These Supermassive Black Holes are About to Collide

4 min readMar 4, 2022

by Jessica Hall

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Scientists have discovered two supermassive black holes, locked together in a final, terrible spiral. They’re about to collide. And when they do, it will shake the fabric of spacetime itself.

Combing through decades of radio telescope observation data, a team of Caltech astronomers discovered a radio pattern from the deep sky unlike anything ever observed before. It was a flickering point of light, a blazar some nine billion light-years away. Every five years, it waxed and waned in brightness in a perfect sine wave, like clockwork. But that’s not what made it special. What made it special is where the signal diverged from the pattern. Over nearly fifty years, this point of light had obeyed a clockwork cycle of five-year pulses — except for the twenty years where it didn’t.

Five other observatories confirmed the readings, including the University of Michigan Radio Astronomy Observatory, MIT’s Haystack Observatory, the National Radio Astronomy Observatory (NRAO), Metsähovi Radio Observatory in Finland, and NASA’s Wide-field Infrared Survey Explorer (WISE) space satellite. This was no error.

While pooling data showed that the oscillation of the light formed a clear and distinct pattern, the mystery remained. In the middle of that elegant sine wave, there’s a big wobble, a dimming that lasted two decades. The fluctuation didn’t match any known astronomical phenomenon. What could explain this departure from the pattern?

“When we realized that the peaks and troughs of the light curve detected from recent times matched the peaks and troughs observed between 1975 and 1983, we knew something very special was going on,” says Sandra O’Neill, lead author of a new study describing the astronomers’ observations. But its identity would come from a surprising direction: not astrophysics, but simple orbital dynamics.

When black holes collide


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