An ‘Impossible’ Black Hole Merger May Be a Spacetime Illusion

On November 23, 2023, both LIGO detectors picked up ripples in spacetime from a black hole merger so massive it shouldn’t have been possible. A new analysis suggests the universe may have simply been playing a trick of the light, or rather, a trick of gravity.

A Signal That Broke the Rules

The gravitational-wave signal GW231123, detected by the two LIGO detectors in the United States, continues to puzzle scientists. Previous studies estimated the two merging black holes at roughly 100 and 140 times the mass of the sun, figures that standard models of stellar evolution simply cannot explain. Black holes of this size are difficult to form through ordinary stellar collapse, and to make matters more puzzling, the two objects also appeared to be spinning unusually fast, suggesting an unconventional formation history for the entire binary system.

In a new study, a research team at the Max Planck Institute for Gravitational Physics, the Albert Einstein Institute, in Potsdam investigated whether the measured masses and spins only appeared this large because of gravitational lensing, rather than reflecting the true properties of the merging black holes.

When Gravity Bends More Than Light

Gravitational lensing is a well-established phenomenon in which massive objects bend the path of light traveling past them, sometimes producing multiple images of the same distant source. Astronomers already account for this effect routinely when studying stars and galaxies, and they’ve long expected it to affect a fraction of gravitational-wave sources too, though it had never been unambiguously confirmed in gravitational-wave data.

“Like light, gravitational waves can also be deflected, magnified and split into multiple signals by massive objects,” said 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 diffraction effects requires both highly sensitive detectors and new data-analysis methods. To make the analysis possible, the team built a mathematical framework for gravitational-wave lensing and developed software fast enough to actually run it against the data.

A Much Less Extreme System, Once You Subtract the Illusion

“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,” said Srashti Goyal, co-lead author of the study and a postdoc at the AEI Potsdam when the research was conducted. “Moreover, the lensing interpretation does not require unusually high spins.”

Once these lensing effects are factored in, the total mass of the source drops considerably, from roughly 230 solar masses down to around 140. A system that size fits far more comfortably within known black-hole formation scenarios, no exotic spin values or unusual stellar evolution required.

Why a Cello Can Sound Like a Double Bass

Part of the illusion comes from an effect entirely separate from lensing. As gravitational waves travel across the expanding universe, their wavelengths stretch and their frequencies drop, a phenomenon called cosmological redshift, similar to how a sound’s pitch can be lowered. Just as a cello recording played back at a lower pitch might be mistaken for a double bass, a distant black-hole binary can end up looking considerably more massive than it actually is. Gravitational magnification compounds that illusion further, making a distant source appear closer, and therefore even more massive, than it really is.

“Our analysis also suggests that the compact lens was embedded in a larger gravitational field, such as that of the galaxy hosting it,” said Héctor Villarrubia-Rojo, the study’s other co-lead author and a postdoctoral scholar at the Universidad Complutense in Madrid. “By including this external potential, we can describe the small-scale diffraction and the large-scale magnification within a single framework.”

The study’s title, “Across the Universe,” is a nod to the Beatles, fitting for a signal that changed appearance during its own journey across the cosmos. It was published August 25 in The Astrophysical Journal Letters.

A Genuine Mystery Either Way

Even if the lensing interpretation holds up, it doesn’t fully resolve the puzzle, it just relocates it. No gravitational-wave signal has ever been unambiguously confirmed as lensed before, and if GW231123 really is the first such case, the object responsible for the lensing effect would itself be a considerable rarity.

“The nature of the lens remains a major mystery in our analysis, as individual compact lenses with 100 to 1,000 solar masses should be exceedingly rare,” Zumalacárregui said. “Future work will need to establish whether such lenses can form, or whether an ensemble of lighter objects, including stars, can explain this event.”

Still an Open Question

Current data don’t yet allow researchers to make an unambiguous claim of gravitational lensing in this particular signal. The team is careful to frame their findings as a plausible explanation rather than a settled conclusion, one that will require further detector upgrades and refined analysis techniques to properly confirm or rule out.

Even short of a definitive answer on GW231123 itself, the broader implications extend well beyond this one signal. If scientists can reliably detect and interpret gravitational waves bent by intervening mass, that capability opens an entirely new observational tool. Gravitational magnification could reveal black hole mergers happening at distances current detectors can’t otherwise reach, while diffraction effects could help probe compact objects and dark matter structures sitting along the line of sight, turning what looks today like a stubborn anomaly into a potential new method for exploring the universe.

Sources

Max Planck Institute for Gravitational Physics (Albert Einstein Institute)