Scientists have discovered two pairs of merging black holes and believe that the larger black hole in each merger is a rare “second generation” veteran of an earlier collision.
The unusual behavior of two large black holes observed through ripples in space-time called gravitational waves was described in The Astrophysical Journal Letters on October 28th.
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The results “provide intriguing evidence that these black holes were formed by past black hole mergers,” study co-author Stephen Fairhurst, a professor at Cardiff University in the UK and a spokesperson for the LIGO scientific collaboration, said in a statement.
consecutive mergers
The study is based on two recently detected mergers just one month apart. By analyzing the gravitational wave signatures from these events, researchers were able to infer the mass, rotation, and distance of the black holes involved.
In the first event, on October 11, 2024, scientists discovered two black holes with six and 20 times the mass of the Sun, respectively, colliding in a merger known as GW241011, about 700 million light-years from Earth. The larger black hole was one of the fastest rotating black holes ever discovered.
The second merger, GW241110, was discovered on November 10, 2024, and discovered black holes with masses eight and 17 times that of the Sun. This merger was even further away, 2.4 billion light years away. The larger black hole was also rotating in the opposite direction to its orbit, something that has never been seen before.
Scientists say each of these mergers has novel properties, with the larger black hole in each merger being almost twice the size of the smaller black hole, and the larger black hole rotating in an odd way compared to hundreds of other mergers observed through gravitational waves since the historic first detection by LIGO (Laser Interferometer Gravitational-Wave Observatory) in 2015.
The scientists suggested that the larger black holes in each merger had previously merged in a process called “hierarchical mergers,” which occurs in dense environments such as star clusters where black holes frequently approach each other.
“This is one of our most exciting discoveries to date,” study co-author Jess McIver, an astrophysicist at the University of British Columbia, said in a statement. “These events provide strong evidence that there is a very dense and crowded part of the universe that moves several dead stars together.”
Apart from the possibility of discovering a second-generation black hole, scientists said the merger of the two validates a physical law predicted by Albert Einstein more than a century ago, and the event is helping scientists learn more about elementary particles.
For example, GW241011 produced a clear signal, and the black hole’s rapid rotation allowed scientists to observe the larger black hole deform as it rotated. The resulting gravitational wave signature matched Einstein and mathematician Roy Carr’s theory of rotating black holes.
The same event also created a “hum” in the gravitational wave signal. This happened because the larger black hole was much larger than the smaller black hole. (The hum, the co-researchers said, is similar to the overtones of a musical instrument.) The observation also helped confirm Einstein’s prediction.
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