The Virgo interferometer is a large-scale scientific instrument near Pisa, Italy, for detecting gravitational waves. The detector is a Michelson interferometer, which can detect the minuscule length variations in its two arms induced by the passage of gravitational waves. The required precision is achieved using many systems to isolate it from the outside world, including keeping its mirrors and instrumentation in an ultra-high vacuum and suspending them using complex systems of pendula.
Between its periodic observations, the detector is upgraded to increase its sensitivity. The observation runs are performed in collaboration with other similar detectors, including the two Laser Interferometer Gravitational-Wave Observatories (LIGO) in the United States and the Japanese Kamioka Gravitational Wave Detector (KAGRA), because cooperation between several detectors is crucial for detecting gravitational waves and pinpointing their origin.
Virgo was conceived and built when gravitational waves were only a prediction of general relativity. The project, named after the Virgo galaxy cluster, was approved in 1992 and construction was completed in 2003. After several years without detection, Virgo was shut down in 2011 for the "Advanced Virgo" upgrades. In 2015, the first observation of gravitational waves was made by the two LIGO detectors, while Virgo was still being upgraded. Virgo resumed observations in early August 2017, making its first detection on 14 August (together with the LIGO detectors); this was quickly followed by the detection of the GW170817 gravitational wave, the only one also observed with classical methods (optical, gamma-ray, X-ray and radio telescopes)
Virgo is hosted by the European Gravitational Observatory (EGO), a consortium founded by the French Centre National de la Recherche Scientifique (CNRS) and the Italian Istituto Nazionale di Fisica Nucleare (INFN). The broader Virgo Collaboration, gathering 940 members in 20 countries, operates the detector, and defines the strategy and policy for its use and upgrades. The LIGO and Virgo collaborations have shared their data since 2007, and with KAGRA since 2019, forming the LIGO-Virgo-KAGRA (LVK) collaboration.
Organisation The Virgo interferometer is managed by the European Gravitational Observatory (EGO) consortium, which was created in December 2000 by the French National Centre for Scientific Research (CNRS) and the Istituto Nazionale di Fisica Nucleare (INFN). Nikhef, the Dutch Institute for Nuclear and High-Energy Physics, later joined as an observer and eventually became a full member in 2021. Institutions from Poland, Spain and Belgium joined EGO as observers in 2023, with the Belgian FWO and FNRS joining as full members in 2025. EGO is responsible for the Virgo site and ensures the detector's commissioning, maintenance, operation and upgrades. By metonymy, the site itself is sometimes referred to as EGO, as the consortium is headquartered there. One of EGO's goals is to promote research on gravity in Europe. Between 2018 and 2024, the budget of EGO fluctuates between 9 and 11.5 million euros per year, employing around 60 people.
The Virgo Collaboration consists of all the researchers working on various aspects of the detector. About 940 members, representing 165 institutions in 20 countries, were part of the Collaboration This includes institutions in France, Italy, the Netherlands, Poland, Spain, Belgium, Germany, Hungary, Portugal, Greece, Czechia, Denmark, Ireland, Monaco, Switzerland, Brazil, Burkina Faso, China, Israel, Japan and South Korea.
The Virgo Collaboration is part of the larger LIGO-Virgo-KAGRA (LVK) Collaboration, which gathers scientists from the other major gravitational-waves experiments to jointly analyse the data; this is crucial for gravitational-wave detection. LVK began in 2007 as the LIGO-Virgo Collaboration, and was expanded when KAGRA joined in 2019.
Science case
Virgo is designed to look for gravitational waves emitted by astrophysical sources across the universe which can be classified into three types: Transient sources, which are objects only detectable for a short period. The main sources in this category are compact binary coalescences (CBC) from binary black holes (or neutron stars) merging, emitting a rapidly-growing signal which only becomes detectable in the last seconds before the merger. Other possible sources of short-lived gravitational waves are supernovas, instabilities in compact astrophysical objects, or exotic sources such as cosmic strings. Continuous sources, emitting a signal observable on a long time scale. Prime candidates are rapidly-spinning neutron stars (pulsars), which may emit gravitational waves if they are not perfectly spherical (e.g. if there are tiny "mountains" on the surface). Stochastic backgrounds, a type of generally-continuous signal diffused across large regions of the sky rather than from a single source. It could consist of a large number of indistinguishable sources from the above categories, or originate from the early moments of the universe.
Detection of gravitational waves from these sources is a new way to observe them (often with different information than classical methods such as telescopes) and to probe fundamental properties of gravity such as the polarisation of gravitational waves, possible gravitational lensing, or determining whether the observed signals are correctly described by general relativity. It also provides a way to measure the Hubble constant.
History The Virgo project was approved in 1992 by the French CNRS and the following year by the Italian INFN. Construction of the detector began in 1996 in Santo Stefano a Macerata in Cascina, near Pisa, Italy, and was completed in 2003. After several observation runs in which no gravitational waves were detected, the interferometer was shut down in 2011 for upgrading as part of the Advanced Virgo project. It began observations again in 2017, and made its first two detections soon after, together with the LIGO detectors.
Conception Although the concept of gravitational waves was presented by Albert Einstein in 1916, serious projects for detecting them only began during the late 1960s. The first were the Weber bars, invented by Joseph Weber; although they could detect gravitational waves in theory, none of the experiments succeeded. However, they sparked the creation of research groups dedicated to gravitational waves.
The idea of a large interferometric detector began to gain credibility during the early 1980s, and the Virgo project was conceptualised by Italian researcher Adalberto Giazotto and French researcher Alain Brillet in 1985 after they met in Rome. A key idea that set Virgo apart from other projects was the targeting of low frequencies (around 10 Hz); most projects focused on higher frequencies (around 500 Hz). Many believed at the time that low-frequency observations were not possible; only France and Italy began work on the project, which was first proposed in 1987. The name Virgo was coined shortly after, in reference to the Virgo galaxy cluster; it symbolizes the aim of the project to detect gravitational waves originating from beyond our galaxy. After approval by the CNRS and the INFN, construction of the interferometer began in 1996 with the aim of beginning observations by 2000.
Virgo's first goal was to directly observe gravitational waves, whose existence was already indirectly evidenced by the three-decade study of the binary pulsar 1913+16: the observed decrease of this binary pulsar's orbital period was in agreement with the hypothesis that the system was losing energy by emitting gravitational waves.
Initial Virgo detector
The Virgo detector was first built, commissioned and operated during the 2000s, and reached its expected sensitivity. This validated its design choices, and demonstrated that giant interferometers were promising devices for detecting gravitational waves in a broad frequency band. This phase is sometimes called the "initial Virgo" or "original Virgo".
Construction of the initial Virgo detector was completed in June 2003, and several data collection periods ("science runs") followed between 2007 and 2011, after 4 years of commissioning. Some of the runs were performed with the two LIGO detectors (which are located in Hanford, Washington and in Livingston, Louisiana). There was a shut-down of a few months in 2010 for an upgrade of the Virgo suspension system, and the original steel suspension wires were replaced by glass fibres to reduce thermal noise. Even after several months of data collection with the upgraded suspension system, no gravitational waves were observed, and the detector was shut down in September 2011 for the installation of Advanced Virgo.
Advanced Virgo detector
The Advanced Virgo detector aimed to increase the sensitivity (and the distance from which a signal can be detected) by a factor of 10, allowing it to probe a volume of the universe 1,000 times larger and making detection of gravitational waves more likely. It benefited from the experience gained with the initial detector and technological advances.
The Advanced Virgo detector kept the same vacuum infrastructure as the initial Virgo, but the rest of the interferometer was upgraded. Four additional cryotraps were added at both ends of each arm to trap residual particles coming from the mirror towers. The new mirrors were larger, with a diameter of and a weight of , and their optical performance was improved. The optical elements used to control the interferometer were under vacuum on suspended mountings. A system of adaptive optics was installed to correct the mirror aberrations in situ. In the original plan, the laser power was expected to reach 200 W in its final configuration.
Advanced Virgo began the commissioning process in 2016, joining the two LIGO detectors (which had gone through similar upgrades with Advanced LIGO, and made their first detection in 2015) on 1 August 2017. Observation "runs" for the Advanced detector era are planned by the LVK collaboration with the goal to maximise the observing time with several detectors, and are labelled O1 to O5; Virgo began participating in these near the end of the O2 run. LIGO and Virgo detected the GW170814 signal on 14 August 2017, which was reported on 27 September of that year. It was the first binary black hole merger detected by both LIGO and Virgo, and the first for Virgo.
GW170817 was detected by LIGO and Virgo on 17 August 2017. The signal, produced by the final minutes of two neutron stars spiralling closer to each other and merging, was the first binary neutron-star merger observed and the first gravitational-wave observation confirmed by non-gravitational means. The resulting gamma-ray burst was also detected, and optical telescopes later discovered a kilonova corresponding to the merger.
After further upgrades, Virgo began its third observation run (O3) in April 2019. Planned to last one year, the run ended early on 27 March 2020 due to the COVID-19 pandemic.
The upgrades following O3 are part of the Advanced Virgo+ program, divided into two phases; the first preceded the O4 run, and the second precedes the O5 run. The first phase focused on the reduction of quantum noise by introducing a more powerful laser, improving the squeezing introduced in O3, and implementing a new technique known as signal recycling; seismic sensors were also installed around the mirrors. The second phase will attempt to reduce the mirror thermal noise by changing the geometry of the laser beam to increase its size on the mirrors (spreading the energy on a larger area and thus reducing the temperature) and improving the coating of the mirrors; the end mirrors will be larger, requiring improvements to the suspension. Further improvements for quantum noise reduction are also expected in the second phase, building on the changes in the first.