A gravitational wave signal detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) on November 23, 2023—named GW231123—may not be from the collision of two unusually massive black holes, as first believed. Recent research suggests that the signal could instead be the result of gravitational lensing, a phenomenon where the path of gravitational waves is bent by a massive object, making the source appear more massive than it actually is. Initially, the signal appeared to indicate a merger between a black hole 140 times the mass of the sun and another about 100 times as massive. However, new analysis suggests that the apparent size of these black holes may be an illusion caused by gravitational lensing. This effect, similar to how light from distant galaxies can be magnified by the gravity of intervening objects, can also distort and amplify gravitational wave signals from faraway sources. A research team led by Miguel Zumalacárregui from the Albert Einstein Institute (AEI) developed a mathematical model and specialized software to explore how gravitational lensing might affect the signal. Their analysis found that if GW231123 was distorted by a compact object with a mass between 190 and 850 times that of the sun, or by a larger structure like a globular cluster, the observed high masses could be explained without assuming the black holes had unusually high spins. According to the team's findings, the actual mass of the system involved in the merger was likely around 140 solar masses, rather than the 240-solar-mass system previously theorized. However, the nature of the massive object responsible for the lensing remains unclear. Zumalacárregui noted that individual compact lenses with masses between 100 and 1,000 solar masses should be extremely rare. Future research will need to determine whether such objects can form or whether a group of lighter objects, such as stars, could explain the event. The study was published on August 25 in the Astrophysical Journal Letters. The researchers emphasized that their findings highlight the potential of gravitational wave astronomy to study the universe's most violent events. They also noted that future discoveries of similar signals will require more sensitive detectors, like upgraded versions of LIGO, to better distinguish between real mergers and lensing effects.