2 supermassive black holes may collide 100 years from now ‪—‬ and Earth would feel it

Astronomers may have discovered an extreme pair of light-spewing black holes that are spiraling toward an enormous collision — the effects of which could be felt in the next century.

Using decades of radio telescope observations, the astronomers studied an ultrabright object that was previously thought to be a blazar — a glowing core of a galaxy usually powered by a black hole — some 500 million light-years from our solar system. The observations revealed a hidden jet of energy that suggests the intensely bright object is actually two black holes on the verge of colliding, perhaps less than 100 years from now.

"We expect one (merged) black hole to remain," study co-author Silke Britzen, an astronomer at the Max-Planck Institute for Radio Astronomy, told BBC Science Focus. "I am really curious to observe how this 'dance' will continue."

The findings were published March 27 in the journal Monthly Notices of the Royal Astronomical Society.

Finding two black holes

Blazars are some of the most luminous objects in the universe. They're classified as active galactic nuclei — actively feeding objects at the centers of galaxies, usually powered by supermassive black holes — and typically shoot jets of high-energy radiation toward Earth. Usually, a central black hole is the source for this jet, but in the case of the blazar in the galaxy Markarian 501, something didn't quite add up.

For years, astronomers had observed different orientations of the jet using radio telescope data, making it difficult to determine if its core really did harbor a supermassive black hole. To answer this question, the researchers analyzed over 83 datasets from the Very Long Baseline Array, an international network of 10 radio telescopes.

The results revealed that, instead of one large jet, there was also a second jet looping counterclockwise around the blazar's center. The team believes each of these jets is powered by a supermassive black hole, each weighing between 100 million and a billion times the sun's mass.

"Realising that [there] was a second jet was awesome," Britzen told BBC Science Focus. "For me it was like: that's how it works? I was so amazed and overwhelmed — and wanted to tell everybody what we just found."

In June 2022, the two black holes lined up perfectly so that the primary black hole's gravity bent the light emitted by the second jet into a near-perfect circle known as an Einstein ring. Thanks to a phenomenon called gravitational lensing — a sort of natural magnifying glass created by intense gravitational forces — this finding adds further evidence to the idea that the blazar is powered by a pair of supermassive black holes.

"Since these jets are directed towards us, an Einstein ring supports the scenario," Britzen said.

The two black holes are thought to circle each other clockwise about once every 121 days and are separated by only 250 to 540 times the distance between Earth and the sun — relatively close in the world of astronomy. Gradually, this distance will close until the two objects ultimately merge.

The researchers think that when the doomed black holes do inevitably collide, they will release gravitational waves — ripples in the fabric of space-time unleashed by the most intense events in the universe — that could be more powerful than those from previously studied black hole mergers. If that's the case, gravitational wave detectors on Earth will pick up the signal, offering new clues about the properties of the original black hole pair.

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