12.09.2025
Observations of gravitational waves produced by 2 black holes colliding and merging have allowed scientists to confirm fundamental predictions made by Albert Einstein and Stephen Hawking about the nature of the universe.
“This is the clearest view yet of the nature of black holes,” says astrophysicist Maximiliano Isi, who co-led the analysis published in the Physical Review Letters.
Cataclysmic cosmological events, such as the merging of 2 black holes, distort the fabric of the universe. This sends out ripples, or ‘gravitational waves’, which stretch and contract space-time.
Gravitational waves produced by merging black holes were first detected by the US National Science Foundation Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015. LIGO detects changes in space-time smaller than 1/10,000 the width of a proton.
Today, it operates with international partners, the Virgo gravitational-wave detector in Italy and KAGRA in Japan. In January 2025, during LVK collaboration’s current observation run, LIGO picked up on the collision of 2 black holes 1.3 billion light-years away.
“This specific collision involved 2 black holes that looked pretty much identical to the first 2 we saw,” says Isi. “Intrinsically, the signal is equally loud, but our detectors are just so much more high fidelity now.
“We’re able to analyse the signal in ways that just weren’t possible 10 years ago.”
Advancements of gravitational wave observatories in observing black holes cosmic collisions, with registered signals shown comparing the 2015 black hole merger (GW150914) and the one observed in 2025 (GW250114). Credit: Dr Derek Davis (Caltech, LIGO Laboratory).
The researchers studied the event from the moment the black holes first collided, until the merged black hole settled into its new state just milliseconds later.
The final reverberations, or ‘ringdown’, can be likened to the way a bell rings when it is struck.
“Ten milliseconds sounds really short, but our instruments are so much better now that this is enough time for us to really analyse the ringing of the final black hole,” Isi says. “With this new detection, we have an exquisitely detailed view of the signal both before and after the black hole merger.”
This allowed the team to calculate the mass and spin of the black hole to determine its surface area. They found that while the initial black holes had a total surface area of 240,000km2, this increased to about 400,000km2 after merging.
The observations confirm a foundational idea proposed by British physicist Stephen Hawking in 1972, which predicts that the surface area of a black hole’s event horizons can never decrease.
Hawking’s area theorem – also known as the second law of black hole mechanics – mirrors the second law of thermodynamics, which says that entropy (disorder) can only increase. The theorem led to the realisation that black holes are thermodynamic objects.
“It’s really profound that the size of a black hole’s event horizon behaves like entropy,” Isi says.
“It has very deep theoretical implications and means that some aspects of black holes can be used to mathematically probe the true nature of space and time.
“It tells us that general relativity knows something about the quantum nature of these objects and that the information, or entropy, contained in a black hole is proportional to its area.”
The researchers also confirmed that the merged black hole was consistent with what is known as the ‘Kerr metric’.
Left panel: Frequency and decay time (half-life) of the different ‘ringdown’ tones measured in the 2025 black hole merger (GW250114). The black markers indicate the values predicted for a Kerr black hole. Right panel: gravitational-wave signal (bottom spiral) emitted by the remnant black hole (bottom sphere) into the different tones, for a numerical simulation matching the measured parameters of GW250114. Credit: Dr. Keefe Mitman (Cornell University), Prof. Harald Pfeiffer (Albert Einstein Institute, Potsdam).
In 1963, New Zealand mathematician Roy Kerr solved Albert Einstein’s field equations of general relativity and showed that black holes can be described by just 2 characteristics: spin and mass.
“We’ve found some of the strongest evidence yet that astrophysical black holes are the black holes predicted from Albert Einstein’s theory of general relativity,” says Isi.
“Two black holes with the same mass and spin are mathematically identical. It’s very unique to black holes.”
Gravitational wave detectors are only expected to become more sensitive in the next decade, which will allow for even more rigorous tests of black hole characteristics.
“Listening to the tones emitted by these black holes is our best hope for learning about the properties of the extreme space-times they produce,” says astrophysicist Will Farr, who co-led the research.
“As we build more and better gravitational wave detectors, the precision will continue to improve.”
Quelle: COSMOS