LIGO’s latest catalog doubles the haul of gravitational-wave detections, revealing new black hole patterns

Ten years ago, scientists celebrated a single faint chirp in their detectors as proof that gravitational waves — ripples in space-time predicted by Albert Einstein — were real. Today, that lone signal has been joined by hundreds more, and researchers say they are finally starting to see patterns in the chaos.

A new catalog from the LIGO–Virgo–KAGRA collaboration, known as GWTC-4.0, now lists 218 collisions between black holes and neutron stars. Based on data from the first part of the network’s fourth observing run, the release more than doubles the number of known gravitational-wave events and turns what was once a series of rare discoveries into a running census of the universe’s most extreme objects.

“In the past decade, gravitational wave astronomy has progressed from the first detection to the observation of hundreds of black hole mergers,” Stephen Fairhurst, a physicist at Cardiff University and spokesperson for the collaboration, said in a recent statement.

What GWTC-4.0 contains

The technical release of the GWTC-4.0 catalog and its underlying data came on Aug. 26, 2025, through the LIGO Laboratory and the Gravitational-Wave Open Science Center. On March 5, 2026, institutions including the Massachusetts Institute of Technology, Caltech and the Dutch research institute Nikhef launched a coordinated push to highlight the key scientific results for a wider audience.

The new catalog draws on observations made between May 24, 2023, and Jan. 16, 2024, a period known as O4a, when only the two U.S. LIGO detectors — near Hanford, Washington, and Livingston, Louisiana — were taking science data. Even without their European and Japanese partners online, the upgraded instruments recorded 128 new compact-binary merger candidates with at least a 50% chance of being astrophysical in origin. Combined with the 90 events listed in the previous catalog, GWTC-3, the tally reaches 218.

Almost all of those detections are mergers of pairs of black holes. Seven involve neutron star–black hole systems, and just two are collisions between two neutron stars, both found in earlier runs.

“What we’re seeing now is a whole new variety of black holes,” said Daniel Williams, an astrophysicist at the University of Glasgow and member of the collaboration. “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.”

Upgrades that made the sky busier

The leap in numbers is tied directly to hardware upgrades between the third and fourth observing runs. During O4a, LIGO’s two detectors could spot the merger of two typical neutron stars out to roughly 150 megaparsecs — about 500 million light-years — compared with shorter ranges in earlier runs.

Engineers installed a new filter cavity to inject “squeezed” quantum light, raising the signal above quantum noise, and increased the power and stability of the main lasers and optics.

The result was a busier sky. Over 237 days of O4a observing, the LIGO detectors issued 81 public alerts for high-significance candidates, compared with 56 similar alerts over 329 days in the previous campaign.

Standout events: mass, spin and extreme pairings

Several detections illustrate the wider range of objects now turning up in the data.

• A record-heavy pair: One event, GW231123_135430, appears to be the most massive binary yet seen. Both black holes weighed in at roughly 130 times the mass of the sun before they collided.

“Some are over 100 times the mass of our sun, others are as small as only a few times the mass of the sun,” said Jack Heinzel, a graduate student at MIT who worked on the analysis.

The unusually heavy pair suggests that each black hole may itself be the product of earlier mergers — hierarchical merging — thought to occur in crowded star clusters.

• Fast-spinning black holes: Another event, GW231028_153006, contains some of the fastest-spinning black holes measured so far, with objects rotating at roughly 40% of the maximum possible spin.

“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,” said Salvatore Vitale, an associate professor of physics at MIT.

• A lopsided merger: A third signal, GW231118_005626, involves a strongly unequal pair, with one black hole about twice as massive as its companion. Such extreme mass ratios are harder to form through the quiet evolution of an isolated pair of stars and may point to dynamic encounters inside dense stellar environments.

The catalog also confirms two neutron star–black hole systems, GW230518_125908 and GW230529_181500, whose masses fall into a debated “gap” between the heaviest neutron stars and the lightest black holes, around three to five solar masses.

Despite the improved sensitivity, the O4a data did not add any new, high-confidence mergers between two neutron stars. Researchers say that could reflect the intrinsic rarity of such events, statistical chance, or the details of detector uptime and search methods — and expect future runs with more detectors to clarify the picture.

From rare detections to population science

Beyond individual case studies, GWTC-4.0’s main impact comes from its sheer size. With more than 150 mergers suitable for population analysis, researchers can begin mapping black-hole masses and spins and inferring how they formed.

The latest analysis reinforces features hinted at in earlier catalogs: clusters of black-hole masses around roughly 10, 20 and 35 times the mass of the sun, and a sharp drop-off at higher masses that likely reflects how the most massive stars explode or collapse.

Spin measurements indicate that most black holes are not spinning near their theoretical maximum. The collaboration reports that 90% of black holes in the sample have dimensionless spin parameters below about 0.57, and that many systems have spins roughly aligned with the orbital axis — consistent with black holes born as pairs of massive stars that evolved together. At the same time, a substantial minority show signs of spins tilted opposite the orbital motion, suggesting dynamical formation channels in star clusters or galactic nuclei.

Testing Einstein — and measuring the universe’s expansion

The growing catalog has also become a tool for testing fundamental physics and cosmology.

Two events, GW230814_230901 and GW231226_01520, reached network signal-to-noise ratios above 30, placing them among the loudest gravitational-wave signals recorded. That clarity enables detailed checks of general relativity, including comparisons between the inspiral and merger phases and searches for unexpected features in the “ringdown,” the dying vibrations of the newly formed black hole.

So far, Einstein’s theory is holding up. Analyses using dozens of events, including the loudest O4a signals, have found no statistically significant deviations from predictions for how black holes orbit, merge and settle.

“Black holes are one of the most iconic and mind-bending predictions of general relativity,” Aaron Zimmerman, a physicist at the University of Texas at Austin, said. “When black holes collide, they ‘shake up’ space and time more intensely than almost any other process we can imagine observing.”

The same dataset is also being used to estimate how fast the universe is expanding. By measuring the strength of the waves, researchers can infer distances to mergers, turning them into “standard sirens” for estimating the Hubble constant.

“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,” said Rachel Gray, a researcher at the University of Glasgow.

A recent analysis using GWTC-4.0 events found a Hubble constant of about 76 kilometers per second per megaparsec — closer to estimates based on nearby supernovae and Cepheid variable stars than to the lower value of about 67 inferred from measurements of the cosmic microwave background. Scientists caution that gravitational-wave estimates still carry large uncertainties and do not yet resolve the so-called Hubble tension.

A global collaboration — and what comes next

GWTC-4.0 also underscores the scale of modern gravitational-wave astronomy. The LIGO Scientific Collaboration, the Virgo Collaboration and the KAGRA Collaboration involve more than a thousand scientists worldwide, supported by agencies including the U.S. National Science Foundation, France’s National Centre for Scientific Research, Italy’s National Institute for Nuclear Physics and Japan’s Ministry of Education, Culture, Sports, Science and Technology.

The catalog is shaping arguments for a new generation of observatories. In India, work is moving ahead on LIGO-India, expected to join the network around the end of the decade. In the United States and Europe, scientists are developing plans for even larger, more sensitive facilities — Cosmic Explorer and the Einstein Telescope — that could detect tens of thousands of mergers per year. In parallel, the European Space Agency and NASA are preparing LISA, a space-based gravitational-wave observatory slated for launch in the mid-2030s.

Nikhef, in a statement on the catalog, called GWTC-4.0 a “kaleidoscope of cosmic collisions” that gives “an even better picture of the enormous variety of sources of gravitational waves in the universe.”

For researchers, that variety is the point. Each event adds another data point to a growing map of the invisible universe — a map that, a decade ago, did not exist.

“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,” said Lucy Thomas of Caltech’s LIGO Laboratory.

With O4a only the first half of LIGO’s fourth observing run and more detectors set to come online, scientists expect future editions of the catalog to grow quickly. If current plans hold, GWTC-4.0 may be remembered less as a culmination than as the moment gravitational-wave astronomy truly became a survey science: listening not for a single historic chirp, but for the constant background hum of a dynamic, merging universe.