This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Gravitational waves are ripples in spacetime created when pairs of black holes spiral into each other and eventually merge. But if this ripple has been warped by another massive object on its way to Earth, a new analysis suggests that these black holes might appear far larger than they really are.
In the analysis published in The Astrophysical Journal Letters, Miguel Zumalacárregui and colleagues at the Max Planck Institute for Gravitational Physics revisit the signal from what appeared to be the largest black hole merger ever recorded. They conclude that the objects were likely far lighter than first thought. When two black holes spiral together and merge, they send gravitational waves rippling outward at the speed of light, creating tiny distortions in spacetime that stretch and squeeze everything they pass through.
Detectors like LIGO, Virgo and KAGRA pick up these distortions as they pass over Earth, and from the pattern of the signal, researchers can work out the masses of the black holes involved. But if the wave passes close to another massive object on its journey, the object's gravity can act like a lens, bending and magnifying the signal. A magnified wave would then appear stronger than when it was originally created, tricking researchers into thinking the source is more massive than it actually is.
In November 2023, the LIGO-Virgo-KAGRA network picked up a signal named GW231123, produced by two black holes merging with a combined mass of around 190 to 265 times that of the sun. It appeared to be the largest binary black hole system ever seen and heavier than standard stellar collapse is thought to produce. However, Zumalacárregui's team suspected that these masses could have been inflated by gravitational lensing.
To explore the idea, they created a model that accounts for a compact object, potentially an intermediate-mass black hole, that sits within a larger galaxy-scale gravitational field along the signal's path. They then tested how well their model's predictions matched the distortions seen in the GW231123 signal. The team found solid statistical support for their lensing explanation, with less than a 1% chance that the pattern arose by coincidence.
Under this scenario, the true combined mass of the merging black holes drops to a more modest 100 to 180 solar masses, easing some of the tension with existing models of black hole formation. The findings suggest that GW231123 may not be quite the record-breaker it first appeared to be and that other exceptionally massive mergers detected so far could be similarly distorted by lensing. With gravitational-wave detectors set to become even more sensitive, the team argues that past detections deserve a second look in case some of the universe's most extreme black holes turn out to be a little less extreme after all.
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