New Simulations Show Black Holes Grow as Universe Expands

Back in 2016 physicists detected gravitational waves—or ripples in the fabric of spacetime—from a merging pair of black holes for the first time. Since then, physicists have detected many more black hole mergers. But there has been a caveat; the black holes have been more massive than they expected. Now, Gizmodo reports that a team of researchers says it may have an answer for this size discrepancy. Black holes grow in mass in lockstep with the expansion of the universe.

A team of researchers from the University of Hawaii at Mānoa, the University of Chicago, and the University of Michigan at Ann Arbor developed the hypothesis. It stands as the latest attempt at explaining the mysterious size difference. Previous explanations have been unable to provide a sound theory for a causative, physically viable phenomenon.

An illustration of a pair of black holes merging, which represents the possibility that merging black holes may grow in size in lockstep with the universe's expansion.

The researchers’ novel hypothesis, which they outlined in a study recently published in The Astrophysical Journal Letters, eschews the tendency to understand merging black holes within the context of a static universe. A common underpinning that simplifies Einstein’s equations. (Einstein’s general theory of relativity predicted the existence of gravitational waves, as well as an expanding universe, back in 1916.)

This simplification has meant, in turn, that predictions for black hole masses only work for short periods of time. A forgivable tradeoff as the spacetime ripples detectable by LIGO—the Laser Interferometer Gravitational Wave Observatory (below)—and Virgo, another instrument physicists use to detect gravitational waves, only last a few seconds.

Despite LIGO and Virgo’s brief detection periods, black hole mergers likely have a lead time of billions of years. In other words, a very long time passes between when a pair of black holes form and when they merge. During this time, the universe expands, potentially forcing black holes to grow in size as it does. Researchers refer to this phenomenon as “cosmological coupling.”

The LIGO instrument, which physicists use to detect black hole mergers, simulations of which now say they may grow in conjunction with the universe's expansion.
Caltech/MIT/LIGO Lab

To test their hypothesis, the researchers simulated the evolution of millions of pairs of large stars. The physicists then took the stellar pairs that “died” and formed black holes and linked them to the universe’s size. In the simulations, as the universe grew, so too did the masses of the black holes. This resulted in, of course, more massive black holes. As well as many more black hole mergers. When the researchers compared their findings to real LIGO and Virgo data, the predictions matched “reasonably” well.

“I have to say I didn’t know what to think [of the agreement between the simulations and the LIGO and Virgo data] at first,” study co-author and University of Michigan Professor Gregory Tarlé said in a press release. “It was a such a simple idea, I was surprised it worked so well,” the professor added.

An illustration of the expanding universe, which physicists hypothesize may increase the mass of black holes.
NASA/WMAP Science Team

Tarlé and his colleagues stress that these findings are preliminary, however. “While we used a simulated stellar population that reflects the data we currently have, there’s a lot of wiggle room,” study co-author and NASA Hubble Fellow Michael Zevin noted in the press release. “We can see that cosmological coupling is a useful idea, but we can’t yet measure the strength of this coupling,” he added. This means merging black holes’ unexpectedly huge sizes mostly remains a mystery for now.

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