Record Black Hole Merger May Have Looked Bigger Than It Really Was

Gravitational waves are ripples in spacetime produced when two black holes spiral into each other and merge. In November 2023, the LIGO-Virgo-KAGRA detector network picked up a signal named GW231123, produced by two black holes merging with a combined mass of around 190 to 265 times that of the sun, making it the largest binary black hole merger ever recorded, heavier than standard stellar collapse is thought to produce.

Miguel Zumalacarregui and colleagues at the Max Planck Institute for Gravitational Physics revisited that signal and suspected its scale could have been inflated by gravitational lensing, in which the gravity of another massive object bends and magnifies a wave passing near it, making the source appear stronger, and therefore more massive, than it actually is.

The team built a model that accounts for a compact object, possibly an intermediate-mass black hole, sitting within a larger galaxy-scale gravitational field along the signal's path, and tested how well it matched the distortions seen in GW231123. They found solid statistical support for the lensing explanation, with less than a 1% chance the pattern arose by coincidence. Under this scenario, the true combined mass of the merging black holes drops to a more modest 100 to 180 solar masses, easing tension with existing models of black hole formation.

The findings, published in The Astrophysical Journal Letters, suggest GW231123 may not be the record-breaker it first appeared to be, and that other exceptionally massive mergers detected so far could be similarly distorted by lensing. The researchers argue that past detections deserve a second look as gravitational-wave detectors become more sensitive.