The international network of gravitational wave detectors LIGO in the United States, Virgo in Italy and KAGRA in Japan (LVK) announced the publication of an updated catalogue of all gravitational events observed to date, called Gravitational-Wave Transient Catalogue-4.0 (GWTC-4).
The results are the fruit of in-depth analyses conducted over more than two years by scientists from the LVK Collaboration on the new signals observed, with the aim of confirming their validity and studying their most important astrophysical and cosmological implications.
Although some of these have already been announced in recent months, the publication of the new catalogue offers a unique perspective on the entirety and scope of all the signals observed by the LVK Collaboration from May 2023 to January 2024 (during the observation cycle called O4a) and shows how the Universe echoes in a veritable kaleidoscope of cosmic collisions.
The catalogue, with a series of articles appearing in Astrophysical Journal Letters, collects 128 new events, more than doubling the number of events in the previous catalogue, which contained the 90 signals detected during the previous three observation campaigns.
The data, which has now been made available for further analysis by research groups external to the LVK collaboration, reveals an even greater variety of binary pairs producing gravitational waves than previously known: these include the most massive black hole binary ever detected with gravitational waves, a black hole binary with the largest mass asymmetry ever observed, and one in which both black holes have exceptionally high spins, as well as two black hole and neutron star binaries.
"In the past decade, gravitational wave astronomy has progressed from the first detection to the observation of hundreds of black hole mergers," says Stephen Fairhurst, a professor at Cardiff University and LIGO Scientific Collaboration Spokesperson. "These observations enable us to better understand how black holes form from the collapse of massive stars, probe the cosmological evolution of the universe and provide increasingly rigorous confirmations of the theory of general relativity."
"The publication of the GWTC-4 catalogue represents a definitive leap forward, adding 128 new signals to our record in less than a year of observation." said Gianluca Gemme, researcher at the Istituto Nazionale di Fisica Nucleare (INFN) and Virgo spokesperson , "This wealth of data reveals a true kaleidoscope of cosmic collisions: from the heaviest binary black holes ever detected, like GW231123, to pairs spinning at nearly half the speed of light. These are no longer just rare anomalies; they provide the statistical foundation we need to test Einstein's General Relativity with unprecedented precision and to provide a new, independent measurement of how fast our Universe is expanding. For Virgo and the LVK network, these results prove that we are now mapping the complex evolution of the cosmos with more clarity than ever before.
Unusual signals
Among the most unusual signals detected in the first phase of the O4 observation campaign is GW231123 (the name refers to the day on which the signal was observed, according to US convention): this signal was generated by the heaviest binary black hole system ever detected to date, each with a mass approximately 130 times that of the Sun. Most of the black holes in the binary systems detected have a mass of about 30 solar masses. The much heavier black holes that generated GW231123 suggest that each of them may be the result of a previous collision between lighter 'progenitor' black holes, probably in extremely crowded and chaotic cosmic environments.
Another case of extraordinary interest is that of GW231028, generated by a pair of black holes with the highest spin ever observed: both black holes rotate very fast, at about 40% of the speed of light. Again, scientists suspect that these black holes may be the product of previous collisions, from which they derived their enormous rotational energy. Among the events in the collection there is also GW231118, generated by an unusually unbalanced pair, with one black hole twice as massive as the other
Thanks to the latest gravitational wave detections and the significant growth in data on black hole mergers, scientists have also begun to study the properties of black holes in terms of populations.
"When we detect unexpected signals like GW231123 and GW231028, we face both a challenge and an exciting opportunity. These discoveries remind us that the Universe can still surprise us. To truly understand it, our scientific models must be able to explain — and even anticipate — the full range of signals nature creates." said Filippo Santoliquido, researcher at Gran Sasso Science Institute
The new data also allows research teams to refine tests and measurements previously made with a smaller data set, continuing to explore and investigate some of the big, unresolved questions in contemporary physics.
Is Einstein still right?
The new findings allow, for example, further and more accurate testing of Einstein's theory of general relativity: the theory that revolutionised our view of the universe a century ago, describing gravity as a geometric property of space and time. Since then, Einstein's theory has been supported by numerous experimental tests and observations, proving to be the best theoretical description of gravity we have.
However, the fact that black hole collisions shake space and time more intensely than almost any other conceivable phenomenon also makes them ideal candidates for testing the theory itself. "When testing our physical theories, it is good to consider the most extreme situations possible, because that is where our theories are most likely to fail and where we have the greatest chance of making discoveries" adds Aaron Zimmerman, associate professor of physics at the University of Texas at Austin.
The researchers tested Einstein's theory using GW230814, one of the 'strongest' gravitational wave signals in this last Catalog. The surprising clarity of the signal allowed it to be examined in detail to see if any aspects of it deviated from Einstein's theory. However, so far, the theory is passing all tests.
"I'm excited to see how the growing number and improving quality of gravitational-wave detections are enabling increasingly sensitive tests of general relativity in the dynamical and strong-field regime of gravity." said Soumen Roy, researcher at UCLouvain, Centre for Cosmology, Particle Physics and Phenomenology (CP3) in Belgium "With future observations covering a broader range of black hole masses, spins, and orbital eccentricities, we will be able to place stronger constraints on alternative theories of gravity or potentially uncover signatures of new physics.
How fast is the universe expanding?
Another major mystery in cosmology concerns the speed at which our universe is expanding today. To answer this question, it is crucial to estimate the Hubble constant, which indicates the expansion speed of the universe today. There are several estimates of this constant which, using different methods and different astrophysical sources, have provided conflicting answers.
Gravitational waves offer an additional way to measure the Hubble constant, since by studying the signal, it is possible to calculate, in a relatively simple way, the distance travelled from its origin.
"There is growing excitement around gravitational wave cosmology." said Ulyana Dupletsa researcher at the Marietta Blau Institute (Austrian Academy of Sciences). "Besides being a new and independent approach, it is highly appealing as it avoids the complex calibration required by the established methods. Even though we do not yet have enough observations to match the precision of the traditional measurements, every new gravitational-wave detection brings us one step closer to shedding light on how fast our universe is expanding."
By analysing all gravitational wave detections in the entire LVK catalogue scientists have developed a new independent estimate of the Hubble constant, which suggests that the universe is expanding at a rate of 76 kilometres per second per megaparsec, meaning that a galaxy one megaparsec away from Earth would move away from us at a speed of 76 km/s.
