That Black Hole Merger That ‘Shouldn’t’ Exist’? Scientists Propose a Wild Explanation

When LIGO broke news of an unintelligibly large black hole merger earlier this year, physicists were stunned but trusted they’d find an explanation someday. They probably didn’t expect the answer this soon, however.

But just as the supposedly impossible merger took place, a possible explanation for it has arrived surprisingly quickly. Astronomers ran different simulations of how a massive star could collapse into black holes that are of a smaller size than expected—including within a “mass gap†where black holes aren’t supposed to exist. The new analysis, published on November 10 in The Astrophysical Journal Letters, demonstrates how magnetic fields can trim some of the mass from black holes, meaning black holes we thought were impossible can actually exist and probably form more often than scientists realized.

“No one has considered these systems the way we did; previously, astronomers just took a shortcut and neglected the magnetic fields,†said Ore Gottlieb, an astrophysicist at the Flatiron Institute’s Center for Computational Astrophysics and the study’s lead author, in a statement. “But once you consider magnetic fields, you can actually explain the origins of this unique event.â€

A “forbidden†merger

Earlier this summer, the LIGO Collaboration released information about GW231123, a gravitational wave signal of two massive galaxies colliding and merging. Gravitational waves—ripples in spacetime from cataclysmic cosmic events—allow researchers to grasp the key properties of black holes without having to rely on light-based sources.

What was shocking about GW231123 was that the merger produced a black hole so gigantic—more than 225 times the mass of our Sun—that its sheer size was “forbidden†according to standard cosmological models, Mark Hannam, LIGO member and physicist at Cardiff University, explained in a previous statement.

It also didn’t make sense how the two black holes, each 137 and 103 times the mass of the Sun, managed to keep themselves together while spinning at 400,000 times the speed of Earth’s rotation. These masses also lie within an infamous “mass gap†for black holes that emerge from massive stars, adding to the mystery.

The destructive collapse of gigantic stars, known as pair-instability supernovas, rarely leaves anything behind. The resulting stellar graveyard prevents the formation of black holes in the mass range of 70 to 140 times the Sun’s mass, Gottlieb explained. This is what’s known as the “mass gap.â€

Cracking the impossible

The team tackled the mystery of these mass gaps by running simulations in two separate stages to test the feasibility of the two black holes from the GW231123 merger. Specifically, the team traced the entire lifespan of a black hole, starting from the birth of a giant star 250 times the mass of the Sun.

By the time this hypothetical star had burnt enough hydrogen to become a supernova, it had slimmed down to around 150 times the Sun’s mass—just above the mass gap. The second stage of the simulations was more complex, tracing the mass, spin, and magnetic field of the black hole following the supernova. This was when the anomaly emerged.

As the dying star spiraled toward an explosive death, the magnetic fields surrounding the stellar graveyard ejected some of the debris away from the black hole at nearly the speed of light. This slight ejection shaved off some of the mass for the final black hole—leaving the final product within the mass gap. Additional simulations revealed that, in extreme cases, the influence of magnetic fields could knock out up to half of the star’s original mass to produce a much smaller black hole, the study noted.

“We found the presence of rotation and magnetic fields may fundamentally change the post-collapse evolution of the star, making black hole mass potentially significantly lower than the total mass of the collapsing star,†Gottlieb said.

But wait, there’s more

The findings challenge previous beliefs that a black hole’s final mass generally matches that of the star it came from. There may be different outcomes for different stars, the researchers admitted in their paper, but the simulations nonetheless present one possible scenario for GW231123.

Still, as the researchers concede, this is just a simulation—an approximation of real-life conditions. Looking ahead, the team plans to search for real black holes formed under similar conditions to GW231123. Supernovas and the black holes that follow are also incredibly energetic events, producing other astrophysical phenomena such as gamma-ray bursts or various energy transients. These could act as signatures to find promising black holes, the paper suggested.

This finding is a remarkable mix of something that both proves and refutes astrophysical consensus: collapsing stars can produce black holes that fit inside the mass gap, and the masses of black holes don’t need to closely match that of the source star. The simulation is based on well-understood theoretical principles, but its results suggest something contrary to what researchers have believed about black holes. In a way, it’s a reminder that the universe is far more complex than we could ever imagine.