This ‘impossible’ black hole merger may be explained by a warp in spacetime
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A “forbidden” merger between two massive black holes may not have been quite as impossible as previously thought. That’s according to new research that suggests the black holes involved were smaller than expected.
On Nov. 23, 2023, the gravitational wave detector LIGO (Laser Interferometer Gravitational-Wave Observatory) detected tiny ripples in spacetime caused by the merger of two black holes. What was incredible about this signal, designated GW231123, was that it seemed to be the result of a black hole with 140 times the mass of the sun colliding with another that holds 100 solar masses.
This raised eyebrows among researchers because usual models of stellar evolution struggle to account for such massive black holes, especially ones that seemed to be spinning as fast as these two. While scientists have been attempting to explain how such an odd black hole binary could form. The team behind this new research suggests it doesn’t need to be explained at all. They think that the masses of these black holes were an illusion.
The key to this illusion is a phenomenon called gravitational lensing, first predicted by Albert Einstein’s 1915 theory of gravity, general relativity, which also first predicted the existence of gravitational waves. This theory says objects with mass cause the curvature of space and time, united as a four-dimensional entity called “spacetime.” The more mass an object possesses, the greater the curvature, and because gravity arises from this curvature, the greater the gravitational influence.
Gravitational lensing occurs when light from a background object passes a massive foreground object. The foreground object can warp the fabric of spacetime in such a way that the light’s path is curved. This means light from the same background source can reach Earth at different times, depending on how much that light was diverted.
This difference in travel time can magnify a background source, and it has been used to great effect to observe distant and ancient galaxies ordinarily too faint to be seen.
This research team thinks the effect also applies to gravitational waves, suggesting the signal GW231123 is an example of gravitationally lensed ripples in spacetime that made the black holes appear larger than they actually are.
“Like light, gravitational waves can also be deflected, magnified and split into multiple signals by massive objects,” team member Miguel Zumalacárregui, group leader in the Astrophysical and Cosmological Relativity Department at the Albert Einstein Institute (AEI), said in a statement. “For gravitational waves, diffraction and interference effects give us an additional way to identify and study lensed signals.”
To investigate this possibility, the team developed a mathematical model of gravitational lensing and created software powerful and fast enough to analyze it.
“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 team member Srashti Goyal, who was based at the AEI when conducting the research. “Moreover, the lensing interpretation does not require unusually high spins.”
When the team modeled the event with that consideration factored in, they found the merger involved a system with a mass of 140 solar masses, rather than the 240-solar-mass system initially theorized.
The team doesn’t quite know what massive object is responsible for lensing the gravitational wave signal GW231123. But no matter what it is, if the team is correct, the lens could be something quite special.
“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.”
The team can’t yet conclusively say if GW231123 is the first gravitationally lensed gravitational wave signal. Finding future signals of the sort will require upgrades to the sensitivity of detectors like LIGO.
Though even before that happens, the research does point to the utility of gravitational wave astronomy to study some of the universe’s most violent events.
The team’s research was published on August 25 in the Astrophysical Journal Letters.
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