Lensed Spacetime May Resolve Mystery of ‘Forbidden’ Black Hole Merger

A gravitational wave event previously deemed nearly impossible may have a simpler explanation rooted in the distortion of spacetime. New research indicates that the merger of two black holes detected by LIGO on November 23, 2023, likely involved objects smaller than initially calculated. The signal, designated GW231123, appeared to show a collision between black holes weighing 140 and 100 times the mass of the sun, respectively.

Such masses posed a significant problem for standard models of stellar evolution, particularly because the black holes appeared to be spinning at extraordinarily high rates. While many scientists have sought mechanisms to explain how such a rare binary system could form, the new study argues that no exotic formation story is necessary. Instead, the team proposes that the apparent massive sizes were an optical illusion caused by gravitational lensing.

First predicted by Albert Einstein’s general theory of relativity in 1915, gravitational lensing occurs when a massive foreground object warps the fabric of spacetime, bending the path of light or other waves from a background source. This effect can magnify distant objects, a technique commonly used to observe ancient galaxies that would otherwise be too faint to detect.

The researchers suggest this phenomenon also affects gravitational waves. Miguel Zumalacárregui, a group leader at the Albert Einstein Institute’s Astrophysical and Cosmological Relativity Department, explained that massive objects can deflect, magnify, and split gravitational wave signals much like they do with light.

“For gravitational waves, diffraction and interference effects give us an additional way to identify and study lensed signals,” Zumalacárregui said in a statement.

To test this hypothesis, the team developed a mathematical model and created specialized software capable of analyzing complex lensing scenarios. Lead researcher Srashti Goyal, who conducted the work while at the Albert Einstein Institute, noted that if the signal was distorted by a compact object weighing between 190 and 850 solar masses, or by an extended structure such as a globular cluster, the observed high masses could be accounted for without requiring unusual spin rates.

When the lensing factor was included in their calculations, the actual mass of the merging system was revised down to 140 solar masses, rather than the 240-solar-mass total originally theorized. This adjustment aligns the event more comfortably with existing astrophysical models.

However, the nature of the lens itself remains unknown. Zumalacárregui highlighted that individual compact lenses in the 100-to-1,000-solar-mass range are expected to be exceedingly rare. Future investigations will need to determine whether such massive lenses can form or if an ensemble of lighter objects, including stars, might explain the anomaly.

It is not yet conclusive whether GW231123 represents the first instance of a gravitationally lensed gravitational wave signal. Confirming such events will require upgraded sensitivity in detectors like LIGO. Nevertheless, the study underscores the growing potential of gravitational wave astronomy to probe violent cosmic phenomena. The findings were published on August 25 in the Astrophysical Journal Letters.