What occurs when the universe plays a cosmic curveball, breaking down long-standing assumptions about how black holes form? On 23 November 2023, the LIGO-Virgo-KAGRA Collaboration observed a gravitational wave signal GW231123 that would become an instant classic in the field of astrophysics. This observation was the first-ever merger of two black holes, both around 100 and 140 solar masses in mass, into a new black hole of around 225 solar masses the largest ever detected using gravitational waves.
This discovery did more than break a record. It forced astrophysicists into a dilemma because both parent black holes are in the so-called “pair-instability mass gap” a gap of about 60 to 130 solar masses where, stellar evolution theory says, black holes are not supposed to exist. The discrepancy comes from the physics of pair-instability supernovae: stars with carbon-oxygen cores in this mass range experience catastrophic explosions that destroy all remnant, in effect eliminating black holes for this range (ADS, MNRAS). That GW231123’s black holes challenge this theoretical limit, however, indicates either a narrower gap than supposed or newly discovered, exotic formation pathways.
The secret to this cosmic enigma could be hidden in the conditions under which these black holes first arose. One theory that’s getting a lot of attention is the hierarchical merger model: black holes formed through previous mergers within packed stellar clusters, like globular clusters, can merge once more, creating increasingly heavier and faster-spinning black holes (arXiv, Astrobites). This is reinforced by the fact that the GW231123 black holes were revolving at levels close to the theoretical maximum specified by Einstein’s general relativity. According to Charlie Hoy of the University of Portsmouth, “The black holes appear to be spinning very rapidly near the limit allowed by Einstein’s theory of general relativity. That makes the signal difficult to model and interpret. It’s an excellent case study for pushing forward the development of our theoretical tools”.
The black holes’ fast spins and high masses test astrophysical theory and technology for detecting them. The LIGO, Virgo, and KAGRA observatories use laser interferometry detecting tiny spacetime distortions produced by gravitational waves. As Caltech’s Sophie Bini pointed out, “This event pushes our instrumentation and data-analysis capabilities to the edge of what’s currently possible. It’s a powerful example of how much we can learn from gravitational-wave astronomy—and how much more there is to uncover”.
Going further into the mass gap, recent hydrodynamical simulations have probed how star collisions in dense clusters could create black holes within this forbidden mass range. For instance, the collision of an evolved star with a carbon-oxygen core and a main-sequence star may produce a remnant so large that it can escape the pair-instability regime and directly collapse into a black hole. These simulations demonstrate that as much as 12 percent of the original mass may be lost in such impacts, with the remnant’s chemical makeup and structure changed in ways that can allow for black hole creation in the gap.
The GW231123 event also emphasizes the role of black hole spin distributions in identifying their sources. Hierarchical cluster mergers should end up with black holes with spins converging to 0.7, but the resulting distribution is contingent on processes such as recoil kicks velocities induced by asymmetric emission of gravitational waves during mergers. The kicks can expel black holes from clusters, skewing the remaining population to merge again.
As the gravitational-wave detectors become increasingly sensitive and theory continues to progress, the field of astrophysics is poised to gain greater understanding about how black holes are born, grow, and rotate. GW231123 is not only a record-smasher it is a challenge to the very framework used to understand the birth and death of the cosmos’s most mysterious objects.
