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Black holes can multiply on their own in massive star clusters, MIT researchers say

A team led by MIT researcher Carl Rodriguez revealed how binary black holes form within dense star clusters.Northwestern Visualization/ Carl Rodriguez

Black holes have always been ominous — a strangely powerful, yet invisible force not even light can escape. Now, scientists have uncovered further details on how they can merge on their own.

An international research team led by MIT astrophysicist Carl Rodriguez recently reported that dense star clusters — which are packed with hundreds of thousands to millions of stars — might be a breeding ground for massive black hole mergers.

The group worked with a Northwestern University supercomputer called Quest to create simulations of star clusters, finding that nearly half of binary black holes merged in these regions, creating new ones even more massive. according to the study, which was published Tuesday in the journal Physical Review Letters.


“We can use these detectors as observatories, detect more black holes, how massive they are, and figure out how stars live and die and how these things evolve over cosmic history,” Rodriguez said. “We were the first to show you could get a large number of binary black holes from these mergers inside the stellar cluster itself.”

With Quest’s help, the researchers discovered stellar clusters actually might help foster a new generation of black holes that could measure up to 50 to 130 solar masses, much larger than ones created by single star collapses, which is how black holes typically form.

“If you have black holes forming binaries inside star clusters, they can create a new black hole that’s more massive,” Rodriguez said. “And the new massive one can find itself another companion and potentially merge again.”

The star, or globular, clusters that house these mergers are found in most galaxies — the Milky Way holds about 200, but bigger galaxies can host tens of thousands, an MIT statement said.

The researchers simulated 24 globular clusters, ranging in size from 200,000 to 2 million stars, to study the interactions within them during the course of 12 billion years, the statement said. These simulations led to the discovery of what unfolds when two black holes join together.


The merging process is a bit complex.

Because Isaac Newton’s theory of gravity didn’t address the existence of gravitational waves when considering black hole activity, scientists initially thought black holes would cross paths without being affected at all. But after factoring gravitational waves into their star cluster simulations — an idea prompted by Albert Einstein’s theory of general relativity — the researchers found that when one black hole passes another, something happens.

“This can subtract enough energy from the system that the two black holes actually become bound, and then they will rapidly merge,” Rodriguez said in the MIT statement.

When binary black holes form, they capture one another and bring in other nearby black holes. It’s like a feedback process, Rodriguez said. They emit gravitational waves, which shrink the orbit, which then cause the black hole to get closer and emit stronger waves.

Rodriguez said these findings have begun to set the stage for intriguing future scientific studies for other gravitational wave detectors, such as LIGO, a physics observatory operated by MIT and the California Institute of Technology.

“It’s very interesting for what LIGO will continue to detect,” he said. “As they upgrade the LIGO instrument, it’ll get more sensitive to a larger region of space and detect more black hole mergers . . . It’s a fun time to be in my job.”


Elise Takahama can be reached at elise.takahama@globe.com. Follow her on Twitter @elisetakahama.