Gravitational-wave analysis narrows the search for black hole impostors

edited by Lisa Lock, reviewed by Robert Egan 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. V. 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. "The combination of its large mass asymmetry, rapidly spinning primary, and high signal-to-noise ratio—about 36 in the detector network—enabled the measurement of the primary's spin-induced quadrupole moment with unprecedented precision," said Krishnendu. "This made GW241011 an ideal system for applying the method we developed almost eight years earlier." In their paper, the researchers tried to use the spin-induced quadrupole moment they measured for the more massive merging object to determine whether it could be something other than a black hole. For instance, they considered the possibility of it being a rotating boson star, a hypothetical spinning compact object made from bosons (i.e., particles that can occupy the same quantum state). "We find that large classes of exotic compact objects, including rotating boson stars with quartic self-interactions, cannot explain the observed properties of the primary," said Krishnendu. "At the same time, sufficiently compact exotic objects, with compactness C≳0.24, remain viable possibilities." Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. Sign up for our free newsletter and get updates on breakthroughs, innovations, and research that matter—daily or weekly. To place constraints on the nature of the more massive object involved in the GW241011 merger, referred to as the "primary," the researchers combined theoretical insights and data analyses. First, they estimated the primary's spin-induced quadrupole moment. [Read Full Story on Phys.Org →](https://phys.org/news/2026-09-gravitational-analysis-narrows-black-hole.html)