When two black holes merge, they form a remnant black hole. But this black hole would not simply sit in place. Instead, due to the gravitational waves emitted during the merger, the conservation of momentum tells us that this black hole will move in a direction to compensate for the loss of linear momentum radiated by the waves.
The direction and speed of this recoil are well understood based on the theory of General Relativity. However, measuring it from gravitational-wave observations requires capturing the subdominant harmonics in the gravitational waves. For years, these subtle details in the data have never been clearly resolved. Then, in 2019, for the first time, we measured the recoil direction from the gravitational-wave event GW190412, which showed strong higher harmonic content. Our research article has recently been published in Nature Astronomy.
In our findings, we determined the direction of the recoil relative to the observer’s position, the orbital angular momentum. Furthermore, we find that the kick velocity of the black hole exceeds 50 km s⁻¹ (that is 180,000 km per hour!), which is enough to eject the black hole from its host galaxy.
Measuring the kick velocity of a black hole is nice. What is even better is that, in the future, we could also utilise this recoil measurement to determine whether flares observed from the centres of galaxies are associated with those produced by escaping black holes after a binary black hole merger event.
See more:
A complete measurement of a black-hole recoil through higher-order gravitational-wave modes
Juan Calderón Bustillo, Samson H. W. Leong, and Koustav Chandra
Nature Astronomy (2025)
Other news coverage:
- The Chinese University of Hong Kong (also available in Chinese)
- Galician Institute for High Energy Physics at the University of Santiago de Compostela (also available in Spanish)
- Penn State Eberly College of Science at the Pennsylvania State University