Lensing could explain the “impossible” GW231123 black hole merger

On Nov. 23, 2023, LIGO recorded a gravitational-wave signal labeled GW231123. Early parameter estimates implied a collision between two black holes with roughly 140 and 100 times the mass of the Sun. Those masses are unusually large for black holes formed by ordinary stellar evolution and raise questions about how such a binary could form, especially with evidence suggesting rapid spins.

Researchers propose that GW231123 might not be intrinsically so massive. Instead, the signal could have been gravitationally lensed by a massive object between the source and Earth. Gravitational lensing — a well-established effect in which foreground mass curves spacetime and alters the path and apparent brightness of background signals — affects gravitational waves in the same ways it affects light: deflection, magnification, time delays, and the possibility of multiple images.

Consequences if GW231123 was lensed

If the event was lensed, the true component masses could be substantially lower than the initially reported ~140 and ~100 solar masses. Lower masses would reduce or remove the tension with standard models of how black holes form and spin, eliminating the need to invoke exotic formation channels for this particular event. The finding would also imply there are foreground lenses strong enough to significantly amplify gravitational waves along some lines of sight.

The team presenting this interpretation emphasizes the parallel between lensing of light and lensing of gravitational waves. Miguel Zumalacárregui, identified as a member of the team and a group leader in Astrophysical and Cosmological Relativity, is quoted describing how gravitational waves can be deflected, magnified, and split by massive objects.

The lensed interpretation requires identifying the lensing configuration and testing for telltale signatures, such as repeated or time-delayed copies of the signal, or consistent population-level signatures of magnified events. The present argument is that lensing is a plausible explanation that removes the apparent need for extraordinary black-hole masses in GW231123.

Adopting a lensing explanation changes how we interpret rare, apparently extreme gravitational-wave events. It affects inferred black-hole mass distributions, the incidence of unexpectedly massive mergers, and how gravitational-wave catalogs constrain stellar and binary evolution models.

Concrete next steps researchers will likely pursue

• Search the gravitational-wave data for repeated, time-shifted copies of GW231123 that would indicate multiple lensed images.
• Cross-match the event's sky localization with galaxy and cluster catalogs to find plausible foreground lenses.
• Reanalyze the signal allowing for magnification to produce revised posterior distributions for masses and distances.

GW231123 looked implausibly massive under the standard, unlensed interpretation. Interpreting it as a lensed gravitational-wave source offers a straightforward way to reduce the inferred masses and reconcile the event with known formation processes.