Amazing stuff!
"How do you measure the distance to far off cosmic objects? That question is key to calculating the speed of the universe’s expansion and hence understanding its evolution and eventual fate. The rate of cosmic expansion, known as the Hubble constant, is so important for cosmologists that the disagreement among researchers over its value has its own name: the Hubble tension.
Astronomers measure it one way, using stars or supernovae with predictable brightness. Cosmologists have another way, studying ripples in the echo of the Big Bang and winding the clock forward to today. The two techniques have become increasingly precise, but they steadfastly disagree with each other.
A third method is needed to break the deadlock . That may come through the magic of gravitational lensing, which can cause a supernova—a star exploding at the end of its life—to appear to explode again and again. If a supernova is situated behind a large mass, such as a galaxy or cluster of galaxies, then as its light passes by the mass, its gravity bends the light along different paths, producing multiple images that show the explosion at different times when viewed from Earth. Using the time delays and path lengths, researchers can calculate the distance to the supernova and so calculate the Hubble constant.
Only a handful of such lensed supernovae have been found so far, but several upcoming survey telescopes, including the Vera C. Rubin Observatory, are expected to find them by the dozen and, hopefully, to ease the Hubble tension."
"... The idea goes back to the early 1960s, when gravitational lenses had only been theorized. But Sjur Refsdal, a graduate student at the University of Oslo, suggested a way to calculate how such a lens would bend light, using the same geometric tools used to model the paths of light through a glass lens. His thesis evaluators weren’t convinced—until he got the result published in the Monthly Notices of the Royal Astronomical Society in 1964. In the same issue, Refsdal also proposed a way to put his technique to work. He suggested time-delayed images of a lensed supernova could offer a handle on the Hubble constant."
A foreground galaxy (center) acts as a lens to produce four images of a background supernova in 2014. The supernova is dubbed SN Refsdal in honor of Sjur Refsdal, who predicted this effect 50 years earlier.
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