Gravitational wave signal GW231123, detected by the Laser Interferometer Gravitational-Wave Observatory on Nov. 23, 2023, originated from a black hole merger that may not have been physically impossible after all, according to research published on Aug. 25 in the Astrophysical Journal Letters. While initial models suggested a collision between a 140-solar-mass black hole and a 100-solar-mass black hole—defying standard stellar evolution models—a team of scientists at the Albert Einstein Institute propose that gravitational lensing made the objects appear larger than they actually were.
The Enigma of GW231123 and the Mass Gap
When the LIGO detector captured GW231123, the signal immediately puzzled researchers. Standard stellar theory dictates that stars massive enough to produce black holes in this range should instead undergo a pair-instability supernova, completely vaporizing the star without leaving a black hole remnant. Furthermore, the objects involved appeared to be spinning rapidly.
How Gravitational Lensing Alters Gravitational Waves
Rather than relying solely on stellar collapse mechanics, the Albert Einstein Institute research team investigated whether an optical illusion was at play. According to general relativity, massive foreground objects warp spacetime, curving the path of light and producing gravitational lensing. Team member Miguel Zumalacárregui stated that gravitational waves can similarly be deflected, magnified, and split into multiple signals by massive objects, creating diffraction and interference effects that allow researchers to study lensed signals.
Did you know? Gravitational lensing doesn’t just apply to light; researchers use mathematical models to show how spacetime curvature can magnify ripples in spacetime, altering how mass measurements appear to detectors on Earth.
Recalculating Mass Through Mathematical Modeling
To test this hypothesis, the research team developed a fast mathematical model and software to analyze gravitational lensing distortions. According to team member Srashti Goyal, if the signal was deflected and distorted by a compact object ranging from 190 to 850 solar masses—or by an extended structure like a globular cluster—the high observed masses can be explained without requiring unusually high spins. When the team factored this into their simulations, they found the actual merger involved a system totaling 140 solar masses, down from the initially theorized 240 solar masses.
Unresolved Questions Surrounding the Lens
Despite the promising model, the identity of the object responsible for lensing GW231123 remains unknown. Zumalacárregui noted that individual compact lenses with masses between 100 and 1,000 solar masses should be exceedingly rare, meaning future work must determine whether such lenses can form or if an ensemble of lighter objects is responsible. While detectors like LIGO will require sensitivity upgrades to conclusively identify future lensed gravitational wave signals, the published research highlights the growing utility of gravitational wave astronomy in probing the universe’s most violent events.
Frequently Asked Questions
What is GW231123?
GW231123 is a gravitational wave signal detected by LIGO on Nov. 23, 2023, originally interpreted as a merger between two unusually massive black holes.
Why were these black holes considered “forbidden”?
Their estimated masses placed them within the upper mass gap, a range where standard stellar evolution models predict stars should be completely destroyed by pair-instability supernovae rather than leaving black hole remnants.
How does gravitational lensing solve the puzzle?
According to research published in the Astrophysical Journal Letters, gravitational lensing may have magnified the gravitational wave signal, making the merging black holes appear significantly more massive than they actually were.
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