Gravitational Waves and the Massive Black Hole Merger
In November 2023, LIGO detectors captured gravitational waves generated by the merger of two black holes, whose combined mass was estimated to be between 190 and 265 solar masses. The event, designated as GW231123, marked the most massive black hole merger ever observed. However, a recent analysis suggests that this merger may not be quite as monumental as initially believed. What could account for such a record-breaking signal? It is possible that the gravitational influence of other objects located between the merger and Earth amplified and distorted the signal.
Srashti Goyal, Héctor Villarrubia Rojo, and Miguel Zumalacárregui, researchers associated with the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), propose that GW231123 may have passed through a kind of cosmic lens. This phenomenon could be attributed to a combination of gravitational lensing and microlensing, which not only amplifies the intensity of gravitational waves but also subtly alters their shape.
The concept stems from a well-known phenomenon in general relativity. The gravity of a massive object can warp spacetime, thereby affecting the trajectory of light or gravitational waves. When a signal travels close to a concentration of matter, it may reach us amplified, as if it has passed through a gigantic lens.
The Implications of Gravitational Lensing
What are the implications of this phenomenon? The expanding universe causes distant sources to appear to have greater masses when their signals are analysed directly, primarily due to the effects of redshift. Concurrently, gravitational lensing can amplify signals, making the source appear closer than it actually is. These factors can combine to yield an exaggerated estimation of mass.
“Like light, gravitational waves can also be bent, amplified, and split into multiple signals by the action of massive objects,” explains Miguel Zumalacárregui, group leader of the Astrophysical Relativity and Cosmology Department at AEI. “In the case of gravitational waves, diffraction and interference effects provide an additional means to identify and study distorted signals.”
To investigate this hypothesis, the researchers analysed GW231123 under three scenarios: without gravitational lensing, with an isolated point lens, and with a point lens embedded within a larger gravitational field, such as that of a galaxy. The latter scenario is the most comprehensive as it simultaneously considers the effects of different gravitational scales.
Reassessing Black Hole Mass Estimates
The results support the interpretation that the signal underwent lensing. Simulations conducted to estimate the likelihood that the detector noise could randomly produce a similar signal found a false alarm probability of less than 1%. Although this provides intriguing evidence, the authors caution that it does not yet constitute definitive detection of gravitational lensing.
The most significant clue lies in a minor distortion of the signal. A compact lens can generate a phenomenon known as diffraction in wave optics. Instead of behaving like a ray following a specific path, the gravitational wave may exhibit interference patterns and other characteristic modifications. In the case of GW231123, an additional structure appears approximately 20 milliseconds from the primary signal. According to the analysis, this feature cannot be fully explained by conventional geometric optics and is essential for producing the observed data.
Rethinking the Nature of the Merging Black Holes
If this interpretation holds true, it also alters the physical history of the merging black holes. Without lensing, the system appears to have a total mass near 232 solar masses, implying extraordinarily heavy black holes and very high spin rates. This would make it an object difficult to explain within typical black hole formation scenarios.
In contrast, including gravitational lensing presents a less extreme picture. The analysis estimates the intrinsic total mass of the system at around 137 solar masses as a median value, with individual components below 100 and 50 solar masses. Concurrently, the source may be located at a redshift close to 1.2, significantly farther than the estimation made without considering the lens.
The Mystery Surrounding the Gravitational Lens
The lens itself may also possess a considerable mass. The study estimates that the compact object responsible for the diffraction could have between 188 and 850 solar masses under the model of a lens embedded within a galaxy. “If we assume that GW231123 was deflected and distorted by a compact object ranging from 190 to 850 solar masses, or by an extended structure like a globular cluster, we can better understand the observed high masses,” states Srashti Goyal, the study’s co-lead author. “Moreover, the interpretation of the gravitational lens effect does not necessitate unusually high spins.”
“Our analysis also suggests that the compact lens was immersed in a larger gravitational field, such as that of the hosting galaxy,” adds Héctor Villarrubia Rojo, co-author and postdoctoral researcher at the Complutense University of Madrid. “By including this external potential, we can describe small-scale diffraction and large-scale magnification within a unified theoretical framework.”
What, then, could the lens be? One possibility is that it could be an intermediate-mass black hole; however, the authors emphasise that its nature remains undetermined. An extended structure, such as a globular cluster, or even a collection of stellar objects could produce a similar signal. “The nature of the lens remains a significant mystery in our analysis, as individual compact lenses with masses between 100 and 1000 solar masses should be extremely rare,” notes Zumalacárregui. “Future research will be necessary to ascertain whether such lenses can form or if a collection of lighter objects, including stars, can account for this phenomenon.”
Future Implications for Gravitational Wave Research
Ultimately, GW231123 occupies a rather peculiar position, as the new analysis does not definitively prove that the signal was distorted by a gravitational lens. However, it suggests that this possibility may simultaneously explain several of its extraordinary characteristics. This case also reveals that gravitational waves could become a valuable tool for studying not only the objects that generate them but also everything they encounter on their journey.
The study has been published in The Astrophysical Journal Letters and is available for further reading.