LIGO's New Catalog Doubles Known Gravitational-Wave Detections to Over 300
In nine months of listening, the world's gravitational-wave observatories heard the universe crash into itself 128 more times. That haul, collected between May 2023 and January 2024, more than doubles the entire previous catalog of gravitational-wave detections and paints a picture of a cosmos teeming with violent, varied collisions.
From 90 to over 300 signals
The new Gravitational-Wave Transient Catalog 4.0 (GWTC-4), published as a special issue of Astrophysical Journal Letters, compiles signals detected during the first segment of the LIGO-Virgo-KAGRA (LVK) collaboration's fourth observing run (O4a). Before this run, three previous campaigns had yielded 90 candidate signals total. The 128 new candidates bring the running total past 300 for the entire O4 run, though not all have yet appeared in the official catalog.
The jump reflects real hardware gains. Recent upgrades to the two LIGO interferometers -- one in Hanford, Washington, and one in Livingston, Louisiana -- extended their reach for binary neutron star signals to roughly 360 megaparsecs, or about 1 billion light-years. For heavier black hole mergers, the detectors can see tens of times farther.
Black holes that break the mold
Most detected merging black holes weigh around 30 times the mass of our sun and look broadly similar. The new catalog shatters that uniformity. Three signals stand out.
GW231123 is the heaviest black hole binary ever detected through gravitational waves. Each black hole in the pair carried roughly 130 solar masses -- more than four times the typical figure. At that size, astrophysicists suspect each was itself the product of a prior merger, built up through successive collisions in dense stellar environments.
GW231028 features the fastest-spinning black holes observed to date. Both objects in this binary were rotating at roughly 40% the speed of light, a spin rate that again suggests they were forged by earlier mergers that wound them up as smaller black holes spiraled inward.
GW231118 is an unusually lopsided pair, with one black hole twice the mass of the other. Such asymmetry is rarer among detected binaries and offers distinct gravitational-wave signatures that let researchers test formation models.
The catalog also includes two black hole-neutron star binaries. Collisions involving neutron stars can produce light as well as gravitational waves, giving astronomers a second channel of information.
Testing Einstein at the extremes
Colliding black holes distort spacetime more violently than almost any other process in the observable universe, making them natural stress tests for general relativity. The collaboration used GW230814, one of the loudest signals in the new catalog, to probe whether any feature of the waveform deviated from Einstein's predictions.
So far, general relativity passes. But the precision of the signal also exposed how environmental noise can challenge certain tests at these extremes -- a useful finding for refining future analyses. Aaron Zimmerman, an associate professor of physics at the University of Texas at Austin and a member of the collaboration, noted that increasingly accurate theoretical predictions will be needed to keep pace with the data.
A new number for how fast the universe expands
Gravitational waves carry distance information baked into their signals. By analyzing the complete LVK catalog, the collaboration produced an independent estimate of the Hubble constant -- the number describing the universe's current expansion rate -- at 76 kilometers per second per megaparsec.
That figure sits within the range of existing estimates from other methods, which have famously disagreed with each other. The gravitational-wave approach avoids the complex calibration chains that plague traditional measurements, though its precision remains limited by sample size. Each new detection tightens the estimate.
Population patterns in the dark
With hundreds of detections in hand, the collaboration is starting to see statistical patterns rather than just individual events. One emerging trend: black holes that collided earlier in the universe's history appear more likely to have had high spins than those merging more recently. If that pattern holds, it could reveal something about the environments and formation mechanisms that prevailed in the younger cosmos.
But the dataset also comes with honest limitations. Gravitational-wave detection remains a probabilistic business. The 128 new entries are "candidates" -- signals with high enough statistical confidence to be very likely astrophysical, but not absolutely certain. And population-level conclusions drawn from a few hundred events, while suggestive, will need larger samples to solidify.
What comes next
The O4 run continued beyond the January 2024 cutoff covered by this catalog. Additional detections from later in the run will appear in future catalog updates. Meanwhile, the Virgo detector in Italy, which was offline during O4a for upgrades, has since rejoined the network. Adding a third detector improves both sensitivity and the ability to localize sources on the sky -- critical for coordinating follow-up observations with telescopes that look for light from neutron star collisions.
The data from GWTC-4 has been made publicly available for independent analysis. That openness matters: many of the most interesting astrophysical inferences in the field have come from groups outside the LVK collaboration working with released data.