When black holes merge with each other, they shake space-time: the collision releases gravitational waves that can be picked up by detectors such as LIGO in the USA, Virgo in Italy and KAGRA in Japan. The characteristics of these gravitational wave events provide valuable information about the black holes involved, including their mass, rotation and area.
Black holes in the “forbidden” zone
But some of these collisions puzzle astronomers: In recent years, gravitational wave detectors have repeatedly registered mergers of unusually heavy stellar black holes. Some of these appear to contradict common astronomical models because they lie in the “forbidden” mass gap of 65 to 120 solar masses.
This gap arises because the supernovae of massive stars can only produce black holes up to around 65 solar masses – this is what the astrophysical models predict. Even heavier predecessor stars are unstable and eject most of their mass before the supernova. This keeps the resulting black hole below this limit. Only with black holes of 120 solar masses or more does a different mechanism take effect: they are not formed by a star explosion, but by the direct collapse of their predecessor star into a black hole.
When two stellar black holes collide and merge, they release gravitational waves. These reveal a lot about their distance and mass. — © brightstars/ iStock
Actually, there should be no stellar black holes that weigh more than 65 and less than 120 solar masses. But according to gravitational wave measurements, they still exist.
Record breakaway GW231123 poses a mystery
One of these difficult-to-explain outliers is the gravitational wave event GW231123, which was detected in November 2023. According to the wave characteristics, two black holes of 100 and 140 solar masses merged together – real giants in their field. “This is the most massive pair of black holes we have ever observed with gravitational waves,” said Mark Hannam of Cardiff University in 2025. The lighter of the two collision partners was also exactly in the mass gap.
But the black hole formed during this merger, around three billion light-years away, was also surprising: with an estimated 232 solar masses, it also set a new mass record for such merger products. According to the gravitational wave signal, the black holes must also be rotating unusually quickly. “This event represents a real challenge to our ideas about black hole formation,” Hannam explained.
Is a gravitational lens to blame?
Now astronomers led by Srashti Goyal from the Max Planck Institute for Gravitational Physics in Potsdam may have found a solution to this puzzle. Behind the exceptional event GW231123 there are therefore not overweight black holes or new astrophysical phenomena, but a foreground effect: a gravitational lens.
“Like light, gravitational waves can be deflected by massive objects, amplified and split into several signals,” explains Goyal’s colleague Miguel Zumalacárregui. The heavy object in the foreground bends space-time and thereby changes the wavelength and shape of the gravitational waves. This can distort the information about distance and masses in a black hole merger. However, the effects of such lensing are difficult to detect.
The astronomers therefore determined whether this could have been the case with GW231123 using a newly developed mathematical model and special software. “In the case of gravitational waves, diffraction and interference effects give us an opportunity to identify and study such altered signals,” explains Zumalacárregui.
Subtle clues
And indeed: The astronomers discovered evidence that points to the distortion of the signals by a strong gravitational lens. Accordingly, a massive but compact object between the merging black holes and the Earth could have distorted the gravitational wave signals. According to the calculations, this gravitational lens on GW231123 would have had a weight of around 180 to 850 solar masses.
“If we take these gravitational lensing effects into account, then the total mass of the event would only be 100 to 180 solar masses,” report Goyal and her team. In addition, the distance to the merging black holes was also distorted by cosmic expansion. The event therefore occurred much further away.
Corrected masses and slower rotation
Taken together, this means that if these results are confirmed, it would solve some of the mysteries surrounding the outlier event GW231123. The two black holes in this collision would then both be less than 100 solar masses, while the smaller partner would probably be less than 50 solar masses, as the astronomers determined. Although it cannot be ruled out that the heavy partner is still in the “forbidden zone”, overall the event fits the picture better.
“If we assume that GW231123 was deflected and distorted by a compact object or by an extended structure such as a globular cluster, we can understand the high masses observed,” says Goyal. “In addition, the interpretation of the signal does not require unusually high rotation speeds when the lens effect is taken into account.” This would make it easier to classify this black hole merger into known scenarios.
Identity of the lens object is still a mystery
However, it is still unclear which object caused the gravitational lensing effect. This foreground object must be significantly smaller than a galaxy, but still massive and compact. “The nature of the lens remains a great mystery, as individual compact lenses with 100 to 1000 solar masses are likely to be extremely rare,” explains Zumalacárregui. “Further studies need to clarify whether such lenses can form or whether a collection of lighter objects, including stars, can explain this event.” It would be conceivable, for example, that a dense star cluster created the lensing effect.
It has not yet been clearly proven that the “collision of the giants” GW231123 was actually distorted by gravitational lensing. The significance of the current analyzes is 2.3 standard deviations and therefore below the threshold for discovery. However, the astronomers hope that the subtle signals of such lensing effects will be better and more clearly recognizable through future optimizations and upgrades of the gravitational wave detectors.
Source: Srashti Goyal (Max Planck Institute for Gravitational Physics, Potsdam) et al., The Astrophysical Journal Letters, doi: 10.3847/2041-8213/ae93b1