LIGO-Virgo-KAGRA Collaboration Releases GWTC-4, More Than Doubling the Catalog of Known Gravitational Wave Events to 218
The fourth gravitational wave transient catalog adds 128 new detections from a nine-month observing run, including the heaviest and fastest-spinning black hole mergers ever recorded.
Overview
The LIGO-Virgo-KAGRA (LVK) Collaboration published the Gravitational-Wave Transient Catalog 4.0 (GWTC-4) on March 5, 2026, adding 128 new gravitational wave candidates detected during the observatories’ fourth observing run. The release more than doubles the 90 events cataloged across the collaboration’s three prior observing runs, according to MIT News, and includes the heaviest black hole merger, the fastest-spinning binary, and the most lopsided pair ever recorded.
What We Know
The 128 candidates in GWTC-4 were detected during a nine-month window from May 2023 to January 2024 using three detector facilities: the twin LIGO observatories in Hanford, Washington and Livingston, Louisiana; the Virgo detector in Italy; and the KAGRA detector in Japan, as detailed by the LIGO Lab at Caltech. The full fourth observing run produced approximately 300 merger signals in total, though only the candidates meeting catalog-quality thresholds appear in GWTC-4.
Among the catalog’s most notable entries is GW231123, a merger of two black holes each approximately 130 times the mass of the Sun, making it the heaviest binary black hole system ever detected through gravitational waves. According to Caltech, the enormous mass of the system suggests these black holes had themselves formed through prior mergers of lighter progenitors.
The catalog also contains GW231028, a binary in which both black holes were spinning at roughly 40 percent the speed of light, the highest inspiral spin measured to date, as reported by MIT News. A third standout is GW231118, the most asymmetric pair yet observed, with one black hole roughly twice as massive as the other.
One of the catalog’s clearest signals, GW230814, provided researchers with an opportunity to conduct one of the most rigorous tests of Einstein’s general theory of relativity using gravitational waves. Salvatore Vitale, an associate professor of physics at MIT, stated that “the theory is passing all our tests,” while cautioning that theorists must develop more accurate predictions to keep pace with the improving quality of observational data, according to MIT News.
The collaboration also used the catalog to produce a new estimate of the Hubble constant, the rate at which the universe is expanding. The gravitational wave data yielded a value of 76 kilometers per second per megaparsec, as reported by the LIGO Lab. This independent measurement contributes to the ongoing effort to resolve the so-called Hubble tension, the discrepancy between expansion rates measured through different astronomical methods.
What We Don’t Know
While the catalog has expanded substantially, the collaboration has noted that roughly 170 additional merger candidates from the fourth observing run have not yet been included. The criteria for future inclusion and the timeline for a more complete release remain unclear.
The heaviest binary in the catalog, with components of approximately 130 solar masses each, falls in a mass range that challenges standard models of stellar evolution. Whether these black holes formed through hierarchical mergers in dense stellar environments, or through some other mechanism, is an open question.
The catalog’s Hubble constant estimate of 76 km/s/Mpc sits close to values derived from supernova observations but above those obtained from cosmic microwave background measurements. Whether future gravitational wave data will favor one side of this tension or suggest new physics remains to be determined.
Analysis
The sheer pace of gravitational wave detection illustrates the field’s rapid maturation. The first detection, GW150914, arrived in September 2015 and was treated as a singular event. A decade later, the collaboration is cataloging more than one event per week on average during active observing runs.
The Machine Herald previously reported on GW250114, an exceptionally clear single event from January 2025 that provided the most precise test of general relativity at the time. GWTC-4 extends this testing capability across a population of events, enabling statistical analyses of black hole properties that individual detections cannot support.
Recent upgrades to LIGO have extended the detectors’ reach for binary neutron star signals to approximately one billion light-years. As the collaboration prepares for its fifth observing run, the growing catalog is shifting gravitational wave astronomy from an era of landmark individual detections to one of population-level astrophysics, where the demographics of black holes and neutron stars across cosmic time become the primary scientific target.
The catalog was published in a special issue of the Astrophysical Journal Letters.