LIGO, Virgo and KAGRA complete the richest observation run to date

Today the international LIGO-Virgo-KAGRA Collaboration announces the completion of the fourth observation campaign (called O4) of the international network of gravitational wave detectors. Launched in May 2023, the campaign ends today after a period of coordinated observations lasting over two years, during which the analysis of the data was also initiated in parallel.
Some 250 new signals were detected in this latest observation run, constituting a significant fraction (over two-thirds) of the approximately 350 gravitational signals detected to date by LIGO, Virgo and KAGRA. The increase in the number of events observed in each cycle is due to the progressive improvement of detector technologies and the resulting increase in their sensitivity.

These developments have led to numerous new discoveries in recent years. Also some of the most significant results of this latest observational cycle have already been announced and published, contributing to a further deepening of our understanding of the nature of compact binary systems and certain fundamental physical processes in the universe.

“The completion of O4 marks a historic milestone: the longest observing run ever conducted by the global gravitational-wave network – said Gianluca Gemme, spokesperson for the Virgo Collaboration and researcher at the Italian National Institute for Nuclear Physics (INFN) – Virgo has played a crucial role, contributing to the detection and characterization of numerous signals. The success of the O4 run reflects the strength of international collaboration and the relentless effort of our teams to push the boundaries of such precise and challenging measurements. Looking ahead, we are preparing for major upgrades that will significantly enhance the sensitivity of our detectors, ensuring a new and even greater scientific impact.”

An analysis of some of the most interesting events of this latest campaign has already yielded particularly significant results. For example the analysis of the event called GW250114 allowed scientists to “hear” with unprecedented accuracy two black holes as they merged into one, providing observational evidence for a theorem put forth by Stephen Hawking in 1971 that says the total surface areas of black holes cannot decrease. In this case the initial black holes had a total surface area of 240,000 square kilometers , while the final area was about 400,000 square kilometers: a clear increase.
Another important outcome, already published in the past months, was the first detection of “second generation” black holes, GW241011 and GW241110, events that present unusual characteristics in terms of the size and rotational orientation of black holes, which can be explained by hypothesising that they are themselves the result of previous mergers. In other words, these would be systems formed in extremely dense and ‘chaotic’ cosmic environments, such as star clusters, where black holes are more likely to collide and merge repeatedly.

A further significant detection, GW231123, marks the observation of the most massive black hole merger to date, which produced a final black hole more than 225 times the mass of our Sun. An event that challenges our current models of stellar evolution and black hole formation. Of course, other significant results and discoveries are also expected from the analysis of the remaining hundreds of events collected over the last two years, which are currently being carefully examined and will be published in the coming months in a detailed collection: the O4 gravitational signal ‘catalogue’. 

The LIGO, Virgo and KAGRA interferometers are preparing for a new phase of technological upgrades and testing over the next few years. However, this upgrade will likely be implemented in several stages, with periods of data collection in between and a new observation campaign starting in late summer/early autumn 2026 and lasting approximately six months.

The LIGO-Virgo-KAGRA Collaboration 

LIGO is funded by the 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.