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Did dark matter cause early supermassive black holes?

One of the biggest mysteries in cosmology may have been solved, reports Michael Lucy.

A computer simulation showing a supermassive black hole at the heart of a galaxy.
A computer simulation showing a supermassive black hole at the heart of a galaxy.

The earliest gigantic black holes in the universe were seeded by dark matter, new research has found.

Ancient supermassive black holes that existed less than a billion years after the Big Bang have long presented a puzzle: how did they get so big so fast? A solution may be at hand.

According to a paper published in Science by Shingo Hirano of the University of Tokyo and colleagues, who have conducted extremely detailed simulations of black hole formation, supersonic streams of gas and vast clumps of dark matter in the early universe may hold the key.

“The origin of the monstrous black holes has been a long-standing mystery and now we have a solution to it,” says Naoki Yoshida, one of the researchers.

How fast a black hole can grow depends on how big it already is. This poses a problem: if it should take more than a billion years to grow a black hole with 10 billion times the mass of the Sun, how come we see such gigantic black holes when the universe itself was less than a billion years old?

Some proposals have suggested that they formed from the remnants of the earliest stars, or directly from the collapse of large clouds of gas, or even from the collisions of smaller black holes. These proposals have difficulty achieving the required black hole mass, or require very particular conditions.

Another idea is that such massive black holes must have grown from seed black holes that were themselves extremely large. But this only kicks the question further down the road. Where did the large seed black holes come from?

According to Hirano’s team, fast relative motion between gas and dark matter may have prevented the formation of stars in some places in the early universe. In these places, dark matter would clump together until it was large enough for its gravity to draw in streams of supersonic gas created by the Big Bang, forming a dense cloud of turbulent gas.

These conditions are ideal to form a proto-star that could grow much larger than usual in a very short period of time without losing much energy as radiation.

“Once reaching the mass of 34,000 times that of our Sun, the star collapsed by its own gravity, leaving a massive black hole,” says Yoshida. “These massive black holes born in the early universe continued to grow and merge together to become a supermassive black hole.”

Their simulation also accurately predicts the approximate number of supermassive black holes in the Universe: around one per three billion cubic light years.

While the model is promising, it will require further study and comparison with the large numbers of ancient supermassive black holes that are expected to be found when NASA’s James Webb Space Telescope launches in 2018.

Michael Lucy is features editor of Cosmos.
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