A catalogue of insights from the fourth LIGO-Virgo-KAGRA observing run
This paper reviews the discoveries and scientific insights from the fourth LIGO-Virgo-KAGRA observing run, highlighting a catalogue of approximately 100 new gravitational-wave detections that enable population-level analyses to constrain astrophysical distributions, test general relativity, and measure the Universe's expansion rate.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
For a century, astronomers have looked up at the night sky, relying on light to tell the story of the universe. They built larger telescopes to see fainter stars and further back in time, mapping the history of galaxies and the life cycles of stars. But in 2015, humanity gained a new sense. We learned to listen to the universe not with light, but with ripples in the fabric of space and time itself. These ripples, called gravitational waves, are created when massive objects, like black holes or neutron stars, crash into one another. Unlike light, which can be blocked by dust or gas, these waves pass through everything, carrying a clear signal from the most violent events in the cosmos. This new way of listening has transformed from a rare, historic miracle into a steady stream of discoveries. What began as a single, Nobel-winning event has evolved into a routine catalog of cosmic collisions, allowing scientists to study the behavior of the universe's most extreme objects in ways that were previously impossible.
Now, researchers have released a comprehensive review of the fourth major period of listening, known as the fourth observing run. This period, which ran from mid-2023 to late 2025, brought the total number of detected gravitational waves to a new high, with nearly one hundred new signals added to the list in the most recent updates alone. The detectors, located in the United States, Italy, and Japan, have become significantly more sensitive, allowing scientists to hear these cosmic crashes from much farther away and with greater clarity. The result is not just a longer list of events, but a deeper understanding of what these events are and how they happen. The most recent data reveals that the universe is filled with a diverse population of colliding black holes and neutron stars, some of which are unlike anything scientists have seen before.
Among the most striking findings are signals from sources that challenge previous expectations. One event, detected in 2023, came from a pair consisting of a neutron star and a black hole. The black hole in this pair was surprisingly small, falling into a range of masses where scientists had previously found almost no black holes at all. This discovery suggests that the gap between the heaviest neutron stars and the lightest black holes is not empty, but rather contains a few rare, elusive objects. Another event, detected in late 2023, involved a pair of black holes so massive that they likely could not have formed from the collapse of a single star. Their immense size and rapid spin suggest they may be the remnants of previous black hole collisions, having merged once before and then merged again in a dense cluster of stars. While the data is compelling, the researchers note that the extreme nature of these signals makes them difficult to model perfectly, leaving open the possibility that the signals were distorted by the bending of space itself, a phenomenon known as gravitational lensing, though this remains unproven.
The sheer volume of data has also allowed scientists to move beyond studying individual events to understanding the population as a whole. By analyzing the properties of hundreds of collisions, researchers have begun to see patterns in how these objects form and evolve. They have found that the spins of black holes—the way they rotate—change depending on their mass. At lower masses, the black holes tend to spin slowly, while at higher masses, they spin faster and in random directions. This shift suggests that different formation processes are at work: some black holes are born from the collapse of single stars, while others are the products of repeated mergers in crowded stellar environments. The data also shows that the distribution of black hole masses is more complex than a simple curve, with distinct peaks and changes that hint at the specific physics of how stars die and how black holes are born.
Perhaps the most powerful application of this growing catalog is in measuring the expansion of the universe. Gravit waves act as "standard sirens," providing a direct measurement of how far away a collision occurred. By combining this distance with the speed at which the host galaxy is moving away from us, scientists can calculate the rate of cosmic expansion, known as the Hubble constant. While this has been difficult to do in the past due to a lack of data, the new, larger catalog is finally allowing these measurements to become precise enough to compete with other methods. The researchers found that by using statistical methods to infer the distances of many dark, unobserved sources, they can now pin down the expansion rate with a level of accuracy that was previously out of reach.
As the field moves forward, the final data from this observing period is scheduled for release in late 2026, promising to add even more detail to this cosmic portrait. The detectors are being upgraded to hear even fainter signals, and future runs are planned to continue this momentum. However, the path ahead is not without challenges. The researchers caution that the future of this field depends on continued funding and support, as budget cuts in various countries could threaten the operation of these massive instruments. Without the resources to keep the detectors running and the scientists working, the progress made in this first decade of gravitational-wave astronomy could stall. For now, the data speaks clearly: the universe is a dynamic place, filled with collisions that tell a complex story of stellar death, rebirth, and the fundamental laws that govern space and time. The story is still being written, and with every new signal, the picture becomes sharper.
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