Twin black hole collisions detected in 2024 have provided the sharpest-ever test of Einstein’s theory while revealing new details about how black holes form and spin. Credit: Carl Knox, OzGrav, Swinburne University of Technology

Two recently detected black hole mergers, observed just a month apart in late 2024, have given scientists their most precise test yet of Einstein’s general relativity.

The twin collisions, recorded by the LIGO-Virgo-KAGRA Collaboration, revealed one of the fastest-spinning black holes ever observed and another that defied expectations by rotating opposite its orbit. These findings confirm key predictions of Einstein’s theory while hinting at “second-generation” black holes—objects formed from earlier mergers.

Twin Black Hole Mergers Rewrite the Rules of Cosmic Collisions

Two massive black hole mergers, recorded just a month apart in late 2024, are helping scientists gain a clearer picture of how the most extreme cosmic collisions unfold. These powerful events are not only refining our understanding of black hole behavior and evolution but also confirming, with unmatched precision, fundamental principles of physics first proposed by Albert Einstein more than a century ago. The new data may also guide future searches for mysterious, still-undiscovered particles that could draw energy from black holes.

A research paper published on October 28 in The Astrophysical Journal Letters by the international LIGO-Virgo-KAGRA Collaboration details the detection of two rare gravitational wave signals from black holes with unusual spin properties, observed in October and November 2024.

Gravitational Waves Confirm Einstein and Reveal the Universe’s Power

Gravitational waves are subtle “ripples” in the fabric of space-time, created when enormous celestial bodies like black holes collide. These waves carry information about the most violent events in the Universe. The first event described in the study, GW241011 (October 11, 2024), occurred about 700 million light-years from Earth. It was caused by the merger of two black holes, roughly 20 and 6 times the mass of the Sun. The larger black hole was found to be spinning at an exceptional rate, making it one of the fastest-rotating black holes ever recorded.

Nearly a month later, another collision—GW241110 (November 10, 2024)—was detected approximately 2.4 billion light-years away. This second merger involved black holes with masses of about 17 and 8 times that of the sun. What made this event especially remarkable was that the larger black hole was spinning in the opposite direction to its orbit, something scientists had never observed before.

GW241011 and GW241110 infographic. Credit: Shanika Galaudage / Northwestern University / Adler Planetarium

“Each new detection provides important insights about the universe, reminding us that each observed merger is both an astrophysical discovery but also an invaluable laboratory for probing the fundamental laws of physics,” says paper co-author Carl-Johan Haster, assistant professor of astrophysics at the University of Nevada, Las Vegas (UNLV). “Binaries like these had been predicted given earlier observations, but this is the first direct evidence for their existence.”

Uncovering Hidden Properties of Black Hole Mergers

Albert Einstein first proposed the existence of gravitational waves in 1916 as part of his general theory of relativity. Although indirect evidence emerged in the 1970s, scientists did not observe these waves directly until 2015, when the LIGO observatory confirmed their detection from a black hole collision.

The LIGO-Virgo-KAGRA Collaboration now operates a global network of highly sensitive detectors that track these faint signals from across the universe. The network is currently in its fourth major observation phase, known as O4, which began in late May 2023 and is scheduled to continue through mid-November 2025. So far, astronomers have identified around 300 black hole mergers through gravitational wave data, including several promising candidates from the ongoing O4 run.

Signs of “Second-Generation” Black Holes Emerge

Together, the detection of GW241011 and GW241110 highlights the remarkable progress of gravitational-wave astronomy in uncovering the properties of merging black holes. Interestingly, both detected mergers point toward the possibility of “second-generation” black holes.

“GW241011 and GW241110 are among the most novel events among the several hundred that the LIGO-Virgo-KAGRA network has observed,” says Stephen Fairhurst, professor at Cardiff University and spokesperson of the LIGO Scientific Collaboration. “With both events having one black hole that is both significantly more massive than the other and rapidly spinning, they provide tantalizing evidence that these black holes were formed from previous black hole mergers.”

Clues Hidden in Unequal Masses and Mysterious Spins

Scientists point to certain clues, including the size differential between the black holes in each merger – the larger was nearly double the size of the smaller – and the spin orientations of the larger of the black holes in each event. A natural explanation for these peculiarities is that the black holes are the result of earlier coalescences. This process, called a hierarchical merger, suggests that these systems formed in dense environments, in regions like star clusters, where black holes are more likely to run into each other and merge again and again.

“These two binary black hole mergers offer us some of the most exciting insights yet about the earlier lives of black holes,” said Thomas Callister, co-author and assistant professor at Williams College. ”They teach us that some black holes exist not just as isolated partners but likely as members of a dense and dynamic crowd. Moving forward, the hope is that these events and other observations will teach us more and more about the astrophysical environments that host these crowds.”

Implications for Fundamental Physics

The precision with which GW241011 was measured also allowed key predictions of Einstein’s theory of general relativity to be tested under extreme conditions.

Because GW241011 was detected so clearly, it can be compared to predictions from Einstein’s theory and mathematician Roy Kerr’s solution for rotating black holes. The black hole’s rapid rotation slightly deforms it, leaving a characteristic fingerprint in the gravitational waves it emits. By analyzing GW241011, the research team found excellent agreement with Kerr’s solution and verified Einstein’s prediction with unprecedented accuracy.

Gravitational “Overtones” Confirm Einstein Once Again

Additionally, because the masses of the individual black holes differ significantly, the gravitational-wave signal contains the “hum” of a higher harmonic – similar to the overtones of musical instruments, seen only for the third time ever in GW241011. One of these harmonics was observed with superb clarity and confirms another prediction from Einstein’s theory.

“The strength of GW241011, combined with the extreme properties of its black hole components provide unprecedented means for testing our understanding of black holes themselves,” says Haster. “We now know that black holes are shaped like Einstein and Kerr predicted, and general relativity can add two more checkmarks in its list of many successes. This discovery also means that we’re more sensitive than ever to any new physics that might lie beyond Einstein’s theory.”

Advanced Search for Elementary Particles

Rapidly rotating black holes like those observed in this study now have yet another application – in particle physics. Scientists can use them to test whether certain hypothesized light-weight elementary particles exist and how massive they are.

These particles, called ultralight bosons, are predicted by some theories that go 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. How much energy is extracted and how much the rotation of the black holes slows down over time depends on the mass of these particles, which is still unknown.

The observation that the massive black hole in the binary system that emitted GW241011 continues to rotate rapidly even millions or billions of years after it formed rules out a wide range of ultralight boson masses.

Next-Generation Detectors Will Push the Limits

“Planned upgrades to the LIGO, Virgo, and KAGRA detectors will enable further observations of similar systems, enabling us to better understand both the fundamental physics governing these black hole binaries and the astrophysical mechanisms that lead to their formation,” said Fairhurst.

Joe Giaime, site head for the LIGO Livingston Observatory, noted that LIGO scientists and engineers have made improvements to the detectors in recent years, which has resulted in precision measurements of merger waveforms that allow for the kind of subtle observations that were needed for GW241011 and GW241110.

“Better sensitivity not only allows LIGO to detect many more signals, but also permits deeper understanding of the ones we detect,” he said.

Reference: “GW241011 and GW241110: Exploring Binary Formation and Fundamental Physics with Asymmetric, High-spin Black Hole Coalescences” by A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I.-L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu, S. Albanesi, W. Ali, S. Al-Kershi, C. Alléné, A. Allocca, S. Al-Shammari, P. A. Altin, S. Alvarez-Lopez, W. Amar, O. Amarasinghe, A. Amato, F. Amicucci, C. Amra, A. Ananyeva, S. B. Anderson, W. G. Anderson, M. Andia, M. Ando, M. Andrés-Carcasona, T. Andrić, J. Anglin, S. Ansoldi, J. M. Antelis, S. Antier, F. Antonini, M. Aoumi, E. Z. Appavuravther, S. Appert, S. K. Apple, K. Arai, C. Araújo-Álvarez, A. Araya, M. C. Araya, M. Arca Sedda, J. S. Areeda, N. Aritomi, F. Armato, S. Armstrong, N. Arnaud, M. Arogeti, S. M. Aronson, K. G. Arun, G. Ashton, Y. Aso, L. Asprea, M. Assiduo, S. Assis de Souza Melo, S. M. Aston, P. Astone, P. S. Aswathi, F. Attadio, F. Aubin, K. AultONeal, G. Avallone, E. A. Avila, S. Babak, C. Badger, S. Bae, S. Bagnasco, L. Baiotti, R. Bajpai, T. Baka, A. M. Baker, K. A. Baker, T. Baker, G. Baldi, N. Baldicchi, M. Ball, G. Ballardin, S. W. Ballmer, S. Banagiri, B. Banerjee, D. Bankar, …, H. Yamamoto, K. Yamamoto, T. S. Yamamoto, T. Yamamoto, R. Yamazaki, T. Yan, K. Z. Yang, Y. Yang, Z. Yarbrough, J. Yebana, S.-W. Yeh, A. B. Yelikar, X. Yin, J. Yokoyama, T. Yokozawa, S. Yuan, H. Yuzurihara, M. Zanolin, M. Zeeshan, T. Zelenova, J.-P. Zendri, M. Zeoli, M. Zerrad, M. Zevin, L. Zhang, N. Zhang, R. Zhang, T. Zhang, C. Zhao, Yue Zhao, Yuhang Zhao, Z.-C. Zhao, Y. Zheng, H. Zhong, H. Zhou, H. O. Zhu, Z.-H. Zhu, A. B. Zimmerman, L. Zimmermann, Y. Zlochower, M. E. Zucker and J. Zweizig, 28 October 2025, The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ae0d54

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