Stephen Hawking passed away in 2018 thinking that one of his theories about black holes could not be proven. He was wrong about that, but he was right in a prediction he made in 1971, which postulated that the area of a black hole's event horizon (its surface) never decreases and can only remain the same or increase.
This has just been demonstrated by scientists dedicated to studying gravitational waves, grouped in the LIGO-Virgo-KAGRA (LVK) collaboration. These are the observatories built in the US, Italy, and Japan respectively to capture these ripples or distortions in space-time that are generated far from Earth. They occur during extremely violent and energetic cosmic events, such as the merger of black holes or neutron stars, and propagate at the speed of light.
On September 14, it will be 10 years since the historic first detection of gravitational waves. A scientific milestone that revolutionized cosmology when it was announced to the world on February 11, 2016. Those first gravitational waves detected from Earth had been generated by the collision of two black holes 1,300 million light-years away, and since then, numerous similar phenomena have been detected, as well as waves generated by mergers of two neutron stars, or by the merger of a black hole and a neutron star. To celebrate the tenth anniversary, this international team presents the best-observed black hole merger to date, as explained in an article published this Wednesday in the journal Physical Review Letters.
The detection from the US observatory LIGO of the gravitational waves whose existence was predicted by Albert Einstein a century ago was made possible thanks to an extraordinary scientific project that took decades. It was led by physicist Rainer Weiss, who passed away last August at the age of 92, along with Kip Thorne and Barry Barish. Weiss proposed the concept of LIGO (Laser Interferometer Gravitational-Wave Observatory) in 1972, and although the team was not sure it would work, it did, and the three shared the Nobel Prize in Physics in 2017 for making this scientific milestone a reality.
The phenomenon described now and observed last January has been named GW250114 and is almost identical to the one observed in 2015: they are also gravitational waves generated by two black holes merging, but thanks to technological advancements and techniques used, "and a bit of luck," they have obtained the best evidence to date on the functioning of these cosmic monsters. Specifically, the gravitational waves captured were emitted by a merger that formed a black hole with a mass of 63 suns and spinning at 100 revolutions per second.
In the words of Maximiliano Isi, the astrophysicist from Columbia University who led the observations alongside Will Farr, "this is the clearest view to date of the nature" of black holes. "The instruments we have now are much better, so I have been able to analyze the signal in a way that would not have been possible 10 years ago," Isi noted in a press release.
In this process, they have obtained the most solid evidence that Stephen Hawking's mentioned theorem was correct, and also that stipulated in Einstein's General Theory of Relativity regarding black holes was true. "One of the most surprising discoveries in the study of black holes is that they behave like thermodynamic systems: they have temperature, emit radiation, and obey laws that closely resemble classical thermodynamic laws. A central aspect of this analogy is the theorem (or law) of the area, formulated by Stephen Hawking in 1971, which states that the area of a black hole's event horizon, and therefore its mass, cannot decrease in any classical process," contextualizes the scientist from the University of Valencia José Antonio Font, a member of the LIGO-Virgo-KAGRA collaboration.
The experimental demonstration of this phenomenon, he explains, "requires access to information on how the event horizon of a black hole evolves in a dynamic process. Therefore, this situation can potentially be investigated when two black holes merge to form another, which is precisely the type of phenomenon that the LVK collaboration has been observing almost routinely since 2015. In the GW250114 detection now published, we have been able to observe this phenomenon with the highest statistical significance to date: the area of the remaining black hole is greater than the sum of the individual areas of the two initial black holes."
Already in 2021, using data from the LIGO detector, Isi and his team made an initial observational confirmation of this theory by Stephen Hawking, which they have now been able to verify with much more precision. After the 2021 result was published, The New York Times wrote that if that verification had come before the genius from Cambridge's death, it could have helped him win the Nobel Prize. Font believes that "the confirmation of his theorem would likely have helped him receive the Nobel Prize," although he considers that "during his career, he obtained other equally spectacular results, such as the quantum phenomenon of black hole evaporation (or Hawking radiation), perhaps also deserving of the award."
The simplicity of black holes
According to Font, the technological advancement in detectors over these 10 years "has been spectacular," and has allowed a significant increase in the number of observations: "They have revealed the existence of new populations of binary systems of compact objects. Most detections have been black hole mergers, which have allowed the exploration of the strong and dynamic gravitational field regime, testing the limits of Einstein's General Theory of Relativity and its paradigmatic predictions," he states.
Thus, in this latest work, researchers have confirmed that the merged black hole is consistent with what is known as a "Kerr black hole." Mathematician Roy Kerr solved Einstein's space-time equations in the 1960s, proposing a mathematical solution for what exactly the gravity, space, and time of a black hole should be. Physicists believe that all black holes should be described by the Kerr solution, but confirming this was very difficult. By studying the vibrations of the final black hole in this clear signal, this team has obtained the most direct evidence to date that black holes behave as Kerr predicted.
As Font explains, that theoretical prediction establishes that the result of a gravitational collapse process is a Kerr black hole, completely characterized by only two parameters, its mass and angular momentum. "This simplicity of black holes is believed to be universal, known as the no-hair hypothesis. If true, it would mean that black holes are extraordinarily simple objects."
On the other hand, in addition to black holes, in the last 10 years, they have also observed mergers of binary systems involving neutron stars: "Undoubtedly, the most important was GW170817, resulting from the merger of two neutron stars, which not only produced gravitational waves but also was accompanied by electromagnetic emission (observed across the entire electromagnetic spectrum) and neutrinos (although the latter were not observed). The analysis of this signal has had a tremendous impact on various areas of physics, such as astrophysics, cosmology, and nuclear physics," the researcher evaluates.
The future generation of detectors
"We are witnessing these pioneering observations right now and are eager for the signals we are receiving and those yet to come," says astrophysicist Isabel Cordero-Carrión, responsible for outreach and scientific communication at the Virgo Group of the University of Valencia. And if the evolution of technology to capture these phenomena is already considered spectacular, future terrestrial detectors will be up to 10 times more sensitive than the current ones.
As Font details, "in the next decade, the so-called third-generation detectors will be operational, specifically the Cosmic Explorer detector in the U.S. and the Einstein Telescope detector in Europe. These observatories (each with more than one detector) will significantly increase the sensitivity of the current detectors (Advanced LIGO, Advanced Virgo, and KAGRA) (by about one or two orders of magnitude), dramatically improving the number of detections."
Among the numerous technological improvements they present, the Valencian scientist highlights two. "In the design of the Cosmic Explorer, the main improvement in sensitivity is achieved by multiplying the length of the arms of the current LIGO detectors by a factor of 10, going from 4 km to 40 km. In the case of the Einstein Telescope, the sensitivity increase is also achieved by increasing the size of the arms (which will be 10 km long) but also by locating the detector underground, thus considerably mitigating seismic noise," he states.
On the other hand, he explains, "the current detectors of the LVK Collaboration are capable of observing distances on the order of about 10 billion light-years, corresponding to a cosmological redshift of 1. It is estimated that third-generation detectors may be able to observe redshift values on the order of 10. If achieved, this would allow observing all stellar-origin black hole mergers occurring throughout the universe," he anticipates.