Ar-Rehman

Scientists with Gravitational-wave analysis have narrowed the search for alternatives to black holes

Gravitational waves can help determine whether the observed compact object is really a black hole or represents another, so far hypothetical structure. Analysis of the GW241011 signal allowed scientists to exclude some variants of exotic objects, although it has not yet been possible to definitively confirm the nature of the object. "How gravitational waves help to search for "impostor black holes", in the material of Izvestia. 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. When two black holes approach and merge, they create gravitational waves, fluctuations in space-time which propagate through the universe. However, similar signals can occur when other compact objects merge. Therefore, scientists are looking for ways to determine whether the source of the signal was really a black hole.

Black holes are regions of space-time where gravity is so strong that nothing, not even light, can pull them out. One of these methods was the analysis of the so-called spin-induced quadrupole moment. It shows how the rotation of an object changes the distribution of its mass and deflects its shape from an ideal sphere. These changes leave a characteristic trace in the gravitational waves, which can be measured. Researchers from the University of Birmingham, the Perimeter Institute for Theoretical Physics, the Canadian Institute for Theoretical Astrophysics and other scientific organizations applied this method to the GW241011 signal. It was detected by the LIGO Hanford detectors in the USA and Virgo in Italy. The signal originated as a result of the merger of two compact objects, which were initially interpreted as black holes. The more massive object had a mass of about 19.6 times that of the Sun, while the second had a mass of about 5.9 times that of the Sun. At the same time, the dimensionless spin of the more massive object was approximately 0.78. The high mass difference, the rapid rotation of the main object, and the strong signal made it possible to measure its spin-induced quadrupole moment with unusually high accuracy. The 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 which 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 said, co-first author of the paper and the corresponding author.

"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 which changes their multipole moments. In particular, their spin-induced quadrupole moment can differ from the prediction for a Kerr black hole." A earlier paper 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 US and the Virgo detector in Italy. It was linked to the merger of two compact objects interpreted as black holes.

"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." 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." According to the general theory of relativity, a black hole is described only by its mass and rotation. Such an object is called a Kerr black hole. Other compact objects may have additional internal structure, so their multipole moments may differ from those predicted by the theory for a black hole. The idea was motivated by fundamental questions: in general relativity, black holes are completely characterized by their mass and spin, whereas exotic compact objects such as bosonic stars may have an additional structure which changes their multipole moments. In particular, their quadrupole moment due to spin may differ from that predicted for a Kerr black hole.

The scientists examined several variants of the object's nature, including bosonic stars, hypothetical compact structures consisting of bosons. For different models, they calculated the expected values of the spin-induced quadrupole moment and compared them with the GW241011 data. The results showed that some classes of exotic objects, in particular rotating bosonic stars with quartic self-action, are inconsistent with the observed signal characteristics. At the same time, it was not possible to completely exclude all alternatives to a black hole: sufficiently compact exotic objects can still correspond to the data obtained. 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. The spin-induced quadrupole moment is a special property that describes how the mass distribution of a compact object is distorted by its rotation, leaving a characteristic imprint on the gravitational waves emitted during the binary inspiral," explained Tamara Evstafyeva, co-first author of the paper. Because this distortion depends on the object's internal structure, what it is made of, different types of compact objects can have different spin-induced quadrupole moments.

Krishnendu, Evstafyeva, Vijaykumar and their colleagues considered various possibilities regarding the nature of the primary object involved in GW241011. For instance, they examined different classes of boson stars and exotic fluid stars, comparing their theoretical predictions for these objects' spin-induced quadrupole moments with the constraints derived from GW241011. "Importantly, when choosing exotic compact objects, we focused on proposals with sufficient physical viability," said Evstafyeva. "In particular, some rotating exotic compact objects are known to suffer from instabilities, making them unlikely to survive on sufficiently long timescales. To identify models that avoid these instabilities, we used results from numerical relativity simulations." The researchers inferred various properties of the primary object in the GW241011 merger from the observed gravitational-wave signal, including its mass, spin and spin-induced quadrupole moment. They used a mathematical approach called Bayesian parameter estimation and waveform models that allowed each object's quadrupole moment to vary independently of the value predicted for a Kerr black hole. "The high signal-to-noise ratio and large mass asymmetry of GW241011 made this measurement particularly precise," said Krishnendu. One of the authors of the study, Tamara Evstafieva, called the spin-induced quadrupole moment a kind of "imprint" of an object in gravitational waves. The quadrupole moment caused by rotation is a special property which describes how the mass distribution of a compact object is distorted by its rotation, leaving a characteristic imprint on the gravitational waves emitted during the approach of a binary system.

The team's predictions and calculations suggest that some exotic objects, including rotating boson stars with quartic self-interactions, would have quadrupole moments which are incompatible with the data recorded by LIGO Hanford and Virgo. This allowed them to rule out the possibility that the primary of the GW241011 merger is one of those boson-star models. Concurrently, they showed that it could potentially be another sufficiently compact exotic object rather than a black hole. "Our work brought together two complementary sides of the problem: theory and numerical simulations told us which exotic objects could plausibly exist and what spin-induced quadrupole moments they should have, while data analysis told us which of those predictions were compatible with the gravitational-wave signal," said Evstafyeva. "Neither side alone would likely have been enough, and GW241011 provided the opportunity to bring the two together. It was this interplay that turned the spin-induced quadrupole moment into a kind of 'fingerprint' in the gravitational-wave data, giving us clues about the nature of the primary object." The researchers' recent work is the culmination of a nearly decade-long research journey aimed at devising a method to test the nature of compact objects. The GW241011 event provided the gravitational-wave data they required to test the approach introduced and refined in their earlier studies.

"We moved from asking whether the gravitational-wave data are consistent with black holes to placing quantitative constraints on specific alternatives to black holes," said Krishnendu. "The particularly favorable properties of GW241011 allowed us to measure the spin-induced quadrupole moment of its primary much more precisely than has been possible with previous gravitational-wave events, giving us a new way to probe the object's internal structure." The team's measurements and analyses suggest that the primary of GW241011 could be a Kerr black hole. Nonetheless, they do not completely rule out the possibility that it could be another type of compact object. "Our result does not prove that the object is a black hole, as sufficiently compact exotic objects can still be consistent with the data," said Krishnendu. "I think this distinction is important, we are constraining the space of possible alternatives rather than simply labeling the object as a black hole." The efforts by Krishnendu, Evstafyeva, Vijaykumar and their collaborators demonstrate that gravitational-wave observations could be used to probe properties of compact objects which are related to their internal structure. As physicists and engineers develop increasingly sensitive gravitational-wave detectors, new measurements could be used to probe the nature of compact objects with greater precision. The gravitational-wave detectors are preparing for their fifth observing run, with substantially enhanced sensitivity expected by around 2029. At the same time, third-generation detectors are being planned, and their science cases are being actively explored worldwide.

"Together with proposed space-based observatories such as LISA, these facilities will give us access to a much larger and more diverse population of gravitational-wave sources. This could allow us not only to place stronger constraints on exotic alternatives to black holes, but potentially also to discover evidence for new forms of compact matter." As part of her future research, Krishnendu plans to develop other techniques which could be used to identify signatures of exotic compact objects from gravitational-wave data. In addition, she would also like to produce more realistic numerical models that predict the internal dynamics of compact objects beyond black holes. "I am genuinely intrigued to discuss these findings with particle physicists who study these objects to get their perspective on how such observations could probe particle physics phenomena in extreme regimes that are impossible to replicate in terrestrial experiments," said Aditya Vijaykumar. "It is important to note, however, that this constraint applies only to the object in this specific binary. We cannot yet make broader claims about the existence of these objects in the universe as a whole. Still, I am confident that these methods will inspire both our team and others to tackle this at the population level by combining data from multiple gravitational-wave events."

The researchers would eventually also like to combine their measurements with analyses of the objects' tidal deformation. Tidal deformation is a change in an object's shape caused by the uneven gravitational pull of another nearby object. "This could grant us access to more stringent constraints on these exotic models," added Vijaykumar. "Modeling the tidal deformation from theory is quite involved for these highly spinning objects, but I am excited about all the new physics we will learn once we have done that!" The authors emphasize that the result does not prove that the GW241011 object is a black hole. The analysis only allows us to narrow down the range of possible explanations. Our result does not prove that the object is a black hole, since sufficiently compact exotic objects can still fit the data. At the same time, the study became an important stage in the development of the method, which scientists proposed earlier. Previously, gravitational-wave observations generally corresponded to the predictions of the general theory of relativity for binary black holes. For the first time, GW241011 provided particularly suitable conditions for a more accurate measurement of the quadrupole moment due to the rapid rotation of one of the objects, a large mass difference and a high signal-to-noise ratio. In the future, the sensitivity of gravitational-wave detectors will increase. Scientists are also planning to create next-generation detectors. Additional observations will allow us to verify more and more models of exotic compact objects. Thus, GW241011 did not become the discovery of a new type of compact object and did not provide definitive evidence of the nature of the observed system. However, scientists were able to rule out some alternatives to a black hole and showed that the analysis of gravitational waves is gradually becoming a tool for studying the internal structure of the most extreme objects.