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Curved Space Lets Astronomers See 1 Supernova Explosion 3 Times — and They Predict a 4th in 2037

Galaxy clusters bend light and space, creating delays in observing phenomena around them.

A supernova that lit up a corner of the universe 10 billion years ago is proving Albert Einstein right, because astronomers have had three separate opportunities to see the explosion. And a fourth one will come 16 years from now.

Einstein’s general theory of relativity predicts that the light from the stellar detonation traveled in every direction at once through empty space that’s curved, not flat. That means its trip to Earth can take any number of paths — or several at once.

Gravity, Einstein wrote, is the result of space and time being “warped” by the presence of mass. It’s this curving of space, caused by heavy objects, that makes planets spin around stars. It also bends beams of light, something astronomers call “gravitational lensing.”

An international team of astronomers observed and recorded it by a tracking the supernova called “SN-Requiem.” Three images of a distant galaxy spotted the explosion, each from a different viewpoint. But in two other images taken at the same time, the explosion had not yet occurred.

By examining how galaxies are distributed within clusters and how curved space distorts the images they see, astronomers can calculate the delay in receiving the images. This has led to a remarkable prediction: light from a single event can reach us many times, sometimes decades apart.

A color composite of a supernova. It took 10 billion years for the light from SN-Requiem to be observed on Earth. (NASA/STScI/J. DePasquale; Las Cumbres Observatory)

“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,” said Gabriel Brammer of the Cosmic Dawn Center at the University of Copenhagen, co-author of the study published in Nature Astronomy. He said the fourth supernova sighting “is roughly 21 years behind, which should allow us to see the supernova explode one more time, sometime around 2037.”

Galaxy clusters, comprising hundreds or thousands of galaxies, are the heaviest of all structures in the universe. They bend light from more distant galaxies behind them so drastically that they appear to be in a completely different location than where they actually are.

Since light can take several paths around a galaxy cluster, astronomers can see objects in more than one place at once using the infrared range of the Hubble Space Telescope. Light takes longer on some routes around a galaxy cluster than others, and stronger gravity results in a slower trip — another consequence of relativity. This staggers the amount of time needed for light to reach a human observer, producing different images.

By analyzing the Hubble data in 2019, researchers noted three bright light sources that had disappeared since they showed up in a previous set of images, captured in 2016. It turned out that the telescope had captured three separate images of a single star’s explosive death.

A still image from the simulation of gas in space shows a large cluster of galaxies, 6 million light-years across. (Anglés-Alcázar et al. 2021, ApJ, 917, 53.)

If astronomers are able to see the SN-Requiem explosion again in 2037, it will confirm their understanding of gravity and also offer clues to the mystery of the universe’s expansion.

“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,” said Brammer.

Dark matter and dark energy are believed to make up 95 percent of the universe, but astronomers can only see the other 5 percent. “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,” said Brammer.

Edited by Siân Speakman and Kristen Butler

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