A Physicist Has Solved How Black Holes Reflect the Universe

And it could provide replays of massive supernovas.
Brad Bergan
Stellar material flung away from a black hole swallowing a star.NASA / JPL-Caltech

When a black hole passes between us and a distant galaxy, the galaxy may be fine, but its image may never escape.

Since light rays may curve around the event horizon of a black hole several times, distant observers may witness multiple versions of the same object. And while this was known for decades, a student of physics at the Niels Bohr Institute has produced the first-ever mathematical expression that adequately models how black holes reflect light from the universe, according to a recent study published in the journal Scientific Reports.

And this accomplishment might one day provide scientists with replays of colossal supernovas.

Black holes reflect the universe in strange ways

Black holes are the result of massive stars collapsing into a singularity of immense gravity so powerful that not even light can escape. The gravity is so immense that the fabric of space-time itself is modified, warped, and altered to exhibit strange behaviors the closer one comes to the event horizon, where space can curve so drastically that light rays are deflected, sometimes so much that a ray of light may traverse the circumference of the black hole multiple times before escaping. If it can.

This is why, when we look to a galaxy (for example) on the opposite side of a black hole, we might see the same image of it several times, although increasingly distorted. When a distant galaxy shines (as always), it does so in all directions. But when some of that light treads too close to a black hole and some of its light is deflected, some of it comes even closer to the hole, orbiting it once before it flings out in our direction. And if we observe space closer to the black hole, there are more and more versions of the galaxy as we approach the event horizon. This left a question in the minds of physicists, namely: how much closer to the black hole must one look before one image of the galaxy is replaced by another? According to earlier studies from 40 years ago, it's roughly 500 times closer, also referred to via the "exponential function of two pi," expressed as e2π.

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However, calculating this process remained too complicated to solve until very recently, which left a mystery surrounding why it had to be exactly this factor. But a master's student named Albert Sneppen of the Cosmic Dawn Center has given scientists the long-sought answer. "There is something fantastically beautiful in now understanding why the images repeat themselves in such an elegant way," said Sneppen in a Phys.org report. "On top of that, it provides new opportunities to test our understanding of gravity and black holes." Beyond the simple intellectual pleasure of proving a theory with elegant mathematics, this development also helps us better grasp how black holes reflect the universe. And Sneppen's new method allows for generalization, applying to all types of black holes.

"It turns out that when the [black hole] rotates really fast, you no longer have to get closer to the black hole by a factor of 500, but significantly less," explained Sneppen in the report. "In fact, each image is now only 50, or 5, or even down to just 2 times closer to the edge of the black hole." In other words, the more a black hole rotates, the more room there is for "extra" images of a background cosmic object. Among other things, this means the light from a background supernova could be witnessed again and again in the presence of an intersecting black hole with a rapid spin.

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