An enormous amount of gravity from a cluster of distant galaxies causes space to curve so much that light from more distant galaxies is bent. This “gravitational lensing” effect has allowed University of Copenhagen astronomers to observe the same exploding star –SN Requiem–in three different places in the heavens, and may help solve the mystery of cosmic expansion and reveal the nature of dark matter and dark energy.
Fourth Image Predicted
The Copenhagen team predicts that a fourth image of the same explosion will appear in the sky by 2037. The study, published in the journal Nature Astronomy, provides a unique opportunity to explore not just the supernova itself, but the expansion of our universe.
One of the most fascinating aspects of Einstein’s famed theory of relativity is that gravity is no longer described as a force, but as a “curvature” of space itself. The curvature of space caused by heavy objects does not just cause planets to spin around stars, but can also bend the trajectory of light beams.
Galaxy Clusters and Curvature of Space Create an Illusion
The heaviest of all structures in the universe—galaxy clusters made up of hundreds or thousands of galaxies—can bend light from more distant galaxies behind them so much that they appear to be in a completely different place than they actually are.
But that’s not the whole story: light can take several paths around a galaxy cluster, making it possible for us to get lucky and make two or more sightings of the same galaxy in different places in the sky using a powerful telescope.
Gravity from the MACS J0138 galaxy cluster shown at the top of the page curves space so much that light from a galaxy behind it is bent down towards us in several different ways. To the left is a picture of the cluster from 2016 in which light from the same exploding star—a supernova—is seen in three places in the night sky. To the right, is the same area in 2019, where the supernova is now gone. Astronomers from the Niels Bohr Institute have calculated that it will reappear in 2037. Credit: S. Rodney (U. of S. Carolina), G. Brammer (Cosmic Dawn Center), J. DePasquale (STScI), P. Laursen (Cosmic Dawn Center)
Single Galaxy Captured in Four Different Places
Some routes around a galaxy cluster are longer than others, and therefore take more time. The slower the route, the stronger the gravity; yet another astonishing consequence of relativity. This staggers the amount of time needed for light to reach us, and thereby the different images that we see, has allowed a team of astronomers at the Cosmic Dawn Center—a basic research center run by the Niels Bohr Institute at the University of Copenhagen and DTU Space at the Technical University of Denmark—along with their international partners, to observe a single galaxy in no less than four different places in the sky.
The observations were made using the infrared wavelength range of the Hubble Space Telescope. By analyzing the Hubble data, researchers noted three bright light sources in a background galaxy that were evident in a previous set of observations from 2016, which disappeared when Hubble revisited the area in 2019. These three sources turned out to be several images of a single star whose life ended in a colossal explosion known as a supernova.
The light of a galaxy with an exploding star takes different paths around an intermediate galaxy cluster before it reaches us. Astronomers from the Cosmic Dawn Center, among others, have calculated that one route is about 21 light years longer than the other. As such, they predict that come 2037, we should be able to spot the supernova yet again. Credit: Peter Laursen, Cosmic Dawn Center).
“A single star exploded 10 billion years ago, long before our own sun was formed. The flash of light from that explosion has just reached us,” explains Associate Professor Gabriel Brammer of the Cosmic Dawn Center, who led the study.
Mirrored Images of “SN-Requiem”
The supernova, nicknamed “SN-Requiem,” can be seen in three of the four “mirrored images” of the galaxy. Each image presents a different view of the explosive supernova’s development. In the final two images, it has not yet exploded. But, by examining how galaxies are distributed within the galaxy cluster and how these images are distorted by curved space, it is actually possible to calculate how “delayed” these images are.
“The fourth image of the galaxy is roughly 21 years behind, which should allow us to see the supernova explode one more time, sometime around 2037,” explains Brammer.
Sheds Light on Unsolved Cosmological Riddle
Should we get to witness the SN-Requiem explosion again in 2037, it will not only confirm our understanding of gravity, but also help to shed light on another cosmological riddle that has emerged in the last few years, namely the expansion of our universe.
We know that the universe is expanding, and that different methods allow us to measure by how fast. The problem is that the various measurement methods do not all produce the same result, even when measurement uncertainties are taken into account. Could our observational techniques be flawed, or—more interestingly—will we need to revise our understanding of fundamental physics and cosmology?
“Understanding the structure of the universe is going to be a top priority for the main earth-based observatories and international space organizations over the next decade. Studies planned for the future will cover much of the sky and are expected to reveal dozens or even hundreds of rare gravitational lenses with supernovae like SN Requiem,” explained Brammer. “Accurate measurements of delays from such sources provide unique and reliable determinations of cosmic expansion and can even help reveal the properties of dark matter and dark energy.”
Connects Our Late Universe with the Early Universe
“Astronomers currently have a very good cosmological model, called the ‘Lambda CDM’ model that incorporates dark energy (lambda) and cold dark matter (CDM),” wrote first author Steven Rodney, assistant professor of astronomy at the University of South Carolina, in an email to The Daily Galaxy.
“This model, if correct, can provide a way to connect the ‘late’ universe with the ‘early’ universe. By the late universe I mean all the galaxies and supernovae and whatnot that formed after the universe had expanded and cooled over a few billion years,” Rodney explained in his email. “For the early universe I mean the era that happened within a few hundred thousand years after the big bang, and most importantly includes the cosmic microwave background (CMB). The current challenge of the ‘Hubble constant tension’ is that some of the most precise measurements of the current rate of cosmic expansion (that is the Hubble-LeMaître constant, H0) are in disagreement. Observations using stars and supernovae in the late universe give one number for H0, while observations using the CMB give a different number. This could be because one (or both) of these observations have some mistakes, like bad assumptions or errors in the setup. Or it could be that our Lambda CDM model that is supposed to be the connecting bridge has some fatal flaw in it. That latter possibility may require us to revise our understanding of dark , dark matter or some aspect of fundamental physics.”
A Powerful Test of Cosmological Models
Steven Rodney elaborates in his email to The Daily Galaxy: “The time delay between appearances of a distant supernovae provides a new way to test our cosmological models. A strongly lensed supernova like SN Requiem appears as multiple points on the sky (multiple images). When we find one of these rare events, we can make a prediction for when the final image will appear. That prediction incorporates the cosmological model (Lambda CDM) and the Hubble constant. So we basically wait on Earth with a stopwatch, and measure the time until the final reappearance. Then we compare that measured time to the predicted time, and that gives us new information about our dark matter and dark energy cosmic model. Any individual lensed supernova is not going to definitely resolve these cosmic mysteries, though. We really need many dozens or hundreds of them, and they should be combined with the well-studied lensed quasars to get a richer and more powerful test of our cosmological models.”
Source: Rodney, S.A. et al. A gravitationally lensed supernova with an observable two-decade time delay. Nat Astron (2021). doi.org/10.1038/s41550-021-01450-9
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