Astronomers have for the first time measured the speed and direction of a newborn black hole, thanks to gravitational waves produced as it bounced away from the site of its parent black holes’ merger. This first complete measurement of black hole recoil comes almost exactly a decade after the first detection of gravitational waves — tiny ripples in spacetime first predicted by Albert Einstein in 1915 — performed by the Laser Interferometer Gravitational-Wave Observatory (LIGO) on Sept. 14, 2015.
Over the last 10 years, a wealth of gravitational wave detections performed by LIGO and its collaboration gravitational wave detectors, Virgo, and Kamioka Gravitational Wave Detector (KAGRA) have painted a more detailed picture of black hole mergers than ever before. However, one of the most fascinating and dramatic aspects of these mergers has remained “unheard” by these detectors that measure the ringing of spacetime caused by the universe’s most extreme events. That is the “kick” delivered to the daughter black hole birthed by these mergers.
This kick causes the newborn black hole to wail out gravitational waves in a preferred direction — an imbalance that causes it to speed away from the site of its birth, sometimes as fast as many millions of miles per hour. That is fast enough for the black hole to escape its home galaxy.
This uneven distribution of gravitational waves from black hole recoil should “sound” very different from regular gravitational waves emitted by black hole mergers as well as ripples in spacetime emitted as black holes in binaries spiral together.
The signal also differs based on the position an observer occupies relative to the black hole’s recoil. That differentiation allows scientists to look at the gravitational wave signal and determine the direction and speed of the kicked black hole.
“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,” Juan Calderon-Bustillo, study team leader and a researcher tat the Instituto Galegode Físicade Altas Enerxías (IGFAE), said in a statement. “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.”
Black hole scientists will get a kick out of this
To investigate the recoil of a newborn black hole, Calderon-Bustillo and colleagues investigated a merger of two black holes of different masses recorded by LIGO and Virgo back in 2019 as the gravitational wave signal GW 190412.
The difference between this study and previous analyses of the signal is this team used a new methodology that enabled them to detect the kick received by the daughter black hole.
“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 [a proposed space-based gravitational wave detector], which was more than a decade away,” Calderon-Bustillo said. “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 actually do it.”
The black hole created in the merger that launched the signal GW190412 was seen racing away from the site of its birth at a staggering 112,000 miles per hour (50 kilometers per second). That’s about 150 times the speed of sound here on Earth.
While that is far from the maximum speed a black hole can reach after a merger-caused kick, it is fast enough to allow this black hole to escape the dense grouping of stars, or globular cluster, in which it was born.
“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,” Koustav Chandra, study team member and a researcher at Penn State University, said in the statement. “It’s a remarkable demonstration of what gravitational waves can do.”
The next step for the team will be to use this recoil as well as the direction and speed measurements of daughter black holes to investigate black hole mergers through both gravitational waves and with electromagnetic radiation, the latter of which is the basis of “traditional astronomy.”
“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),” study team member Samson Leong of the Chinese University of Hong Kong explained in the statement. “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 gravitational wave-electromagnetic signal pair that comes from a binary black hole and a just random coincidence.”
The team’s research was published on Tuesday (Sept. 9) in the journal Nature Astronomy.
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