When the densest objects in the universe collide and merge, the violence sets off ripples in the form of gravitational waves that reverberate across space and time, over hundreds of millions and even billions of years. By the time they reach Earth, these cosmic ripples are barely discernible. Yet, thanks to a global network of gravitational-wave observatories, scientists can detect them. These observatories include the U.S.-based National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO), the Virgo interferometer in Italy, and the Kamioka Gravitational Wave Detector (KAGRA) in Japan. Together, they “listen” for faint wobbles in the gravitational field that could have originated from far-off astrophysical events.
The LIGO-Virgo-KAGRA (LVK) Collaboration has now published its latest compilation of gravitational-wave detections, presented in a forthcoming special issue of Astrophysical Journal Letters. The findings suggest that the universe is echoing with a kaleidoscope of cosmic collisions. The Gravitational-Wave Transient Catalog-4.0 (GWTC-4) includes detections from the fourth and most recent observing run, conducted between May 2023 and January 2024. During this nine-month period, the observatories detected 128 new gravitational-wave “candidates,” indicating signals likely from extreme, distant astrophysical sources. This newest collection more than doubles the size of the gravitational-wave catalog, which previously contained 90 candidates from all three prior observing runs.
Advancements in Gravitational-Wave Detection
“The beautiful science that we are able to do with this catalog is enabled by significant improvements in the sensitivity of the gravitational-wave detectors as well as more powerful analysis techniques,” says LVK member Nergis Mavalvala, dean of the MIT School of Science and Curtis and Kathleen Marble Professor of Astrophysics.
In the past decade, gravitational-wave astronomy has evolved from the first detection to the observation of hundreds of black hole mergers. According to 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.”
Pushing the Boundaries of Astrophysics
Black holes are created when all the matter in a dying star collapses into a single point, making them among the densest objects in the universe. Often forming in pairs, black holes are bound together through gravitational attraction. As they spiral toward each other, they emit enormous amounts of energy in the form of gravitational waves before merging into a more massive black hole.
A binary black hole was the source of the very first gravitational-wave detection by NSF’s LIGO observatories in 2015, and colliding black holes have been the source of many gravitational waves detected since. These “bread-and-butter” binaries typically consist of two black holes of similar size (usually several tens of times more massive than the sun) that merge into one larger black hole.
Gravitational waves can also result from the collision of a black hole with a neutron star, an extremely dense remnant core of a massive star. While black hole collisions only produce gravitational waves, a smash-up involving a neutron star can also generate light, providing more information about the event. In its first three observing runs, the LVK observatories detected signals from a handful of collisions involving black holes and neutron stars, as well as two collisions between neutron stars.
Unveiling New Cosmic Phenomena
The latest detections reveal a greater variety of binaries producing gravitational waves. In addition to black hole binaries, the updated catalog includes the heaviest black hole binary, a binary with black holes of asymmetric masses, and a binary where both black holes have exceptionally high spins. The catalog also holds two black hole-neutron star binaries.
“The message from this catalog is: We are expanding into new parts of what we call ‘parameter space’ and a whole new variety of black holes,” says co-author Daniel Williams, a research fellow at the University of Glasgow and a member of the LVK. “We are really pushing the edges and are seeing things that are more massive, spinning faster, and are more astrophysically interesting and unusual.”
Detecting Unusual Signals
The LIGO, Virgo, and KAGRA observatories detect gravitational waves using L-shaped, kilometer-scale instruments called interferometers. Scientists send laser light down the length of each tunnel and precisely measure the time it takes each beam to return to its source. Any slight difference in their timing can indicate that a gravitational wave passed through and minutely wobbled the laser’s light.
For the first segment of the LVK’s fourth observing run, gravitational-wave detections were made using only LIGO’s identical interferometers—one located in Hanford, Washington, and the other in Livingston, Louisiana. Recent upgrades to LIGO’s detectors enabled them to search for signals from binary neutron stars as far out as 360 megaparsecs, or about 1 billion light-years away, and for signals from binaries including black holes tens of times farther away.
“You can’t ever predict when a gravitational wave is going to come into your detector,” says co-author and LVK member Amanda Baylor, a graduate student at the University of Wisconsin at Milwaukee involved in the signal search process. “We could have five detections in one day, or one detection every 20 days. The universe is just so random.”
Among the more unusual signals detected in the first phase of the O4 observing run was GW231123_135430, the heaviest black hole binary detected to date. Scientists estimate that the signal arose from the collision of two heavier-than-normal black holes, each roughly 130 times as massive as the sun. The much heavier black holes of GW231123_135430 suggest that each may be a product of a prior collision of lighter “progenitor” black holes.
Another standout is GW231028_153006, a black hole binary with the highest inspiral spin, meaning that both black holes appear to be spinning very fast, at about 40 percent the speed of light. Scientists suspect these black holes were also products of previous mergers that spun them up as they were created from two smaller, inspiraling black holes. The O4 run also detected GW231118_005626, an unusually lopsided pair, with one black hole twice as massive as the other.
“One of the striking things about our collection of black holes is their broad range of properties,” says co-author LVK member Jack Heinzel, an MIT graduate student who contributed to the catalog’s analysis. “Some of them are over 100 times the mass of our sun, others are as small as only a few times the mass of the sun. Some black holes are rapidly spinning, others have no measurable spin. We still don’t completely understand how black holes form in the universe, but our observations offer crucial insight into these questions.”
Cosmic Connections and Future Implications
From the newest gravitational-wave detections, scientists have begun to make connections about the properties of black holes as a population. “For instance, this dataset has increased our belief that black holes that collided earlier in the history of the universe could more easily have had larger spins than the ones that collided later,” says LVK member Salvatore Vitale, associate professor of physics at MIT and member of the MIT LIGO Lab. This idea raises interesting questions about what conditions could have spun up black holes in the early universe.
The new detections have also allowed scientists to test Albert Einstein’s general theory of relativity, which describes gravity as a geometric property of space and time. “Black holes are one of the most iconic and mind-bending predictions of general relativity,” says co-author and LVK member Aaron Zimmerman, associate professor of physics at the University of Texas at Austin. When black holes collide, they “shake up space and time more intensely than almost any other process we can imagine observing. When testing our physical theories, it’s good to look at the most extreme situations we can, since this is where our theories are most likely to break down, and where we have the best chance of discovery.”
Scientists put Einstein’s theory to the test using GW230814_230901, one of the “loudest” gravitational-wave signals observed to date. The surprisingly clear signal gave scientists a chance to probe it in detail, to see if any aspects of the signal might deviate from what Einstein’s theory predicts. This signal pushed the limits of their tests of general relativity, passing most with flying colors but illustrating how environmental noise can challenge others in such an extreme scenario.
“So far, the theory is passing all our tests,” Zimmerman says. “But we’re also learning that we have to make even more accurate predictions to keep up with all the data the universe is giving us.”
The updated catalog is also helping scientists to nail down a key mystery in cosmology: How fast is the universe expanding today? Scientists have tried to answer this by measuring a rate known as the Hubble constant. Various methods, using different astrophysical sources, have given conflicting answers. Gravitational waves offer an alternative way to measure the Hubble constant since scientists can work out, in relatively straightforward fashion, how far these waves traveled from their source.
“Merging black holes have a really unique property: We can tell how far away they are from Earth just from analyzing their signals,” says co-author and LVK member Rachel Gray, a lecturer at the University of Glasgow involved in the cosmological interpretations of the catalog’s data. “So, every merging black hole gives us a measurement of the Hubble constant, and by combining all of the gravitational wave sources together, we can vastly improve how accurate this measurement is.”
By analyzing all the gravitational-wave detections in the LVK’s entire catalog, scientists have come up with a new, independent estimate of the Hubble constant, suggesting the universe is expanding at a rate of 76 kilometers per second per megaparsec (a square volume of about half a billion light-years wide).
“It’s still early days for this method, and we expect to significantly improve our precision as we detect more gravitational wave sources,” Gray says.
“Each new gravitational-wave detection allows us to unlock another piece of the universe’s puzzle in ways we couldn’t just a decade ago,” says Lucy Thomas, who led part of the catalog’s analysis and is a postdoc in the Caltech LIGO Lab. “It’s incredibly exciting to think about what astrophysical mysteries and surprises we can uncover with future observing runs.”
