Astronomers just solved the mystery of “impossible” black holes

In 2023, astronomers witnessed a dramatic event: two extraordinarily massive black holes collided roughly 7 billion light-years away. Their immense size and rapid spin defied explanation. According to existing theories, black holes like these simply shouldn't exist.

Researchers from the Flatiron Institute's Center for Computational Astrophysics (CCA) and partner institutions have now uncovered how such cosmic giants might form and eventually collide. By tracing the life cycles of the stars that gave rise to these black holes, the team discovered that magnetic fields -- long overlooked in earlier models -- play a crucial role.

"No one has considered these systems the way we did; previously, astronomers just took a shortcut and neglected the magnetic fields," explains Ore Gottlieb, an astrophysicist at the CCA and lead author of the study, which appears in The Astrophysical Journal Letters. "But once you consider magnetic fields, you can actually explain the origins of this unique event."

The 2023 Collision That Challenged Black Hole Theory

The cosmic crash, now known as GW231123, was detected by the LIGO-Virgo-KAGRA observatories, which measure gravitational waves -- the ripples in space-time produced by massive celestial motions.

At the time of detection, astronomers could not understand how such enormous and fast-spinning black holes had formed. When a massive star exhausts its fuel, it typically collapses and explodes in a supernova, leaving behind a smaller black hole. However, stars within a specific mass range experience an especially violent type of explosion called a pair-instability supernova, which destroys the star completely.

"As a result of these supernovae, we don't expect black holes to form between roughly 70 to 140 times the mass of the sun," Gottlieb says. "So it was puzzling to see black holes with masses inside this gap."

Simulations Reveal a Hidden Force at Work

One possible explanation is that black holes within this "mass gap" form indirectly, through the merger of smaller black holes. But in the case of GW231123, this seemed unlikely. Mergers are typically chaotic, disrupting the spin of the resulting black hole. Yet the two black holes involved in GW231123 were spinning near the speed of light -- the fastest ever observed -- making such a scenario improbable.

To solve the mystery, Gottlieb and his team performed a two-stage simulation. First, they modeled a massive star 250 times the mass of the Sun through its life and death. By the time it exploded as a supernova, it had burned enough fuel to shrink to about 150 solar masses -- just above the theoretical mass gap, leaving behind a black hole.

The next phase introduced magnetic fields into the picture. The model began with the remnants of the supernova: a swirling cloud of stellar debris containing magnetic fields and a newborn black hole at the center. Earlier theories assumed all the remaining material would fall into the black hole, but the new simulations painted a different picture.

How Magnetism Reshapes the Fate of a Collapsing Star

If a collapsing star is not rotating, the surrounding matter falls straight into the black hole. But when a star spins rapidly, that material forms a disk around the black hole, feeding it over time and increasing its spin. Magnetic fields, however, disrupt this process. Their pressure can blast some of the material outward at nearly the speed of light, preventing it from falling in.

This ejection of matter reduces the amount of material the black hole absorbs. The stronger the magnetic fields, the more mass gets expelled. In extreme cases, up to half of the original star's mass can be lost to these outflows. In the team's simulations, this mechanism naturally produced a black hole whose mass fell within the once "forbidden" range.

"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 says.

Linking Black Hole Mass and Spin

The results point to an intriguing relationship between a black hole's mass and how quickly it spins. Stronger magnetic fields can slow a black hole's rotation and remove more stellar mass, leading to smaller, slower black holes. Weaker fields, on the other hand, allow heavier, faster-spinning ones to form. This pattern could reveal a broader law connecting mass and spin -- a relationship that future observations might confirm.

Currently, no other known black hole systems can test this connection, but astronomers hope upcoming detections will uncover more examples like GW231123.

Bursts of Light From the Darkest Events

The simulations also predict that these magnetic processes produce bursts of gamma rays during black hole formation. Detecting such gamma-ray flashes could help confirm the theory and show how common these massive black holes really are.

If verified, these findings would not only explain an "impossible" collision but also reshape how scientists understand one of the universe's most extreme and fascinating objects.