In October 2024, two dead stars slammed into each other about a billion light-years away, sending ripples through the very fabric of spacetime. The signal, called GW241011, reached Earth and was caught by the LIGO Hanford detector in the U.S. and the Virgo detector in Italy. But something about this cosmic crash caught the attention of physicists far beyond the numbers: one of the objects involved might not have been a black hole at all.
That is because black holes — regions of space where gravity is so strong that even light cannot escape — can be mimicked by other strange, hypothetical objects that send out nearly identical gravitational waves. These "impostors" could fool even careful listeners. So an international team of researchers set out to tell them apart, and their new findings, published in Physical Review Letters, give the strongest evidence yet that one such object really is what it appears to be: a true black hole.
Astrophysicists have long hunted for a reliable test to separate real black holes from lookalikes. In 2017, a team including N. V. Krishnendu of the University of Birmingham and the Perimeter Institute for Theoretical Physics proposed a clever method: measure something called the spin-induced quadrupole moment. In plain terms, this describes how an object's rotation squashes it away from a perfect sphere. A real, rotating black hole — one first described by physicist Roy Kerr — has a very precise, predictable squash. But exotic objects, like the hypothetical "boson stars" made of particles that can pile into the same quantum state, would squish differently. Measuring the squash can reveal the truth.
The problem was that the trick needed a special kind of event to work well: one with a fast-spinning heavy object, a big mismatch between the two merging masses, and a strong, clean signal. GW241011 delivered all three. Calculations show the two objects had masses of about 19.6 and 5.9 times that of our Sun, and the heavier one spun at a rate of roughly 0.78. The signal was so loud — about 36 times stronger than the noise of the detectors — that the team could measure the object's spin-induced squash with unprecedented precision.
The verdict? The massive primary is consistent with a true Kerr black hole. Large families of exotic impostors, including rotating boson stars with quartic self-interactions, simply cannot explain what the researchers saw.
The discovery does not close the book entirely. The team notes that sufficiently compact exotic objects — those with a compactness of about 0.24 or higher — could still potentially hide in the data.
For Krishnendu, the moment felt like vindication. "This made GW241011 an ideal system for applying the method we developed almost eight years earlier," he said. The work narrows the list of places a black hole impostor could hide, sharpening our view of one of the universe's strangest and most spectacular events.