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: Black holes are regions of spacetime where gravity is so strong that nothing, not even light, can escape. When two black holes orbit each other and merge, they produce gravitational waves (i.e., ripples in spacetime) that can resemble those emitted by mergers involving other exotic compact objects.
Astrophysicists have therefore been trying to devise methods to distinguish real black holes from "impostors" with similar gravitational-wave signatures. One proposed approach entails measuring an object's spin-induced quadrupole moment, which describes how an object's rotation deforms its mass distribution away from a perfect sphere. Researchers at the University of Birmingham, the Perimeter Institute for Theoretical Physics, the Canadian Institute for Theoretical Astrophysics and other institutes recently relied on this method to analyze GW241011, a gravitational-wave signal linked to a binary compact-object merger detected by the LIGO Hanford and Virgo detectors.
Their paper, published in Physical Review Letters, shows that the more massive object involved in the observed merger is consistent with a so-called Kerr black hole, a rotating black hole with properties determined entirely by its spin and mass. In addition, it sets constraints on other exotic compact objects that could have constituted this object. "The paper builds on a method we originally proposed in 2017 to use gravitational-wave observations to test whether compact objects are truly black holes," N.
Krishnendu, co-first author of the paper and the corresponding author, told Phys.org. "The idea was motivated by a fundamental question: black holes are completely characterized by their mass and spin in general relativity, whereas exotic compact objects—such as boson stars—can have additional structure that changes their multipole moments. In particular, their spin-induced quadrupole moment can differ from the prediction for a Kerr black hole." A 2017 paper published in Physical Review Letters introduced the idea of examining the spin-induced quadrupole moments of compact objects to determine whether they are black holes or other exotic objects producing similar gravitational-wave signals.
Krishnendu and other researchers have since been using this property in gravitational-wave tests to probe the nature of the objects producing recorded signals. "The observations made so far have been consistent with the predictions of general relativity for binary black holes," explained Krishnendu. "However, we knew that the method would be particularly powerful for an event with a rapidly spinning primary, a significant mass asymmetry, and a high signal-to-noise ratio, because these conditions make the spin-induced multipole moment much easier to measure.
That opportunity came with GW241011, detected in October 2024 and subsequently reported by the LIGO-Virgo-KAGRA collaboration." GW241011 is a gravitational-wave event recorded by the LIGO Hanford detector in the U.S. and the Virgo detector in Italy. It was linked to the merger of two compact objects interpreted as black holes. Calculations suggest that the two merging objects had masses of about 19.6 and 5.9 solar masses, respectively, with the more massive object exhibiting a dimensionless spin of approximately 0.78.
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