Scientists Solve the Mystery Behind LIGO’s “Forbidden” Black Hole Pair

Late in 2023, LIGO and Virgo detected an extraordinary event – a collision between two massive black holes, each weighing around 100 times more than the Sun. By the current understanding, stars of this size should explode entirely, leaving no black holes behind, which creates what scientists call a 'mass gap'. But this event, dubbed GW231123, appeared to challenge that idea. Recently, though, some computer simulations have shown that stars that spin quickly and have strong magnetic fields can actually collapse into black holes, even when they're in that 'forbidden' mass range.

The forbidden merger

According to the observatories' records in November 2023, GW231123 was a gravitational-wave signal from two merging black holes of roughly 100 and 130 solar masses. This put them in the 'mass gap' (70–140 solar masses), where stars should end in a violent pair-instability supernova, leaving no black holes. Experts thus called these black holes "forbidden," because theory predicted no black hole could survive from a star of that size. Finding this double merger was a surprise, defying previous expectations.

Spinning stars and magnetic fields

New simulations by researchers at the Flatiron Institute made them discover that everything is different with rapid rotation. When a huge star is rotating fast, it forms a rotating disk around the black hole that is being formed. The magnetic forces in this disk disperse the gas of the star to a large extent, and hence the black hole is left significantly smaller compared to the core. This leads to a black hole moving into the mass gap. This naturally created a black hole of greater mass and slower spin and another, lighter, faster-spinning black hole - just what GW231123 had predicted.