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CUHK cracks a mystery of the universe
World’s first precise measurement of a black hole’s recoil
A research team led by the Department of Physics at The Chinese University of Hong Kong (CUHK) has made a major breakthrough in measuring the speed and direction of the recoil of the remnant black hole from a binary black-hole merger. The team revealed that the recoil of that black hole exceeded 50 km s-1, as well as determined its direction relative to Earth, the orbital angular momentum of the system, and the binary’s line of separation a couple of seconds before merger. The research findings, which have been published in the prestigious journal Nature Astronomy, provide invaluable insights into some of the mysteries of the universe.
The research was led by Professor Juan Calderón Bustillo, Adjunct Assistant Professor at CUHK’s Department of Physics and Assistant Professor at the University of Santiago de Compostela, in collaboration with Mr Samson Leong Hin-wai, a PhD student at CUHK’s Department of Physics, and Dr Koustav Chandra from the Pennsylvania State University and the Indian Institute of Technology Bombay. Gravitational waves are tiny ripples in the fabric of spacetime that travel at the speed of light, encoding information about their sources. These waves offer a completely unique channel of information that allows scientists to observe astrophysical phenomena that do not emit light, such as black hole mergers, and to gather new knowledge about processes that do, like supernovae or neutron star mergers.
Although Albert Einstein predicted the existence of gravitational waves in 1916, they were not detected until 2015, when two black holes merged and sent out a signal recorded by the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) detectors in the United States. Since then, nearly 300 such events have been recorded, helping scientists better understand black holes and the nature of gravity. However, accurately measuring the recoil of the remnant black hole, the powerful push it gets after two black holes collide and merge, required observing a merger with the right characteristics. When two black holes merge, the uneven release of gravitational waves can produce a kick that is enough to eject the black hole from a globular cluster.
Professor Calderón Bustillo explained this with a musical analogy: “Gravitational waves from black-hole mergers can be understood as a superposition of different signals, much like an orchestra composed of music played by various instruments. However, this orchestra is special: audiences located in various positions will hear slightly different combinations of instruments, helping them understand their exact location in relation to it.”
Measuring a black hole’s recoil
Gravitational waves look different depending on the locations in space from which they are observed. By analysing these differences, scientists can figure out where the waves came from and how the black hole moved after merging. In this new study, the research team focused on a black hole merger detected in 2019, known as GW190412. He added: “We came up with this method back in 2018. We showed it would enable kick measurements with our current detectors at a time when other existing methods require future detectors like LISA, which will only operate in more than a decade away. Unfortunately, Advanced LIGO had not detected a signal with ‘music from various instruments’ that could enable a kick measurement. Just a year later, we successfully detected GW190412 and noticed that the kick could be measured.”
Dr Chandra added: “This is one of the few astrophysical phenomena where we are not just detecting something but actually reconstructing the motion of an object billions of light years away, using only ripples in spacetime. Moreover, unlike most other astronomical observations, this is a full 3D reconstruction, not just a 2D projection on the sky. It’s a remarkable demonstration of what gravitational waves can achieve.”
Broader scientific applications and future research directions
This technique for measuring the direction of black-hole recoil could prove useful in unexpected avenues in the future. One example is its application in studies of black-hole mergers detected with both gravitational and electromagnetic signals.
Mr Leong said: “When a black hole moves through a dense environment, like the centre of a galaxy, it can create a visible flare. But whether we can see that flare depends on the direction the black hole is moving. Therefore, if we know the recoil direction, we can tell whether the flare and the gravitational wave came from the same event or if it is just a coincidence.”
Beyond that, the tools and techniques developed in this research can be used to explore other mysteries of the Universe, such as how symmetrically black holes are distributed in the Universe on the large scale.
A research team led by the Department of Physics at CUHK has made a major breakthrough in measuring the speed and direction of the recoil of the remnant black hole from a binary black-hole merger. The team revealed that the recoil of that black hole exceeded 50 km s-1, as well as determined its direction relative to Earth, the orbital angular momentum of the system, and the binary’s line of separation a couple of seconds before merger.
A research team led by the Department of Physics at CUHK has made a major breakthrough in measuring the speed and direction of the recoil of the remnant black hole from a binary black-hole merger. The team revealed that the recoil of that black hole exceeded 50 km s-1, as well as determined its direction relative to Earth, the orbital angular momentum of the system, and the binary’s line of separation a couple of seconds before merger.
