The international LIGO–Virgo–KAGRA Collaboration has completed its fourth and longest observing campaign, O4, marking a major milestone in the study of gravitational waves.
Running from May 2023 to late 2025, the two-year effort brought together the world’s leading gravitational wave detectors for a continuous search for ripples in spacetime, dramatically expanding our catalogue of cosmic events.
During this campaign, scientists detected around 250 new gravitational-wave signals, representing a significant majority of the approximately 350 events observed so far by the global detector network.
This surge places O4 among the most productive observational periods since humanity first detected gravitational waves in 2015.
Gravitational waves explained
Gravitational waves are ripples in spacetime generated when massive objects – like black holes or neutron stars – accelerate violently.
First predicted by Albert Einstein, these waves stretch and compress space as they travel at the speed of light. Unlike light, gravitational waves pass almost unhindered through matter, carrying untouched information about events that may be invisible to telescopes.
Detecting gravitational waves requires ultra-precise interferometers capable of sensing distortions far smaller than a proton.
By comparing the timing and shape of signals detected across LIGO, Virgo and KAGRA, scientists can pinpoint where the gravitational waves originated and reconstruct the events that created them.
Detector upgrades drive a wave of new discoveries
The rapid increase in observed gravitational waves is tied directly to ongoing improvements in detector sensitivity.
As the interferometers’ mirrors, lasers and isolation systems become more refined, they can detect fainter distortions in spacetime and capture more black-hole and neutron-star mergers.
Early analysis from O4 has already produced headline discoveries:
Testing Hawking’s black-hole theorem through gravitational waves
The event known as GW250114 offered the clearest gravitational-wave signal yet of two black holes merging.
By analysing this remarkably sharp recording, researchers discovered strong evidence supporting Stephen Hawking’s 1971 prediction that a black hole’s total surface area cannot shrink during a merger.
The final merged black hole showed a substantial area increase, confirming this principle through gravitational-wave observation.
Spotting second-generation black holes
Two additional detections, GW241011 and GW241110, revealed what appear to be second-generation black holes – objects created not from collapsing stars but from previous black-hole mergers.
Their unusual characteristics suggest they formed in dense, turbulent regions where repeated interactions lead to multiple merger cycles. Such systems are only visible to us through gravitational waves, making O4 essential for uncovering these rare phenomena.
The most massive black-hole merger ever
Another signal, GW231123, marked the detection of the most massive black-hole merger ever recorded through gravitational waves, producing a final object more than 225 times the mass of the Sun.
This event challenges existing astrophysical models, pushing scientists to reconsider how such enormous black holes can form and grow.
Hundreds of remaining O4 detections are still being analysed, with a full gravitational-wave catalogue expected soon.
Next steps for the global gravitational wave network
With O4 complete, the collaboration is preparing a series of major technological upgrades to enhance sensitivity to gravitational waves further.
These improvements will be installed over several phases, interspersed with shorter data-collection periods. A new full observing run – O5 – is expected to begin in late summer or early autumn 2026.
As detection capabilities continue to improve, researchers anticipate an even greater influx of gravitational waves, promising deeper insights into black holes, exotic cosmic environments and the hidden dynamics shaping the universe.