The international LIGO-Virgo-KAGRA gravitational wave detector network concluded its fourth observing run on Nov. 18, marking the end of the most productive cosmic surveillance campaign in the history of gravitational wave astronomy. The collaboration now shifts its focus to implementing major technological upgrades ahead of the fifth observing run, scheduled to begin in late summer or early fall 2026.

The O4 observing run, which began in May 2023, detected approximately 250 new gravitational wave signals over its 30-month duration. This represents a dramatic acceleration in discovery rates compared to the first three observing runs combined, which detected just 90 events between 2015 and 2020. The surge brings the total number of gravitational wave events observed since humanity's first detection in 2015 to approximately 350.

Breakthrough Discoveries Define O4 Era

The fourth observing run produced several landmark discoveries that pushed the boundaries of astrophysics and fundamental physics. Among the most significant findings was GW250114, detected on Jan. 14, 2025, which generated the clearest gravitational wave signal ever recorded. With a signal-to-noise ratio of approximately 80, far exceeding the 26 recorded for the historic first detection in 2015, this event allowed researchers to test Einstein's theory of general relativity with unprecedented precision.

Analysis of GW250114 provided the first observational confirmation of Stephen Hawking's 1971 area theorem, which predicts that the total surface area of black holes cannot decrease during mergers. The final merged black hole, with a mass of 62.7 solar masses, showed a substantial area increase, validating this fundamental principle through direct gravitational wave observation.

Two additional detections in late 2024 revealed evidence of second-generation black holes, objects formed not from collapsing stars but from previous black hole mergers. GW241011, detected on Oct. 11, 2024, occurred approximately 700 million light years away and featured unusually high spin characteristics. The precision of this measurement allowed researchers to identify overtones in the gravitational wave signal, similar to harmonics in musical instruments, confirming another key prediction of Einstein's theory.

GW241110, detected on Nov. 10, 2024, from roughly 2.4 billion light years away, marked another historic first. The primary black hole in this merger was spinning in the opposite direction to its orbit, a configuration never before observed in gravitational wave astronomy. These unusual spin properties suggest the black holes likely formed in dense, turbulent cosmic environments where repeated mergers are common.

Another remarkable detection, GW231123, identified the most massive black hole merger ever recorded through gravitational waves. The event produced a final object exceeding 225 solar masses, challenging existing astrophysical models about how such enormous black holes can form and grow.

Network Performance and Detector Evolution

The O4 run demonstrated the growing maturity and capability of the global gravitational wave detector network. LIGO's two interferometers in Hanford, Washington, and Livingston, Louisiana, operated near their planned sensitivity goals of 160 to 190 megaparsecs for binary neutron star detections. The Virgo detector in Italy achieved sensitivities between 80 and 115 megaparsecs, while KAGRA in Japan gradually improved its performance throughout the run.

KAGRA faced significant challenges during O4, including damage from the January 2024 Noto earthquake that affected nine of its 20 mirror suspension systems. Despite these setbacks, the Japanese detector participated in data collection during multiple phases of O4, operating at approximately 6 to 7 megaparsecs sensitivity.

The network's improved sensitivity translated directly into increased detection rates. During O4, the collaboration detected gravitational wave events approximately every two to three days, compared to weekly or less frequent detections in earlier runs. This acceleration reflects continuous improvements in laser systems, mirror coatings, seismic isolation, and quantum noise reduction technologies.

Planned Upgrades and O5 Preparations

With O4 complete, the collaboration now enters an intensive upgrade phase aimed at further enhancing detector sensitivity. According to recent assessments and discussions with funding agencies, the collaboration envisions a six-month observing run beginning in late summer or early fall 2026, with detectors participating as they become available.

The planned upgrades will be implemented in several phases, interspersed with shorter data collection periods to test improvements and maintain scientific output. For LIGO, the A+ upgrade program aims to push sensitivity toward the design goal of 330 megaparsecs for binary neutron star detections. Virgo plans to implement Phase 2 of its AdV+ upgrade, targeting sensitivities between 150 and 260 megaparsecs.

Engineering alerts may be sent during the initial upgrade phase through Dec. 9, 2025, in case extraordinary events are detected while detectors are being commissioned. The collaboration emphasizes that upgrade timelines and O5 plans remain subject to refinement as work progresses.

Multimessenger Astronomy Advances

Beyond pure gravitational wave detections, O4 continued the search for multimessenger events where gravitational waves are accompanied by electromagnetic signals or neutrinos. While no definitive multimessenger detections occurred during O4, the improved sky localization capabilities of the four-detector network significantly enhanced follow-up observing campaigns.

The combination of data from LIGO, Virgo and KAGRA allows researchers to pinpoint source locations more accurately than any single detector could achieve alone. This improved localization is crucial for directing telescopes across the electromagnetic spectrum toward promising candidates, potentially enabling the discovery of optical, radio or gamma-ray counterparts to gravitational wave events.

Two or three binary neutron star mergers were detected during O4, though determining precise classifications remains challenging for some events. The most scientifically valuable multimessenger detection remains the August 2017 observation of GW170817, which was observed in both gravitational waves and electromagnetic radiation across the spectrum.

Scientific Impact

The accelerated detection rate during O4 is transforming gravitational wave astronomy from a novel discovery science into a mature observational field. Researchers are now building comprehensive catalogs of black hole and neutron star populations, studying their mass distributions, spin properties and formation channels.

The unusual black hole configurations discovered in O4 provide compelling evidence for hierarchical merger scenarios in dense cosmic environments such as globular clusters or galactic nuclei. These discoveries address fundamental questions about how the most massive black holes in the universe formed and evolved.

Rapidly rotating black holes like those observed during O4 also serve as laboratories for testing exotic physics beyond Einstein's general relativity. Researchers can use these systems to search for hypothetical ultralight boson particles predicted by theories extending the Standard Model of particle physics.

As the collaboration prepares for O5, researchers anticipate an even greater influx of gravitational wave detections. The improved sensitivity will enable observations of more distant events and fainter signals, potentially revealing entirely new classes of cosmic phenomena. With approximately 300 black hole mergers now observed, patterns are emerging that were impossible to discern from the handful of detections available just a few years ago.

The planned addition of LIGO-India in future observing runs will further enhance the network's capabilities, providing better sky localization and expanding coverage to include more of the Southern Hemisphere. Combined with next-generation detectors under development worldwide, the field of gravitational wave astronomy is poised for continued rapid growth throughout the coming decade.

The completion of O4 represents not an ending but a transition point in humanity's exploration of the universe through gravitational waves. As the detectors undergo their technological evolution, the scientific community eagerly anticipates the discoveries that await when observations resume in 2026.