Artist's conception of two merging black holes like many of those detected by the LIGO-Virgo-KAGRA (LVK) collaboration in the fourth observing run (O4) of the current era of gravitational wave detection. In this illustration, the black holes are spinning in a non-aligned fashion, meaning, they have different orientations relative to the overall orbital motion of the pair. So far in O4, the LVK has confirmed the detection of 250 merger events. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)
LIGO – Virgo – KAGRA Complete Fourth Observing Run
News Release • November 18, 2025
On November 18, 2025, the world's laser interferometer gravitational wave detectors in the United States, Italy, and Japan concluded the fourth observing run of the "advanced detector era" in coordinated celebrations. Both the LVK collaboration and the Rapid Response Team released statements marking the occasion. Read them below.
LIGO-Virgo-KAGRA Collaboration Successfully Wraps Up its Fourth Observing Run
LIGO-Virgo-KAGRA Rapid Response Team Celebrates the Conclusion of the Fourth Observing Run
LIGO-Virgo-KAGRA Collaboration Successfully Wraps Up its Fourth Observing Run
Today, the international LIGO-Virgo-KAGRA Collaboration celebrates the successful completion of its fourth observing run (O4), marking another major step forward in the exploration of the gravitational wave universe. The run, which began May 24, 2023 and continued through November 18, 2025, relied on coordinated observations and analyses by teams across the world.
In Japan, KAGRA staff gathered in the control room to celebrate the end of O4. This was the first run that KAGRA took science data simultaneously with Virgo and LIGO.
During O4, the detectors observed roughly 250 candidate signals in real time. Comprehensive analysis of the first (of three) segments of O4 yielded 128 significant events (an increase of around 50% compared to those announced in real time). These results were reported in the most recent update to the Gravitational-Wave Transient Catalog (GWTC-4.0). Data from the remaining two segments of O4 are currently under detailed examination. The collaboration expects that further results will emerge in the coming months as analyses continue.
Compared to the first three observing runs, which detected 90 signals combined, O4 has already more than doubled the number of confident gravitational wave detections, with many more left to analyze. This demonstrates a continuous increase in both detector sensitivity and scientific return. Indeed, throughout the four observing runs to date, the collaboration has significantly improved its detector technologies, theoretical modeling, low-latency alert system, and data analysis methods. These developments have already led to numerous new discoveries and refined our understanding of, for example, the nature of compact binary systems and fundamental physical processes in the universe.
Staff at LIGO Livingston Observatory in Louisiana gathered in the control room to say farewell to O4
“This is the longest observing run undertaken in either the initial or advanced LIGO era,” said Stephen Fairhurst, spokesperson of the LIGO Scientific Collaboration. “It has been a fantastically successful run, with over 250 significant gravitational wave candidates released in low latency and already over twenty papers released describing the scientific results from O4, with many more to come.”
Following the conclusion of O4, the observatories will undergo a planned break for maintenance and upgrade work. With these improvements, the collaboration anticipates even more frequent and detailed detections, offering new opportunities to probe the most extreme and fascinating phenomena in nature.
After recent assessments of upgrade phasing and discussions with funding agencies, we now expect to have a period of science observing in 2026-27. We currently envision a six-month observing run to begin in the late summer/early fall of 2026, with detectors participating as available. We look forward to sharing more details about this observing run as they emerge.
In Italy, Virgo staff gathered in the control room to celebrate the end of O4.
LIGO-Virgo-KAGRA Rapid Response Team Celebrates the Conclusion of the Fourth Observing Run
As the LIGO-Virgo-KAGRA (LVK) collaboration concludes its fourth observing run, O4, members of the LVK Rapid Response Team (RRT) are celebrating the success of the Rapid Response program all over the world.
RRT is a team of scientists that collectively checks the validity of all significant gravitational wave (GW) candidates and issues public alerts and statements to help astronomers observe events using different means like electromagnetic waves and neutrinos. In O4, the RRT processed a total of 283 candidates over the course of two and a half years.
“Providing 24/7 coverage for such a long period to process as many events as we did was no small task. Cooperation of the entire collaboration was the key”, said Keita Kawabe, a scientist at LIGO Hanford Observatory and the coordinator of the RRT program for the US and Americas time zone.
Staff at LIGO Hanford Observatory in Washington State gathered in the control room at 8am to celebrate the end of O4. Everyone enjoyed morning coffee and cake.
“Indeed, we are extremely grateful that everybody was willing to support this important endeavor. More than 600 individuals have contributed either by taking shifts or agreeing to be on-call.”, added Francesco DiRenzo, a scientist at the University of Florence and the RRT coordinator for the European time zone.
“This was the first Observing Run during which KAGRA took science data simultaneously with LIGO and Virgo. Seeing how the gravitational-wave candidates are vetted was a super exciting experience for me, and I’m proud that many of the events were handled by our KAGRA colleagues.”, said Takahiro Sawada, a scientist at the University of Tokyo and the RRT’s Asia-Pacific time zone coordinator.
RRT members enjoyed sharing the final moments of O4 with others in the control rooms of LIGO Hanford and LIGO Livingston Observatories (USA), Virgo (Italy), and KAGRA (Japan). Enjoy the pictures to celebrate together!
The LIGO-Virgo-KAGRA Collaboration
LIGO is funded by the United States National Science Foundation (NSF), and operated by Caltech and MIT, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,600 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional member institutions are listed at https://my.ligo.org/census.php.
The Virgo Collaboration is currently composed of approximately 1000 members from over 150 institutions in 15 different (mainly European) countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the National Institute of Nuclear Physics (INFN) in Italy, the National Institute of Subatomic Physics (Nikhef) in the Netherlands, The Research Foundation – Flanders (FWO) and the Belgian Fund for Scientific Research (F.R.S.–FNRS) in Belgium. More information is available on the Virgo website at https://www.virgo-gw.eu.
KAGRA is the laser interferometer with a 3 km arm-length in Kamioka, Gifu, Japan. The host institute is Institute for Cosmic Ray Research (ICRR), the University of Tokyo, and the project is co-hosted by National Astronomical Observatory of Japan (NAOJ) and High Energy Accelerator Research Organization (KEK). KAGRA collaboration is composed of over 400 members from 128 institutes in 17 countries/regions. KAGRA’s information for general audiences is available at https://gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for researchers are accessible from http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.
