The LIGO–Virgo–KAGRA collaboration has detected strong evidence for second-generation black holes, which were formed from earlier mergers of smaller black holes. The two gravitational wave signals provide one of the strongest confirmations to date for how Einstein’s general theory of relativity describes rotating black holes. Studying such objects also provides a testbed for probing new physics beyond the Standard Model.
Over the past decade, the global network of interferometers operated by LIGO, Virgo, and KAGRA have detected close to 300 gravitational waves (GWs) – mostly from the mergers of binary black holes.
In October 2024, the network detected a clear signal that pointed back to a merger that occurred 700 million light–years away. The progenitor black holes were 20 and 6 solar masses and the larger object was spinning at 370 Hz, which makes it one of the fastest-spinning black holes ever observed.
Just one month later, the collaboration detected the coalescence of another highly imbalanced binary (17 and 8 solar masses), 2.4 billion light–years away. This signal was even more unusual – showing for the first time that the larger companion was spinning in the opposite direction of the binary orbit.
Massive and spinning
While conventional wisdom says black holes should not be spinning at such high rates, the observations were not entirely unexpected. “With both events having one black hole, which is both significantly more massive than the other and rapidly spinning, [the observations] provide tantalizing evidence that these black holes were formed from previous black hole mergers,” explains Stephen Fairhurst at Cardiff University, spokesperson of the LIGO Collaboration. If this were the case, the two GW signals – called GW241011 and GW241110 – are first observations of second-generation black holes. This is because when a binary merges, the resulting second-generation object tends to have a large spin.
The GW241011 signal was particularly clear, which allowed the team to make the third-ever observation of higher harmonic modes. These are overtones in the GW signal that become far clearer when the masses of the coalescing bodies are highly imbalanced.
The precision of the GW241011 measurement provides one of the most stringent verifications so far of general relativity. The observations also support Roy Kerr’s prediction that rapid rotation distorts the shape of a black hole.
Kerr and Einstein confirmed
“We now know that black holes are shaped like Einstein and Kerr predicted, and general relativity can add two more checkmarks in its list of many successes,” says team member Carl-Johan Haster at the University of Nevada, Las Vegas. “This discovery also means that we’re more sensitive than ever to any new physics that might lie beyond Einstein’s theory.”
This new physics could include hypothetical particles called ultralight bosons. These could form in clouds just outside the event horizons of spinning black holes, and would gradually drain a black hole’s rotational energy via a quantum effect called superradiance.
The idea is that the observed second-generation black holes had been spinning for billions of years before their mergers occurred. This means that if ultralight bosons were present, they cannot have removed lots of angular momentum from the black holes. This places the tightest constraint to date on the mass of ultralight bosons.
“Planned upgrades to the LIGO, Virgo, and KAGRA detectors will enable further observations of similar systems,” Fairhurst says. “They will enable us to better understand both the fundamental physics governing these black hole binaries and the astrophysical mechanisms that lead to their formation.”
Haster adds, “Each new detection provides important insights about the universe, reminding us that each observed merger is both an astrophysical discovery but also an invaluable laboratory for probing the fundamental laws of physics”.
The observations are described in The Astrophysical Journal Letters.
The post Rapidly-spinning black holes put new limit on ultralight bosons appeared first on Physics World.