Scientists 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.
Gravitational waves helped to study an unusual object
When two black holes approach and merge, they create gravitational waves— fluctuations in space-time that 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 results are published in the journal Phys.org .
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.
Scientists have checked the "fingerprint" of the rotation of the black hole
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.
N.V. Krishnendu, co-author of the study
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 that changes their multipole moments. In particular, their quadrupole moment due to spin may differ from that predicted for a Kerr black hole.
In their work, 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.
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.
Tamara Evstafieva
The quadrupole moment caused by rotation 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 approach of a binary system.
Why it is important to search for "imposter black holes"
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.
N.V. Krishnendu
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 back in 2017. 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. The fifth observation cycle of LIGO and Virgo should receive significantly more sensitive data by about 2029, and 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 in the universe.
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