Astronomers have recently unveiled the solution to one of the most puzzling phenomena observed in the universe: the merger of two black holes that were once considered "impossible" or "forbidden" due to their unusual combination of mass and spin.
The collision, detected via gravitational waves by the LIGO–Virgo–KAGRA collaboration in 2023 (known as GW231123), involved two black holes whose characteristics seemed to defy the standard models of stellar evolution.
The Enigma of GW231123
The gravitational wave signal from GW231123 revealed two highly unusual features of the merging black holes:
"Forbidden" Mass Range (The Mass Gap): The merging black holes had masses that placed them squarely within the "upper mass gap," a range (roughly 70 to 140 times the mass of the Sun) where black holes are not expected to form directly from the collapse of a single star.
The Theory Challenge: Standard stellar theory predicts that stars massive enough to produce black holes in this range would instead undergo a catastrophic explosion called a pair-instability supernova. This explosion is so violent that the entire star is completely annihilated, leaving behind no black hole remnant at all. Seeing black holes in this range directly challenged this fundamental understanding.
Extreme, Rapid Spin: Both black holes were measured to be spinning at an extreme rate, close to the theoretical maximum speed allowed by general relativity (near the speed of light). This was puzzling because another potential explanation for the "mass gap" objects—hierarchical mergers (where smaller black holes merge to form a larger one)—typically disrupts or "scrambles" the final black hole's spin, leading to slower or misaligned rotation.
The existence of two such massive, rapidly spinning black holes colliding was considered highly improbable.
The Missing Piece: The Role of Magnetic Fields
A comprehensive set of computational simulations conducted by astrophysicists, including a team from the Flatiron Institute's Center for Computational Astrophysics (CCA), uncovered the crucial factor that earlier studies had overlooked: magnetic fields.
The simulations followed the entire lifecycle of a massive progenitor star (around 250 solar masses) through its collapse and subsequent supernova explosion to the final formation of the black hole.
How Magnetism Alters Black Hole Formation:
Spinning Star Collapse: When a rapidly spinning massive star collapses, the leftover stellar material doesn't just fall straight in. Instead, it forms a rapidly rotating disk of debris around the newborn black hole.
Magnetic Pressure and Outflow: If strong magnetic fields are present within this debris disk, they exert immense pressure. This pressure is powerful enough to drive large amounts of material away from the black hole in fast-moving outflows or jets, nearly at the speed of light.
Mass and Spin Reduction: This mass ejection process significantly reduces the amount of matter that ultimately falls into the black hole, dramatically lowering its final mass. The stronger the magnetic fields, the greater this mass-loss effect. This mechanism is key: it allows a massive star that would normally be too large to form a black hole to shed enough mass to fall into the "forbidden" mass gap. Furthermore, the magnetic fields simultaneously influence the angular momentum, connecting the black hole's final mass and spin.
A New Black Hole Formation Pattern
The simulations suggest a direct link between the strength of the magnetic field during the collapse and the resulting black hole's properties:
| Magnetic Field Strength | Final Black Hole Mass | Final Black Hole Spin |
| Strong | Lighter (in the mass gap) | Slower |
| Weaker | Heavier | Faster (near maximum) |
The black holes in GW231123, being both very massive (though in the 'gap') and extremely fast-spinning, could be explained by a collapse scenario involving weaker, but still significant, magnetic fields. The weak fields allowed for more mass to be swallowed (making them heavier) and resulted in a faster spin than in the strong-field case, but still ejected enough material to push them into the mass gap.
🌟 Future Observational Tests
This new model provides a concrete hypothesis for the formation of these rare, massive black holes and suggests a way to confirm it through future observations:
Gamma-Ray Bursts: The simulations predict that the formation of black holes in this specific mass range through this magnetically-driven process should be accompanied by short, intense bursts of high-energy radiation known as gamma rays.
Correlating Events: Detecting a gravitational wave event like GW231123 that is coincident in space and time with a gamma-ray burst would provide powerful, direct evidence supporting the new formation mechanism.
This solution not only resolves the long-standing puzzle of the "forbidden" mass gap but fundamentally transforms our understanding of how the most massive stars end their lives and seed the universe with colossal black holes .