⚫ Observation of "second-generation black holes"

The detection of two singular black hole mergers, only one month apart in late 2024, improves our understanding of the nature and evolution of the most violent collisions in the Universe. Some characteristics of these mergers suggest the possibility of "second-generation black holes," which would be the result of prior coalescences, likely produced in very dense and crowded cosmic environments, such as stellar clusters, where black holes are more likely to meet and merge repeatedly.

In a new article published in The Astrophysical Journal Letters, the international LIGO-Virgo-KAGRA collaboration announces the detection of two gravitational wave events in October and November of last year featuring unusual black hole spins. An observation that brings a new important piece to our understanding of the most elusive phenomenon in the Universe.

Gravitational waves are "ripples" in spacetime resulting from cataclysmic events in deep space. The strongest waves are produced by black hole collisions. Using sophisticated algorithmic techniques and mathematical models, researchers are able to reconstruct many physical characteristics of the detected black holes from the analysis of gravitational signals, such as their masses and the distance of the event from Earth, or the speed and direction of their rotation around their axis, called spin.

The first of the two mergers in the article was detected on October 11, 2024 (GW241011). It occurred about 700 million light-years away and resulted from the collision of two black holes weighing about 17 and 7 times the mass of the Sun. The spin of the larger black hole in GW241011 is one of the fastest ever observed.

Almost a month later, on November 10, 2024, the second merger, named GW241110, was detected about 2.4 billion light-years away and involved the merger of black holes of about 16 and 8 solar masses. While most observed black holes spin in the same direction as their orbit, the primary black hole in GW241110 was observed spinning in a direction opposite to its orbit - a first of its kind.

"Each new detection provides important information about the Universe, reminding us that every observed merger is both an astrophysical discovery and an invaluable laboratory for studying the fundamental laws of physics," says Carl-Johan Haster, co-author of the article and a research faculty member in astrophysics at the University of Nevada, Las Vegas (UNLV). "Binaries like these had been anticipated based on previous observations, but this is the first direct evidence of their existence."

These two events seem to indicate that they could be "second-generation black holes." "Among the hundreds of events that the LIGO-Virgo-KAGRA network has observed, GW241011 and GW241110 are among the most groundbreaking," says Stephen Fairhurst, a research faculty member at Cardiff University and spokesperson for the LIGO Scientific Collaboration. "The fact that both events feature a black hole much more massive than the other and with rapid spin seems to indicate that these black holes were formed from previous black hole mergers."

Scientists highlight certain particular characteristics, especially the size difference between the black holes in each merger - the larger one was almost double the size of the smaller one - and the spin orientation of the larger black holes in each event. A natural explanation for these peculiarities is that the black holes are the result of prior coalescences. This process, called hierarchical merging, suggests that these systems formed in dense environments, such as stellar clusters, where black holes are more likely to meet and merge repeatedly.

"These discoveries highlight the extraordinary capabilities of our global gravitational wave observatories," says Gianluca Gemme, a researcher at INFN and spokesperson for the Virgo collaboration. "The unusual spin configurations observed in GW241011 and GW241110 not only challenge our understanding of black hole formation but also offer compelling evidence of hierarchical mergers in dense cosmic environments: they teach us that some black holes exist not only as isolated partners but probably also as members of a dense and dynamic crowd. These discoveries underscore the importance of international collaboration in unveiling the most elusive and energetic phenomena in the Universe."

Discovering the hidden properties of black hole mergers

Gravitational waves were first predicted by Albert Einstein as part of his general theory of relativity in 1916, but their presence - although proven in the 1970s - was not directly observed by scientists until just 10 years ago, when the LIGO and Virgo scientific collaborations announced the detection of waves following a black hole merger.

Today, LIGO-Virgo-KAGRA is a global network of advanced gravitational wave detectors, nearing the end of its fourth observation run, O4. The current run began in late May 2023 and is expected to continue until mid-November of this year. To date, about 300 black hole mergers have been observed via gravitational waves, including candidates identified in the ongoing observation run awaiting final validation.

On the other hand, the precision with which GW241011 was measured has also allowed for testing some predictions of Einstein's general theory of relativity under extreme conditions.

In fact, this event can be compared to the predictions of Einstein's theory and the solution by mathematician Roy Kerr for rotating black holes. The black hole's rapid spin slightly distorts it, leaving a characteristic imprint in the gravitational waves it emits. By analyzing GW241011, the research team found a very good agreement with the Kerr solution and once again verified Einstein's prediction, with unprecedented precision.

Furthermore, because the masses of the individual black holes differ considerably, the gravitational wave signal contains the "hum" of a higher harmonic - similar to harmonics in musical instruments - observed only for the third time with GW241011. One of these harmonics was observed with great clarity and confirms another prediction of Einstein's theory.

"This discovery also means we are more sensitive than ever to any new physics that might lie beyond Einstein's theory," concludes Haster.

Advanced elementary particle search

Rapidly spinning black holes like those observed in this study now have a new utility in particle physics. Scientists can use them to test the existence of certain hypothetical elementary particles.

These particles, called ultralight bosons, are predicted by some theories beyond the Standard Model of particle physics, which describes and classifies all known elementary particles. If ultralight bosons exist, they can extract rotational energy from black holes. The amount of energy extracted and the rate at which the black holes' spin slows over time would therefore depend on the mass of these particles. The fact that the massive black hole involved in GW241011 continues to spin rapidly, even millions or billions of years after its formation, rules out a wide range of ultralight boson masses.

"The detection and inspection of these two events demonstrates how important it is to operate our detectors in synergy and strive to improve their sensitivity," says Francesco Pannarale, a research faculty member at the University of Rome and co-chair of the LIGO-Virgo-KAGRA collaboration's observational sciences division. "The LIGO and Virgo instruments have taught us even more about how black hole binaries can form in our Universe, as well as the fundamental physics governing them. By improving our instruments, we will be able to delve deeper into these and other aspects with increased accuracy."