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A single supernova flare appeared in four places in the sky — due to a galaxy that bent its light.
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A distant lamp shines through uneven glass. The glass bends the rays, and instead of one flame on the wall, several appear — some brighter, some fainter. In much the same way, a massive galaxy bends the light of an exploded star. There was one explosion, but the images show four glowing points.
Astronomers noticed the first point by chance: in data from the ZTF telescope, a blue speck appeared near a distant galaxy. Then spectroscopy showed the light had been traveling for more than eight billion years. Images from the James Webb Space Telescope resolved the picture into details: two points very close together, a third thirty times fainter on the other side of the galaxy. The fourth, faintest one was found in spectrograph data.
Light from the explosion traveled different paths, so each image shows the supernova at a different age. The faintest point is the "youngest": its light arrived first, while the bright ones came later.
The most astonishing part is the amplification. The gravitational lens acted like a magnifying glass: one of the points turned out to be more than a hundred times brighter than it should be. To explain such brightness, an amplification of about 250 times was needed. For comparison, it's as if a candle on the far shore of a lake suddenly blazed like a searchlight.
Such "multiplied" supernovae are rare. They serve as natural rulers: knowing how much light was delayed in each image, one can measure the rate at which the universe is expanding.
So far, the exact time delay hasn't been measured: the first image is too faint to catch the moment of its appearance. But spectra taken at different times offer hope: by comparing how the color and brightness of each copy changed, astronomers can calculate how much the galaxy bent the different paths. This is another step toward a precise map of the invisible mass that governs the motion of everything in the universe.
Light from this supernova traveled to us for more than 8 billion years — over half the age of the universe.
In the movie "Interstellar," the characters fly near a black hole that warps the light of stars — here it's the same effect, but the lens is an entire galaxy.