New LIGO–Virgo–KAGRA Catalog Lists 218 Gravitational-Wave Events, Nearly Doubling Known Cosmic Mergers

From first chirp to cosmic census

When scientists first heard the faint “chirp” of two colliding black holes in 2015, the signal lasted just a fraction of a second and was hailed as a once-in-a-generation breakthrough. Little more than a decade later, those once-rare crashes have become routine. A new catalog released this month now lists 218 gravitational-wave events, turning a singular discovery into an unfolding census of the most violent mergers in the universe.

The LIGO–Virgo–KAGRA (LVK) collaboration on March 5 rolled out its fourth Gravitational-Wave Transient Catalog, known as GWTC-4, combining almost nine years of observations from detectors in the United States, Italy and Japan. The catalog adds 128 new candidate mergers recorded between May 24, 2023, and Jan. 16, 2024, roughly doubling the number of known gravitational-wave events since the last major release.

“We are expanding into new parts of what we call ‘parameter space’ and a whole new variety of black holes,” said Daniel Williams, a physicist at the Massachusetts Institute of Technology and member of the LVK collaboration. “We are really pushing the edges.”

A new phase for gravitational-wave astronomy

The latest entries come from the first segment of the network’s fourth observing campaign, called O4a. During that period, the upgraded instruments were sensitive enough to register gravitational-wave signals from colliding black holes or neutron stars roughly every two to three days.

The Laser Interferometer Gravitational-wave Observatory (LIGO) operates two 4-kilometer-long interferometers in Hanford, Washington, and Livingston, Louisiana. Virgo, near Pisa, Italy, and KAGRA, in Japan’s Kamioka mine, complete the international network. Together, they measure minute changes in the lengths of their vacuum-filled arms, caused when passing gravitational waves stretch and squeeze space-time by less than the width of a proton.

“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,” said Nergis Mavalvala, dean of the School of Science at MIT and a longtime LIGO researcher. Those upgrades, she said, include more powerful lasers, better vibration isolation and advanced quantum techniques such as “squeezed light” to reduce noise.

The new catalog’s 128 O4a additions join 90 earlier events from observing runs between 2015 and 2020, bringing the total to 218 candidates with at least a 50% probability of being astrophysical in origin. Eighty-six of the O4a events are classified as high-confidence detections, each with an expected false-alarm rate of less than once per year.

Heavyweights, loud signals and odd couples

Most of the newcomers are mergers of binary black holes — pairs of stellar remnants spiraling together and fusing into a single, larger black hole. The data extend the known range of black-hole masses seen in gravitational waves, from about 5.8 times the mass of the sun up to roughly 137 solar masses for the heaviest component in a binary.

One event in particular, recorded on Nov. 23, 2023, stands out as likely the most massive binary system yet seen in gravitational waves, with its primary black hole weighing in at about 137 times the mass of the sun. Such hefty objects encroach on a theoretically predicted “mass gap” where certain types of stellar explosions are not expected to leave behind black holes, raising questions about how they formed.

The catalog also includes the loudest gravitational-wave signals detected so far. Two mergers from August and December 2023 reached a network signal-to-noise ratio above 30, meaning the telltale chirp of the inspiraling black holes towered over the background noise. Those exceptionally clear waveforms give researchers a rare chance to scrutinize the ringdown — the fading vibrations of the newly formed black hole — and test general relativity in the regime of extreme gravity.

Beyond sheer size and volume, GWTC-4 highlights a richer variety of systems. The catalog features binary black holes with highly unequal masses, pairs in which both black holes appear to spin rapidly, and two newly identified mergers between a black hole and a neutron star. These neutron star–black hole events are relatively rare and can offer clues about how matter behaves at nuclear densities.

Reading the population: how black holes are born

With more than 200 events now on record, the LVK team can move beyond individual showpieces to study black holes and neutron stars as a population.

Analyzing 158 well-measured mergers from the catalog, researchers report that the distribution of black-hole masses is not a smooth curve but shows clear structure. There are pronounced peaks around 10 and 35 times the mass of the sun, and indications of another cluster near 20 solar masses.

Those patterns offer hints about how massive stars live and die. Black holes of around 10 solar masses likely come from relatively ordinary massive stars that collapse at the end of their lives. Heavier black holes may trace back to stars born in low-metallicity environments, where weaker stellar winds allow them to retain more mass, or to successive mergers in dense star clusters.

The new data also sharpen the picture of how these binaries assemble. At lower masses, black-hole pairs tend to have companions that are only slightly smaller, with mass ratios clustering around 0.7 to 0.8. That tendency matches models in which two stars are born together and exchange matter as they evolve, ending up as a moderately unequal black-hole pair.

Spin measurements add another piece of forensic evidence. Many binaries show spins that are broadly aligned with their orbital motion, consistent with an origin in isolated stellar binaries that evolved together. But the LVK team finds that roughly one-quarter to two-fifths of systems have a negative effective inspiral spin, a parameter that indicates the black holes’ spins are tilted or even counter-aligned with the orbit.

That fraction points to a substantial population of binaries that likely formed dynamically — for example, when unrelated black holes wander close enough in a dense star cluster to become gravitationally bound and eventually merge.

Taken together, the mass and spin distributions support a picture in which multiple formation channels, both quiet and chaotic, shape the universe’s black-hole population.

A new tool in the Hubble tension

The enlarged catalog is not just about stellar graveyards. It also allows gravitational waves to play a more active role in cosmology, particularly in efforts to pin down the Hubble constant, the current rate of cosmic expansion.

Traditional methods rely either on detailed maps of the cosmic microwave background, which encode conditions in the early universe, or on a “distance ladder” built from nearby stars and supernovae. Those approaches disagree by several kilometers per second per megaparsec, a discrepancy known as the Hubble tension.

Gravitational-wave sources offer an independent route. Because the shape and amplitude of a merger’s signal can be used to infer its absolute luminosity distance, such events act as “standard sirens,” analogous to standard candles in optical astronomy. To translate distance into an expansion rate, astronomers still need a redshift, which can come from an electromagnetic counterpart, as in the famous 2017 neutron star collision, or statistically by comparing the localization region to galaxy catalogs.

Using 142 events from GWTC-4, together with the well-studied neutron star merger and a comprehensive galaxy database, the LVK collaboration estimates the Hubble constant at about 76.6 kilometers per second per megaparsec. The uncertainty remains large — roughly plus 13 and minus 9.5 at 68% confidence — but the central value is closer to local distance-ladder measurements than to cosmic microwave background inferences.

The same analysis tests whether gravitational waves fade with distance in a way that deviates from Einstein’s theory. A parameter that measures modified propagation, labeled Ξ₀, is found to be consistent with the general relativity prediction within current errors, offering no sign yet of exotic behavior over cosmological scales.

Global infrastructure, open data and what comes next

Behind the numbers, GWTC-4 reflects a sprawling, publicly funded effort spanning three continents. The LIGO Scientific Collaboration counts more than 1,600 scientists. Virgo involves about 1,000 members from roughly 20 countries, and KAGRA brings together around 400 researchers from more than a dozen nations and regions.

Funding agencies including the U.S. National Science Foundation, Europe’s CNRS and INFN, the Dutch institute Nikhef, Japanese institutions such as the University of Tokyo’s Institute for Cosmic Ray Research, and partners in the United Kingdom, Germany, Australia and elsewhere support the construction and upgrades of the detectors.

All of the data underlying the new catalog have been released through the Gravitational Wave Open Science Center, allowing independent teams worldwide to reanalyze the signals or search for additional events using their own methods.

The O4a results do not mark the end of the story. Later segments of the fourth observing run, which continued into 2025, produced dozens more candidate mergers that are still being vetted. A new observing period, known as the Interim Run 1, is planned to begin in late 2026, with further sensitivity gains expected as commissioning work continues.

Looking further ahead, concepts for even larger ground-based instruments — the Einstein Telescope in Europe and Cosmic Explorer in the United States — and the space-based LISA mission aim to probe a broader range of frequencies and sources, from the slow dance of supermassive black holes to the earliest moments after the Big Bang.

For now, the GWTC-4 catalog offers a snapshot of a universe where black-hole and neutron-star collisions, once purely theoretical, are recorded in growing detail.

Just over a decade after the first faint ripple from a single merger confirmed Einstein’s prediction, astronomers now keep a running tally of such events — 218 and counting, with the next cosmic crash likely already on its way to Earth as a whisper in space-time.