128 New Gravitational-Wave Signals Reveal a Universe Ringing with Collisions

What does the universe sound like when you listen with instruments sensitive enough to detect a change in distance smaller than a proton's width? Noisy. Very noisy.

The LIGO-Virgo-KAGRA (LVK) collaboration has released its Gravitational-Wave Transient Catalog 4.0, or GWTC-4, adding 128 new events detected between May 2023 and January 2024. The previous three observing runs, spanning nearly a decade, had accumulated 90 candidates. This single nine-month window more than doubled that number.

The heaviest, the fastest, the most lopsided

Among the 128 new signals, several push into territory never seen before. GW231123 was produced by the heaviest binary black hole system detected with gravitational waves -- each black hole approximately 130 times the mass of the sun, compared with the roughly 30 solar masses typical of previously detected mergers. Scientists suspect these massive objects are themselves products of prior collisions, built up through what might be described as a chain of cosmic pile-ups in dense stellar environments.

Then there is GW231028, a binary where both black holes spin at about 40% the speed of light -- the highest inspiral spin recorded. The physics suggests these, too, were assembled through previous mergers that transferred enormous rotational energy. And GW231118 stands out as an unusually unbalanced pair, with one black hole twice as massive as the other.

The catalog also contains two black hole-neutron star binaries. Unlike pure black hole mergers, which produce only gravitational waves, collisions involving neutron stars can emit light, giving astronomers two different kinds of signal to study.

Stress-testing general relativity

Black hole collisions warp spacetime with an intensity unmatched by nearly any other physical process. That makes them ideal laboratories for testing Einstein's general theory of relativity, which has described gravity as a geometric property of space and time for over a century.

The collaboration zeroed in on GW230814, one of the clearest gravitational-wave signals in the catalog, examining it for any deviation from Einstein's predictions. So far, the theory holds up. But the exercise also showed that at this level of signal clarity, environmental noise begins to challenge certain tests -- a useful lesson for future analyses.

Measuring cosmic expansion with ripples in spacetime

One of cosmology's persistent headaches is the Hubble tension: different methods of measuring how fast the universe is expanding give different answers. Gravitational waves offer a fresh approach, because the distance to a source can be extracted directly from the signal itself, bypassing the complex calibration ladders that other techniques require.

Using every detection in the full LVK catalog, the collaboration now estimates the Hubble constant at 76 kilometers per second per megaparsec. That number is broadly consistent with existing measurements but not yet precise enough to settle the tension. Each new detection adds to the precision, and the collaboration expects significant improvement as the catalog grows.

From individual events to population science

With hundreds of detections now in hand, researchers can begin to study black holes as a population rather than one event at a time. An intriguing early finding: black holes that merged earlier in cosmic history appear to have had higher spins than those merging more recently. If confirmed with additional data, this trend could reveal important information about the conditions that shaped black hole formation in the young universe.

But there are important caveats. These are still statistical candidates -- highly confident signals, but not absolute certainties. The population-level trends are suggestive, not definitive. And the dataset, while large by gravitational-wave standards, remains modest compared with what optical and radio astronomy can muster.

Where the field goes from here

The O4a data released in GWTC-4 covers only the first segment of the fourth observing run. Detections from later in the run will follow. The Virgo detector, which was undergoing upgrades during O4a, has since resumed operations, adding a third detector to the network and improving the ability to pinpoint where signals originate on the sky.

All data from GWTC-4 has been made available for analysis by researchers outside the LVK collaboration -- a practice that has consistently yielded significant independent discoveries in the field.

The catalog appears in a special issue of Astrophysical Journal Letters.