Gravitational wave detectors LIGO and Virgo have observed two recent collisions of black holes that appear to originate from previous collisions.
The paper today in the Astrophysical Journal Letters describes two gravitational wave events on 11 October and 10 November last year, originating from colliding black holes. It is striking that in both cases one of the black holes is much heavier than the other, with in addition an unusual rotation or spin.
The assumption is that the heavy component is the product of an earlier merger of black holes, somewhere in a dense and crowded environment such as a star cluster. In theory, black holes can merge with other black holes time and time again, creating a new and larger black hole, known as hierarchical mergers. The new observations confirm this.
Interferometers
LIGO and Virgo are interferometers in the US and Italy that use lasers and mirrors to measure small variations in distances when a gravitational wave from the universe distorts space and time. Such space-time vibrations are caused by colliding black holes or neutron stars. Nikhef is a partner in Virgo near Pisa and a member of the LIGO-Virgo-KAGRA collaboration that analyses the measurements.
The first wave signal was measured on 11 October 2024 and is attributable to the merger of a black hole with a mass of 17 solar masses and one with a mass of 7 solar masses at a distance of 700 million light years. The heavy specimen of GW241011 had the highest spin observed to date.
Unique direction of rotation
A month later, on 10 November, a signal GW24 was detected 2.4 billion light-years away from the collision of a black hole with a mass of 16 solar masses and one with a mass of 8 solar masses. What was remarkable was that the heavy specimen rotated in the opposite direction to the normal direction of rotation, which was a first.
The heavy components in both mergers are thought to stem from earlier black hole mergers. Until now most black holes were expected to originate from star collapses. These new observations demonstrate the first alternatives for how some black holes grow merger after merger, says Nikhef gravitational wave expert Chris Van Den Broeck in Utrecht.
Van Den Broeck: “This is very exciting, also because it points to what future gravitational-wave observatories such as Einstein Telescope and LISA will be capable of delivering. Together, these will be able to reconstruct the merger history of black holes going from hundreds to more than a billion solar masses, throughout the entire lifetime of the Universe – and study other formation scenarios, such as primordial black holes being forged immediately after the Big Bang.”
300 collisions
Gravitational waves were first predicted by Albert Einstein as part of his general theory of relativity in 1916. Their existence – although proven in the 1970s – was only directly observed by scientists for the first time ten years ago, when the scientific collaborations LIGO and Virgo announced the detection of waves from a merger of black holes.
Today, LIGO-Virgo-KAGRA is a global network of advanced gravitational wave detectors and is nearing the end of its fourth observation period, O4. The current period began in late May 2023 and is expected to last until mid-November this year.
To date, approximately 300 black hole collisions have been detected via gravitational waves, including candidates identified in the ongoing O4 run that are awaiting final validation.
Kerr effect
The accuracy with which GW241011 was measured made it possible to test important predictions of Einstein’s general theory of relativity under extreme conditions. Due to its rapid rotation, the black hole deforms slightly, leaving a characteristic fingerprint in the gravitational waves it emits, the so-called Kerr effect. The GW241011 shows an excellent match with Kerr’s solution and reconfirmed Einstein’s prediction with unprecedented accuracy.
