A closer look at the gravitational-wave event GW231123
© Simulation: I. Markin (University of Potsdam), H. Pfeiffer (Max Planck Institute for Gravitational Physics), T. Dietrich (University of Potsdam and Max Planck Institute for Gravitational Physics); Collage: B. Knispel, M. Zumalacárregui (Max Planck Institute for Gravitational Physics)
To the point
- The signal: On 23 November 2023, both LIGO observatories in the US detected gravitational waves from the most massive black hole merger to date.
- The mystery: According to previous studies, the two black holes had masses approximately 100 and 140 times that of our Sun, respectively. However, according to standard models of stellar evolution, black holes of these masses are difficult to form. How they could have evolved remains a mystery.
- A possible solution: Scientists at the Max Planck Institute for Gravitational Physics in Potsdam have now proposed an explanation for these unusual masses. The signal may have been magnified by a galaxy and diffracted by an object within it, making the black holes appear more massive than they really are.
A potential explanation for a mysterious gravitational wave signal.
The gravitational-wave signal GW231123, detected by the two LIGO detectors in the US on 23 November 2023 from the merger of two black holes, continues to puzzle scientists. Given their large masses, the existence of these black holes cannot be explained with the standard understanding of stellar evolution. However, they are not only very massive, but also spin very quickly. This makes interpreting the signal particularly challenging and suggests an unusual formation history for the binary system.
In a new study, a research team at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in the Potsdam Science Park investigated whether the measured masses and spins appeared larger than they actually were due to the effect of a gravitational lens.
In a new study, a research team at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in the Potsdam Science Park investigated whether the measured masses and spins appeared larger than they actually were due to the effect of a gravitational lens.
Light on Curved Paths and Distorted Gravitational Waves
Light traveling through the universe is deflected by massive objects, resulting sometimes in multiple images of the same astrophysical source. This gravitational lensing effect needs to be accounted for to correctly interpret observations. Lensing is routinely observed on electromagnetic sources such as stars and galaxies, and expected to occur in a fraction of gravitational-wave sources.
“Like light, gravitational waves can also be deflected, magnified and split into multiple signals by massive objects,” says Miguel Zumalacárregui, group leader in the Astrophysical and Cosmological Relativity Department at the AEI. “For gravitational waves, diffraction and interference effects give us an additional way to identify and study lensed signals.”
Detecting these subtle lensing diffraction effects requires sensitive detectors and novel data-analysis methods. To make this analysis possible, the team developed a mathematical description of gravitational-wave lensing and software fast enough to analyze the data.
“If we assume that GW231123 was deflected and distorted by a compact object of about 190 to 850 solar masses—or by an extended structure such as a globular cluster—we can understand the observed high masses,” says Srashti Goyal, co-lead author and postdoc at the AEI Potsdam when working on the topic. “Moreover, the lensing interpretation does not require unusually high spins.” Taking these effects into account, the total mass of the source would be around 140 solar masses rather than about 230 solar masses. The system would then be much less extreme and easier to accommodate within known black-hole formation scenarios.
“Like light, gravitational waves can also be deflected, magnified and split into multiple signals by massive objects,” says Miguel Zumalacárregui, group leader in the Astrophysical and Cosmological Relativity Department at the AEI. “For gravitational waves, diffraction and interference effects give us an additional way to identify and study lensed signals.”
Detecting these subtle lensing diffraction effects requires sensitive detectors and novel data-analysis methods. To make this analysis possible, the team developed a mathematical description of gravitational-wave lensing and software fast enough to analyze the data.
“If we assume that GW231123 was deflected and distorted by a compact object of about 190 to 850 solar masses—or by an extended structure such as a globular cluster—we can understand the observed high masses,” says Srashti Goyal, co-lead author and postdoc at the AEI Potsdam when working on the topic. “Moreover, the lensing interpretation does not require unusually high spins.” Taking these effects into account, the total mass of the source would be around 140 solar masses rather than about 230 solar masses. The system would then be much less extreme and easier to accommodate within known black-hole formation scenarios.
Across the Universe
The study’s title, “Across the Universe”, is a nod to the Beatles — but here the journey really does change how the source appears. As gravitational waves travel through the expanding Universe, their wavelengths are stretched and their frequencies lowered (an effect known as cosmological redshift) — the equivalent of lowering a sound’s pitch. Just as a cello played back at a lower pitch might be mistaken for a double bass, a distant black-hole binary can appear more massive than it really is. Gravitational magnification adds to the illusion by making the distant source appear closer.
“Our analysis also suggests that the compact lens was embedded in a larger gravitational field, such as that of the galaxy hosting it,” adds Héctor Villarrubia-Rojo, co-leading author of the study and a postdoctoral scholar at the Universidad Complutense in Madrid, Spain. “By including this external potential, we can describe the small-scale diffraction and the large-scale magnification within a single framework.”
“Our analysis also suggests that the compact lens was embedded in a larger gravitational field, such as that of the galaxy hosting it,” adds Héctor Villarrubia-Rojo, co-leading author of the study and a postdoctoral scholar at the Universidad Complutense in Madrid, Spain. “By including this external potential, we can describe the small-scale diffraction and the large-scale magnification within a single framework.”
New insights from lensed gravitational waves
So far, researchers have not unambiguously detected any signals affected by gravitational lensing in the data from gravitational-wave detectors. According to the new study, GW231123 is a promising candidate for such an event.
“The nature of the lens remains a major mystery in our analysis, as individual compact lenses with 100–1,000 solar masses should be exceedingly rare,” adds Zumalacárregui. “Future work will need to establish whether such lenses can form, or whether an ensemble of lighter objects, including stars, can explain this event.”
Current data do not yet allow an unambiguous claim of gravitational lensing. Following further upgrades to the detectors, however, the scientists expect to be able to detect and interpret lensed gravitational waves using new data analysis methods.
The detection — or lack thereof — of gravitational waves deflected by the gravity of other objects will provide new insights into gravitational-wave astronomy: Gravitational magnification may reveal black-hole mergers beyond the distance current detectors can normally reach, while diffraction can probe compact objects and dark-matter structures along the line of sight. Consequently, these deflected gravitational waves could become a new method for exploring the universe.
“The nature of the lens remains a major mystery in our analysis, as individual compact lenses with 100–1,000 solar masses should be exceedingly rare,” adds Zumalacárregui. “Future work will need to establish whether such lenses can form, or whether an ensemble of lighter objects, including stars, can explain this event.”
Current data do not yet allow an unambiguous claim of gravitational lensing. Following further upgrades to the detectors, however, the scientists expect to be able to detect and interpret lensed gravitational waves using new data analysis methods.
The detection — or lack thereof — of gravitational waves deflected by the gravity of other objects will provide new insights into gravitational-wave astronomy: Gravitational magnification may reveal black-hole mergers beyond the distance current detectors can normally reach, while diffraction can probe compact objects and dark-matter structures along the line of sight. Consequently, these deflected gravitational waves could become a new method for exploring the universe.
Media contact:
Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
Tel: +49 331 567-7303
Email: elke.mueller@aei.mpg.de
Science contacts:
Dr. Srashti Goyal
Postdoc
Email: srashti.goyal@iucaa.in
Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, India
Dr. Héctor Villarrubia-Rojo
Postdoctoral scholar
Email: hectorvi@ucm.es
Universidad Complutense, Madrid, Spain
Dr. Miguel Zumalacarregui
Group Leader
Tel:+49 331 567-7322
Fax: +49 331 567-7298
Email: miguel.zumalacarregui@aei.mpg.de
Publication
Goyal, S.; Villarrubia-Rojo, H.; Zumalacarregui, M.
Across the Universe: GW231123 as a magnified and diffracted black hole merger. The Astrophysical Journal Letters 1008, L12 (2026)
MPG.PuRe - DOI - pre-print
Related information
1. Homepage of the “Astrophysical and Cosmological Relativity” department
2. YouTube video: The Next Frontier: Lensing of Gravitational Waves
