'Impossible' Black Hole Merger May Be a Trick of Warped Spacetime

A "forbidden" merger between two massive black holes, detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) on Nov. 23, 2023, may not have been as extreme as first thought, according to new research. The signal, designated GW231123, appeared to come from a black hole with 140 times the mass of the sun colliding with another holding 100 solar masses, both spinning faster than existing models of stellar evolution can easily explain.

The new study, published in the Astrophysical Journal Letters, suggests the unusual masses were an illusion caused by gravitational lensing. Predicted by Albert Einstein's 1915 theory of general relativity, gravitational lensing occurs when light — or, the team argues, gravitational waves — passes a massive foreground object that warps spacetime and bends its path, which can magnify and split the signal.

"Like light, gravitational waves can also be deflected, magnified and split into multiple signals by massive objects," said team member Miguel Zumalacarregui, a group leader in the Astrophysical and Cosmological Relativity Department at the Albert Einstein Institute (AEI). The researchers built a mathematical model and software to test whether GW231123 could be explained this way.

Team member Srashti Goyal, who was based at the AEI during the research, said that if GW231123 was deflected by a compact object of about 190 to 850 solar masses, or by an extended structure such as a globular cluster, the high masses could be explained without the black holes needing to spin unusually fast. Modeled this way, the merger involved a system with a total mass of 140 solar masses rather than the 240 solar masses first calculated.

The team does not yet know what object might be responsible for the lensing, and Zumalacarregui noted that individual compact lenses with 100 to 1,000 solar masses should be exceedingly rare. The researchers say they cannot yet conclusively say GW231123 is the first gravitationally lensed gravitational-wave signal, and confirming future examples will require upgrades to detectors such as LIGO.