the universe gives us remarkable insights into how the whole cosmos operates. As such, scientists will always point their eyes in the sky towards these cosmic explosions, revealing the story of the star that came before them. With enough luck, experts can even spot a supernova as it happens—or, really, happened, given how long it takes for its light to reach their eyes—giving them a unique perspective into the inner workings of stars.
One such encounter was with University of Copenhagen astronomer Gabriel Brammer back in 2016. At the time, he was combing over images taken by the Hubble Space Telescope at one particular area of the sky. Comparing this to previous images taken from the same spot, he noticed that three bright spots of light went missing in the newer image, while they actually appeared in the older images taken before. He took this as a sign of the three spots of light being from one of the strongest explosions in the universe—a supernova. “It was purely an accident,” says fellow astronomer Steve Rodney, from the University of South Carolina in Columbia.
Thing is, those three spots weren’t exactly three supernovae that just coincidentally exploded at the exact same time; only one supernova—a star from a distant galaxy codenamed MRG-M0138—actually went off. The three points of light were actually three duplicate images of just one supernova. But as far as we know, there aren’t really any cosmic funhouse mirrors just floating around in space, ready to fool our astronomers by giving them multiple images of the same thing. As it turns out, this triple-image of a supernova was birthed by a phenomenon known to scientists as gravitational lensing. Rodney’s and Brammer’s new study, formed together with a team of scientists, was published just this 2021 in the journal Nature Astronomy.
Gravitational Lensing
Gravity, being a distortion of spacetime caused by the presence of massive entities in space, can be simplified like placing a bowling ball in the middle of a stretched-out piece of fabric. The mass of the bowling ball causes the fabric around it to warp, creating a well in the middle. Any additional items that will be rolled into the fabric, given they are significantly lighter than the bowling ball, will simply roll around according to the “well” caused by the presence of the heavy bowling ball. This, scientists say, is analogous to how compared to more massive cosmic objects like the Sun, lighter planets like Earth simply “roll around” our star’s gravity well, forming the planet’s orbit around the star.
Light, much like the rest of the cast in this cosmic stage, will also tend to follow these “wells” of gravity caused by the presence of massive objects like stars and galaxies; light will then bend and distort, sometimes even merging and diverging, sometimes causing the appearance of multiple images of the same light source appearing. (Think of how light from the dining room distorts as you stare down the tube of a clear drinking glass.) In a way, the gravity of massive objects bend the light of any object whose light just so happens to cross their path—a gravitational “lens.” (In fact, an observation of just how our very Sun functioned as a gravity well for light from other stars, during the 1919 total solar eclipse, served as proof of Einstein’s Theory of General Relativity, giving the scientist his worldwide fame.)
Back to the Matter at Hand
Light from MRG-M0138 was lensed by a cluster of galaxies, sending the supernova light on several diverging paths, causing multiple images of the same supernova to appear. However, follow-up calculations by Rodney, Brammer, and team, revealed that there are duplicate images of the supernova that have yet to appear. In fact, they predict that the supernova is set to appear in the sky again around the year 2037, give or take a few years. A fifth image was calculated to follow sometime in 2042. As the very Hubble Space Telescope they used to gather these images is expected to have been taken down by this time, the research team designated the supernova SN Requiem. “It’s a requiem for a dying star and a sort of elegy to the Hubble Space Telescope itself,” Rodney says.
University of Minnesota astronomer Patrick Kelly, who’s unrelated to the work, added that the prediction by Rodney and team would be “a phenomenally precise measurement” if proven true. Kelly believes that the margin of error for the actual prediction of the reappearance of the supernova is tiny compared to the actual amount of time needed to elapse before it even happens. Since the supernova’s first appearance back in 2016, it was calculated that its next appearance in the sky wouldn’t be until 2037—a 21-year time delay. To compare, a gravitationally-lensed quasar spotted back in 1979 only had a time delay of around a year between its reappearances.
Rudolph Schild, an astrophysicist from the Harvard-Smithsonian Center for Astrophysics in Cambridge who’s also unrelated to the work, also casts doubt on the prediction by Rodney and team. “It is very difficult to predict what the time delay will be,” he added. Schild noted that the prediction of SN Requiem’s reappearance may well be distorted by the presence of an unexplainable source of mass within the galaxy MRG-M0138 and the cluster of galaxies distorting the light of the supernova within it: dark matter. Dark matter, in simplified terms, is a hypothetical form of matter present and apparently ubiquitous in space, and is said to comprise around 27% of the total mass of the universe; it’s called “dark” since it appears to not interact with any electromagnetic fields at all, meaning we can’t observe it directly using any wavelengths of light.
References
- Burns, D. (2012, March 11). Gravity visualized [YouTube]. apbiolghs. https://www.youtube.com/watch?v=MTY1Kje0yLg
- Croswell, K. (2021, September 13). A supernova’s delayed reappearance could pin down how fast the universe expands. Science News. Retrieved September 29, 2021, from https://www.sciencenews.org/article/supernova-sn-requiem-gravitational-lens-hubble-constant-universe-expansion
- Dark matter. (n.d.). CERN. Retrieved September 29, 2021, from https://home.cern/science/physics/dark-matter
- Gravitational lensing. (2019, May 30). HubbleSite.Org. Retrieved September 29, 2021, from http://hubblesite.org/contents/articles/gravitational-lensing
- Rodney, S. A., Brammer, G. B., Pierel, J. D. R., Richard, J., Toft, S., O’Connor, K. F., Akhshik, M., & Whitaker, K. E. (2021). A gravitationally lensed supernova with an observable two-decade time delay. Nature Astronomy, 1–8. https://doi.org/10.1038/s41550-021-01450-9
- Testing General Relativity. (n.d.). Eclipse 2017. Retrieved September 29, 2021, from https://eclipse2017.nasa.gov/testing-general-relativity
