In February 2016, scientists at the Laser Interferometer Gravitational Wave Observatory (LIGO) announced the first-ever detection of gravitational waves (GWs). Originally predicted by Einstein’s theory of general relativity, these waves are ripples in spacetime that occur whenever massive objects (like black holes and neutron stars) merge. Since then, countless GW events have been detected by observatories around the world – to the point where they have become an almost daily occurrence. This has given astronomers a better understanding of some of the most extreme objects in the Universe.
In a recent study, an international team of researchers led by Cardiff University observed a binary black hole system initially detected in 2020 by the Advanced LIGO, Virgo and Kamioki Gravitational Wave Observatory (KAGRA). During the process, the team noticed a peculiar twisting motion (aka precession) in the orbits of the two colliding black holes that was 10 billion times faster than what has been noted with other precessing objects. . This is the first time that a precession has been observed with binary black holes, confirming yet another phenomenon predicted by General Relativity (GR).
The team was led by Professor Mark Hannam, Dr Charlie Hoy and Dr Jonathan Thompson of Cardiff University’s Gravity Exploration Institute. They were joined by researchers from the LIGO laboratory, the Barcelona Institute of Science and Technology, the Max Planck Institute for Gravitational Physics, the Institute of Gravitational Wave Astronomy, the ARC Center of Excellence for the discovery of gravitational waves, the Scottish Universities Physics Alliance (SUPA), and other GW research institutes.
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Binary black holes (BBH) are considered a prime candidate for GW research since astronomers expect some to consist of precessing binaries. In this scenario, the black holes will circle each other in tighter and tighter orbits, generating a stronger and stronger GW signal until they merge. However, no definitive evidence of orbital precession has been observed from the 84 BBH systems detected by Advanced LIGO and Virgo so far. However, the team noticed something different when examining event GW200129 detected by the LIGO-Virgo-KAGRA collaboration during its third operational phase (O3).
One of the black holes in this system (~40 solar masses) is believed to be the fastest spinning black hole ever detected by gravitational waves. Unlike all of BBH’s previous observations, the rapidly rotating system has such a profound effect on spacetime that the entire system oscillates back and forth. This form of precession is known as Frame Dragging (aka the Lense-Thirring effect), an interpretation of GR where gravitational forces are so strong that they “drag” with them the very fabric of spacetime. .
This same phenomenon is observed when observing the orbit of Mercury, which periodically precedes in orbit around the Sun. In short, Mercury’s path around the Sun is very eccentric, and the furthest point in its orbit (perihelion) also moves in time, spinning around the Sun like a top. These observations are one of the ways GR was tested (and confirmed) after Einstein formalized it in 1916. In general, precession in general relativity is usually such a small effect that it is almost imperceptible. As Dr Thompson explained in a recent press release from Cardiff University:
“It’s a very difficult effect to identify. Gravitational waves are extremely weak and their detection requires the most sensitive measuring device in history. Precession is an even weaker effect buried in the already weak signal, so we had to do some careful analysis to find out.
Previously, the fastest known example was a binary pulsar that took over 75 years for orbit to be processed. In this case, the BBH known as GW200129 (observed January 29, 2020) processes several times per second, an effect 10 billion times stronger than the binary pulsar. Even so, confirming that black holes in this system preceded was a tall order. Says Dr Hoy, who is now a researcher at the University of Portsmouth:
“Until now, most of the black holes we’ve found with gravitational waves have been spinning quite slowly. The largest black hole in this binary, which was about 40 times more massive than the Sun, was spinning almost as fast as physically possible. Our current patterns of binary formation suggest this was extremely rare, possibly one in a thousand events. Or it could be a sign that our patterns need to change.
These results confirm that before black hole mergers – the most extreme gravitational event ever observed by astronomers – BBHs can undergo orbital precession. It’s also the latest in a long line of examples that demonstrate how GW Astronomy allows astronomers to probe the laws of physics in the most extreme conditions imaginable. With a network of advanced LIGO, Virgo and KAGRA detectors in the United States, Europe and Japan, it is also one of the fastest growing areas of astronomical research.
This array is currently being upgraded to improve its sensitivity to GW events and will begin its fourth round of observations (O4) in 2023. When this happens, it is hoped that several hundred black hole collisions will be detected and added. in the GW catalog. This will allow astronomers to better understand the most extreme gravitational phenomenon in the Universe and let them know if GW200129 was an outlier or if such extreme events are common.
This research was funded by the Science and Technology Facilities Council (STFC) – part of the UK Research and Innovation Organization (UKRI) – and the European Research Council (ERC) of the European Commission. The paper describing their findings, titled “General-relativistic precession in a black-hole binary,” recently appeared in the journal Nature.
Further reading: Cardiff University, Nature
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