How black holes grow

As different as they may seem, black holes and Las Vegas have one thing in common: What happens there stays there -- much to the frustration of astrophysicists trying to understand how, when and why black holes form and grow. Black holes are surrounded by a mysterious, invisible layer -- the event horizon -- from which nothing can escape, be it matter, light or information. The event horizon swallows every bit of evidence about the black hole’s past.

“Because of these physical facts, it had been thought impossible to measure how black holes formed,” said Peter Behroozi, an associate professor at the University of Arizona Steward Observatory and a project researcher at the National Astronomical Observatory of Japan.

Together with Haowen Zhang, a doctoral student at Steward, Behroozi led an international team to use machine learning and supercomputers to reconstruct the growth histories of black holes, effectively peeling back their event horizons to reveal what lies beyond.

Simulations of millions of computer-generated “universes” revealed that supermassive black holes grow in lockstep with their host galaxies. This had been suspected for 20 years, but scientists had not been able to confirm this relationship until now. A paper with the team’s findings has been published in Monthly Notices of the Royal Astronomical Society.

“If you go back to earlier and earlier times in the universe, you find that exactly the same relationship was present,” said Behroozi, a co-author on the paper. “So, as the galaxy grows from small to large, its black hole, too, is growing from small to large, in exactly the same way as we see in galaxies today all across the universe.”

Most, if not all, galaxies scattered throughout the cosmos are thought to harbor a supermassive black hole at their center. These black holes pack masses greater than 100,000 times that of the sun, with some boasting millions, even billions of solar masses. One of astrophysics’ most vexing questions has been how these behemoths grow as fast they do, and how they form in the first place.

To find answers, Zhang, Behroozi and their colleagues created Trinity, a platform that uses a novel form of machine learning capable of generating millions of different universes on a supercomputer, each of which obeys different physical theories for how galaxies should form. The researchers built a framework in which computers propose new rules for how supermassive black holes grow over time. They then used those rules to simulate the growth of billions of black holes in a virtual universe and “observed” the virtual universe to test whether it agreed with decades of actual observations of black holes across the real universe. After millions of proposed and rejected rule sets, the computers settled on rules that best described existing observations.

Science Daily