Imagine the universe as a vast, stretchy fabric, like a cosmic trampoline. When something massive moves or collides, it sends ripples across space and time. These ripples are called gravitational waves.
They’re not made of sound or light. Instead, they’re like invisible tremors that race across the cosmos at the speed of light, carrying clues about the wildest events in the universe, like black holes crashing into each other or stars exploding. Even when these events don’t shine, gravitational waves let us “hear” them.
Back in 1916, Albert Einstein imagined these ripples while working on his theory of general relativity. But they’re incredibly faint, like trying to detect the flutter of a butterfly’s wings from across the planet. It took 100 years and some of the most sensitive instruments ever built to catch one.
In 2015, two detectors called LIGO, located in Washington and Louisiana, finally picked up a signal: GW150914. It came from two black holes, each about 30 times the mass of our Sun, merging in deep space. That moment was like hearing the universe’s heartbeat for the first time.
Since then, scientists have recorded nearly 300 gravitational wave events, opening a brand-new window into the universe.
When two black holes crash into each other, it’s not just a cosmic hug; it’s more like a chaotic dance that ends with a dramatic exit. The newly formed black hole doesn’t always stay put. Instead, it can get a powerful kick, shooting off through space at thousands of kilometers per second, fast enough to escape its own galaxy!
Why does this happen? Because the gravitational waves aren’t always released evenly. If one black hole is bigger or spins differently, the waves push harder in one direction, giving the final black hole a shove. Scientists call this wild phenomenon a black hole recoil.
And here’s the exciting part: in 2019, during a black hole merger called GW190412, researchers finally measured both the speed and direction of this cosmic kick. The event involved two black holes of different sizes, and detectors from LIGO and Virgo picked up the signal.
Prof. Juan Calderon-Bustillo, IGFAE researcher and leading author, explains this with a music analogy: “Black-hole mergers can be understood as a superposition of different signals, just like the music of an orchestra consistent with the combination of music played by many different instruments. However, this orchestra is special: audiences located in different positions around it will record different combinations of instruments, which allows them to understand where exactly they are around it.”
The team concluded that the recoil of the remnant of GW190412 surpassed 50 km/s – enough to expel the black hole from a globular cluster – and determined its recoil direction with respect to the Earth, the orbital angular momentum of the system, and the binary’s separation line a couple of seconds before the merger.
“We came out with this method back in 2018. We showed it would enable kick measurements using our current detectors at a time when other existing methods required detectors like LISA, which was more than a decade away”, Calderon-Bustillo says. “Unfortunately, by that time, Advanced LIGO and Virgo had not detected a signal with ‘music from various instruments’ that could enable a kick measurement. However, we were sure one such detection should happen soon. It was extremely exciting to detect GW190412 just one year later, notice the kick could probably be measured, and we actually did it.!”
Dr. Koustav Chandra, postdoctoral researcher at Penn State, says: “This is one of the few phenomena in astrophysics where we’re not just detecting something, we’re reconstructing the full 3D motion of an object that’s billions of light-years away, using only ripples in spacetime. It’s a remarkable demonstration of what gravitational waves can do.”
Measuring the direction of black-hole recoils can open avenues to study black-hole mergers with both gravitational and electromagnetic signals.
“Black-hole mergers in dense environments can lead to detectable electromagnetic signals – known as flares – as the remnant black hole traverses a dense environment like an active galactic nucleus (AGN),” says Samson Leong, Ph.D student at the Chinese University of Hong Kong and co-author of the article. “Because the visibility of the flare depends on the recoil’s orientation relative to Earth, measuring the recoils will allow us to distinguish between a true GW-EM signal pair that comes from a BBH and a just random coincidence.”
Journal Reference:
- Calderón Bustillo, J., Leong, S.H.W. & Chandra, K. A complete measurement of a black-hole recoil through higher-order gravitational-wave modes. Nat Astron (2025). DOI: 10.1038/s41550-025-02632-5