Max Planck team proposes a gravitational lens to shrink GW231123's forbidden black holes

The lens has to be heavier than the black hole it explains away. Srashti Goyal, the paper's first author and a postdoctoral researcher at the Max Planck Institute for Gravitational Physics, said the observed masses make sense if the signal was "deflected and distorted by a compact object of about 190 to 850 solar masses" ^[13]. That floor sits 60 solar masses above the heavier of the two progenitors ^[15]. Goyal's second option is "an extended structure such as a globular cluster" ^[14].

The range is wide: 850 divided by 190 is about 4.5, so the proposal constrains the foreground object only loosely ^[16]. Live Science's account of the study says the signal could have been magnified enough to greatly exaggerate the masses, without giving a magnification factor ^[18]. The masses themselves are quoted as round numbers, and 100 plus 130 is exactly the 230 solar masses given for the remnant ^[17].

Magnification, as described, is an argument about the masses. The other oddity in the event is the spin: both black holes were turning much faster than expected when they merged ^[6], and the explanation Goyal gives covers "the observed high masses" ^[13].

The competing explanation has its own problem. A study released last year proposed that black holes of this size could form when larger stars collapse instead of exploding as supernovas, an idea that runs against a lot of earlier observational evidence ^[7]. So the choice on the table is between a formation channel that conflicts with observations ^[7] and a lensing regime that, until this work, no one had found evidence for in gravitational waves ^[9]. Lensing of light, by contrast, has been measured many times over ^[9].

What would settle it is a signature in the waveform itself. "Like light, gravitational waves can also be deflected, magnified and split into multiple signals by massive objects," study co-author Miguel Zumalacarregui, an astrophysicist at the same institute, said in a statement ^[10]. "For gravitational waves, diffraction and interference effects give us an additional way to identify and study lensed signals," he said ^[11].