Virgo interferometer

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).{{cite web |date=11 December 2000 |title=Communique de Presse – Le CNRS Signe l'Accord Franco-Italien de Création du Consortium EGO European Gravitational Observatory |trans-title=Press Release – The CNRS Signs the Franco-Italian Agreement on the Creation of the EGO (European Gravitational Observatory) Consortium. |url=http://www.cnrs.fr/cw/fr/pres/compress/Ego.htm |url-status=dead |archive-url=https://web.archive.org/web/20160305011822/http://www.cnrs.fr/cw/fr/pres/compress/Ego.htm |archive-date=5 March 2016 |access-date= |website=cnrs.fr |language=fr}} Nikhef, the Dutch Institute for Nuclear and High-Energy Physics, later joined as an observer and eventually became a full member in 2021.{{Cite web |last=Rossi |first=Guglielmo |date=2022-03-24 |title=NWO-I, new Associate member of the European Gravitational Observatory |url=https://www.ego-gw.it/blog/2022/03/24/nwo-i-new-associate-member-of-the-european-gravitational-observatory/ |access-date=2025-05-28 |website=EGO - European Gravitational Observatory |language=en-GB}} Institutions from Poland, Spain and Belgium joined EGO as observers in 2023,{{Cite web |last=Rossi |first=Guglielmo |date=2023-08-04 |title=EGO welcomes new observer members from Belgium, Spain and Poland |url=https://www.ego-gw.it/blog/2023/08/04/ego-welcomes-new-observer-members-from-belgium-spain-and-poland/ |access-date=2025-05-28 |website=EGO - European Gravitational Observatory |language=en-GB}} with the Belgian FWO and FNRS joining as full members in 2025.{{Cite web |last=Rossi |first=Guglielmo |date=2025-03-19 |title=Belgium is the fourth European country to join the EGO consortium |url=https://www.ego-gw.it/blog/2025/03/19/belgium-is-the-fourth-european-country-to-join-the-ego-consortium/ |access-date=2025-05-28 |website=EGO - European Gravitational Observatory |language=en-GB}} 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.{{Cite web |last=Carpinelli |first=Massimo |date=12 July 2023 |title=EGO Council Report |url=https://indico.ego-gw.it/event/636/contributions/5541/attachments/2962/5213/Council_0712.pdf |publisher=European Gravitational Observatory |website=ego-gw.it |access-date=15 November 2024}}

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 {{as of|2024|12|lc=y|post=.}}{{Cite web |last=The Virgo Collaboration |date=7 November 2024 |title=Winners of the Virgo Award 2024 announced |url=https://www.virgo-gw.eu/news/winners-of-the-virgo-award-2024-announced/ |access-date=2024-12-15 |website=Virgo |language=en-GB}} 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.{{cite web |title=The Virgo Institutions |url=https://apps.virgo-gw.eu/vmd/public/institutions |access-date=28 October 2024 |website=virgo-gw.eu |publisher=The Virgo Collaboration |archive-date=16 July 2024 |archive-url=https://web.archive.org/web/20240716195936/https://apps.virgo-gw.eu/vmd/public/institutions |url-status=live }}

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.{{Cite web |title=Scientific Collaboration – Virgo |url=https://www.virgo-gw.eu/about/scientific-collaboration/ |access-date=31 March 2023 |website=virgo-gw.eu |publisher=The Virgo Collaboration |archive-date=31 March 2023 |archive-url=https://web.archive.org/web/20230331111528/https://www.virgo-gw.eu/about/scientific-collaboration/ |url-status=live }} LVK began in 2007{{Cite web |date=March 2019 |title=LIGO-M060038-v5: Memorandum of Understanding (MoU) Between VIRGO and LIGO |url=https://dcc.ligo.org/LIGO-M060038/public |access-date=4 July 2023 |website=dcc.ligo.org |archive-date=8 December 2015 |archive-url=https://web.archive.org/web/20151208065730/https://dcc.ligo.org/LIGO-M060038/public |url-status=live }} as the LIGO-Virgo Collaboration, and was expanded when KAGRA joined in 2019.{{Cite web |title=LIGO Scientific Collaboration – Learn about the LSC |url=https://www.ligo.org/about.php |access-date=31 March 2023 |website=ligo.org |publisher=LIGO Lab {{!}} Caltech |archive-date=3 April 2023 |archive-url=https://web.archive.org/web/20230403153715/https://www.ligo.org/about.php |url-status=live }}{{Cite web |date=4 October 2019 |title=KAGRA to Join LIGO and Virgo in Hunt for Gravitational Waves |url=https://www.ligo.caltech.edu/news/ligo20191004 |access-date=4 July 2023 |publisher=LIGO Lab {{!}} Caltech |website=ligo.caltech.edu |archive-date=18 November 2020 |archive-url=https://web.archive.org/web/20201118152839/https://www.ligo.caltech.edu/news/ligo20191004 |url-status=live }}

File:MergingBlackHoles V2.jpg of gravitational waves emitted by the orbital decay and merger of two black holes]]

File:GW170817 Gravitational Wave Chirp Spectrogram.jpg event. The x axis represents time, and the y axis the frequency. The frequency increase over time is typical of gravitational waves from binary compact objects, and its shape is primarily determined by the objects' mass.{{Cite web |title=Sources and Types of Gravitational Waves |url=https://www.ligo.caltech.edu/page/gw-sources#CBC |access-date=21 October 2024 |publisher=LIGO Lab {{!}} Caltech |website=ligo.caltech.edu |archive-date=1 October 2024 |archive-url=https://web.archive.org/web/20241001000745/https://www.ligo.caltech.edu/page/gw-sources#CBC |url-status=live }}]]

{{Main|Ground-based interferometric gravitational-wave search#Science case}}

Virgo is designed to look for gravitational waves emitted by astrophysical sources across the universe which can be classified into three types:{{Cite web |title=Astrophysical Sources of Gravitational Waves |url=https://www.virgo-gw.eu/science/gw-universe/astrophysical-sources-of-gw/ |access-date=17 May 2024 |website=virgo-gw.eu |publisher=The Virgo Collaboration |language=en-GB |archive-date=24 May 2024 |archive-url=https://web.archive.org/web/20240524094125/https://www.virgo-gw.eu/science/gw-universe/astrophysical-sources-of-gw/ |url-status=live }}

• 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,{{Cite journal |last1=Eardley |first1=Douglas M. |last2=Lee |first2=David L. |last3=Lightman |first3=Alan P. |last4=Wagoner |first4=Robert V. |last5=Will |first5=Clifford M. |date=30 April 1973 |title=Gravitational-Wave Observations as a Tool for Testing Relativistic Gravity |url=https://link.aps.org/doi/10.1103/PhysRevLett.30.884 |journal=Physical Review Letters |volume=30 |issue=18 |pages=884–886 |bibcode=1973PhRvL..30..884E |doi=10.1103/PhysRevLett.30.884 |s2cid=120335306 |hdl-access=free |hdl=2060/19730012613}} possible gravitational lensing,{{Cite journal |last=Abbott |first=R. |display-authors=etal |date=2021 |title=Search for Lensing Signatures in the Gravitational-Wave Observations from the First Half of LIGO–Virgo's Third Observing Run |journal=The Astrophysical Journal |volume=923 |issue=1 |page=14 |arxiv=2105.06384 |bibcode=2021ApJ...923...14A |doi=10.3847/1538-4357/ac23db |s2cid=234482851 |doi-access=free}} or determining whether the observed signals are correctly described by general relativity.{{cite book |last=Van Den Broeck |first=Chris |chapter=Probing Dynamical Spacetimes with Gravitational Waves |date=2014 |title=Springer Handbook of Spacetime |pages=589–613 |editor-last=Ashtekar |editor-first=Abhay |chapter-url=https://doi.org/10.1007/978-3-642-41992-8_27 |access-date=23 April 2023 |series=Springer Handbooks |place=Berlin, Heidelberg |publisher=Springer |language=en |arxiv=1301.7291 |bibcode=2014shst.book..589V |doi=10.1007/978-3-642-41992-8_27 |isbn=978-3-642-41992-8 |s2cid=119242493 |editor2-last=Petkov |editor2-first=Vesselin |editor-link=Abhay Ashtekar}} It also provides a way to measure the Hubble constant.{{Cite journal |last1=The LIGO Scientific Collaboration |last2=the Virgo Collaboration |last3=the KAGRA Collaboration |last4=Abbott |first4=R. |last5=Abe |first5=H. |last6=Acernese |first6=F. |last7=Ackley |first7=K. |last8=Adhikari |first8=N. |last9=Adhikari |first9=R. X. |last10=Adkins |first10=V. K. |last11=Adya |first11=V. B. |last12=Affeldt |first12=C. |last13=Agarwal |first13=D. |last14=Agathos |first14=M. |last15=Agatsuma |first15=K. |year=2023 |title=Constraints on the Cosmic Expansion History from GWTC–3 |journal=The Astrophysical Journal |volume=949 |issue=2 |page=76 |arxiv=2111.03604 |bibcode=2023ApJ...949...76A |doi=10.3847/1538-4357/ac74bb |s2cid=243832919 |doi-access=free}}

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,{{Cite web |title=Cascina, Santo Stefano a Macerata {{!}} Virgo Center |url=https://www.terredipisa.it/en/attrazione/cascina-santo-stefano-a-macerata-virgo-research-center/ |access-date=17 November 2024 |website=Terre di Pisa |language=en-US |archive-date=21 February 2024 |archive-url=https://web.archive.org/web/20240221080757/https://www.terredipisa.it/en/attrazione/cascina-santo-stefano-a-macerata-virgo-research-center/ |url-status=live }} 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.{{Cite web |title=Virgo History |url=https://www.virgo-gw.eu/about/virgo-history/ |access-date=1 October 2024 |website=virgo-gw.eu |publisher=The Virgo Collaboration |language=en-GB |archive-date=1 August 2024 |archive-url=https://web.archive.org/web/20240801123609/https://www.virgo-gw.eu/about/virgo-history/ |url-status=live }}

Although the concept of gravitational waves was presented by Albert Einstein in 1916,{{Cite journal |last=Einstein |first=Albert |date=1 January 1916 |title=Näherungsweise Integration der Feldgleichungen der Gravitation |trans-title=Approximative Integration of the Field Equations of Gravitation |url=https://ui.adsabs.harvard.edu/abs/1916SPAW.......688E |journal=Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften (Minutes of the Royal Prussian Academy of Sciences) |language=de |pages=688–696 |bibcode=1916SPAW.......688E |access-date=5 March 2023 |archive-date=7 March 2023 |archive-url=https://web.archive.org/web/20230307155936/https://ui.adsabs.harvard.edu/abs/1916SPAW.......688E |url-status=live }} serious projects for detecting them only began during the late 1960s.{{cite journal |last=Lindley |first=David |date=22 December 2005 |title=A Fleeting Detection of Gravitational Waves |url=http://focus.aps.org/story/v16/st19 |url-status=live |journal=Physics |volume=16 |page=19 |doi= |archive-url=https://web.archive.org/web/20110927072008/http://focus.aps.org/story/v16/st19 |archive-date=27 September 2011 |access-date=2006-05-06 |doi-access=}} The first were the Weber bars, invented by Joseph Weber;{{Cite journal |last=Weber |first=J. |author-link=Joseph Weber |date=3 June 1968 |title=Gravitational-Wave-Detector Events |url=https://link.aps.org/doi/10.1103/PhysRevLett.20.1307 |journal=Physical Review Letters |volume=20 |issue=23 |pages=1307–1308 |bibcode=1968PhRvL..20.1307W |doi=10.1103/PhysRevLett.20.1307|url-access=subscription }} 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,{{Cite book |last=Giazotto |first=Adalberto |author-link=Adalberto Giazotto |title=La Musica Nascosta dell'Universo: La Mia Vita a Caccia delle Onde Gravitazionali |publisher=Einaudi |year=2018 |location=Turin |language=it |trans-title=The Hidden Music of the Universe: My Life of Running After Gravitational Waves |asin=B07FY52PGV |bibcode=2018lmnd.book.....G}} which was first proposed in 1987.{{Cite tech report |title=Proposta di Antenna Interferometrica a Grande Base Per la Ricerca di Onde Gravitazionali |last=Giazotto |first=Adalberto |last2=Milano |first2=Leopoldo |date=12 May 1987 |language=it |trans-title=Proposition for an Interferometric Antenna with Long Arms for Searching Gravitational Waves |last3=Bordoni |first3=Franco |last4=Brillet |first4=Alain |author-link4=Alain Brillet |last5=Tourrenc |first5=Philippe |url=https://www.ego-gw.it/wp-content/uploads/sites/12/2020/06/VIRGO_Proposta_1987_VIR-0473B-15_clean.pdf |website=ego-gw.it }} 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.{{Cite journal |last1=Caron |first1=B. |last2=Dominjon |first2=A. |last3=Drezen |first3=C. |last4=Flaminio |first4=R. |last5=Grave |first5=X. |last6=Marion |first6=F. |last7=Massonnet |first7=L. |last8=Mehmel |first8=C. |last9=Morand |first9=R. |last10=Mours |first10=B. |last11=Yvert |first11=M. |last12=Babusci |first12=D. |last13=Giordano |first13=G. |last14=Matone |first14=G. |last15=Mackowski |first15=J. -M. |date=1 May 1996 |title=Status of the VIRGO Experiment |url=https://dx.doi.org/10.1016/0920-5632%2896%2900220-4 |journal=Nuclear Physics B: Proceedings Supplements |series=Proceedings of the Fourth International Workshop on Theoretical and Phenomenological Aspects of Underground Physics |language=en |volume=48 |issue=1 |pages=107–109 |bibcode=1996NuPhS..48..107C |doi=10.1016/0920-5632(96)00220-4 |issn=0920-5632|url-access=subscription }}

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.{{cite journal |author1=J.M. Weisberg and J.H. Taylor |year=2004 |title=Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis |journal=ASP Conference Series |volume=328 |pages=25 |arxiv=astro-ph/0407149 |bibcode=2005ASPC..328...25W}}

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.{{cite journal |last1=Riles |first1=K. |year=2013 |title=Gravitational Waves: Sources, Detectors and Searches |journal=Progress in Particle and Nuclear Physics |volume=68 |pages=1–54 |arxiv=1209.0667 |bibcode=2013PrPNP..68....1R |doi=10.1016/j.ppnp.2012.08.001 |s2cid=56407863}}{{cite journal |author=Sathyaprakash and |first1=B.S. |last2=Schutz |first2=Bernard F. |author-link2=Bernard F. Schutz |year=2009 |title=Physics, Astrophysics and Cosmology with Gravitational Waves |journal=Living Reviews in Relativity |volume=12 |issue=1 |page=2 |arxiv=0903.0338 |bibcode=2009LRR....12....2S |doi=10.12942/lrr-2009-2 |pmc=5255530 |pmid=28163611 |doi-access=free}} This phase is sometimes called the "initial Virgo" or "original Virgo".{{Cite web |last=Williams |first=Matt |date=28 September 2017 |title=LIGO and Virgo Observatories Detect Black Holes Colliding |url=https://www.universetoday.com/137319/ligo-virgo-observatories-detect-black-holes-colliding/ |access-date=21 October 2024 |website=Universe Today |language=en-US}}{{Cite web |title=Virgo – European Gravitational Observatory {{!}} Department of Physics |url=https://df.units.it/en/research/researchareas/researchgroups/24248 |access-date=21 October 2024 |website=df.units.it |archive-date=17 July 2024 |archive-url=https://web.archive.org/web/20240717111448/https://df.units.it/en/research/researchareas/researchgroups/24248 |url-status=live }}

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.{{cite web |date=22 May 2007 |title=Ondes Gravitationnelles : Virgo Entre dans sa Phase d'Exploitation Scientifique – Communiqués et Dossiers de Presse |trans-title=Gravitational Waves : Virgo Enters in its Scientific Exploitation Phase – Press Releases and Communications |url=http://www2.cnrs.fr/sites/communique/fichier/dpvirgo_1.pdf |access-date=21 February 2024 |website=cnrs.fr |language=fr}} Some of the runs were performed with the two LIGO detectors (which are located in Hanford, Washington and in Livingston, Louisiana).{{Cite web |title=What Is LIGO? |url=https://www.ligo.caltech.edu/page/what-is-ligo |access-date=26 November 2024 |website=ligo.caltech.edu |publisher=LIGO Lab {{!}} Caltech |archive-date=11 October 2024 |archive-url=https://web.archive.org/web/20241011214115/https://www.ligo.caltech.edu/page/what-is-ligo |url-status=live }} 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.{{Cite journal |last=Lorenzini |first=Matteo |date=April 2010 |title=The Monolithic Suspension for the Virgo Interferometer |url=https://iopscience.iop.org/article/10.1088/0264-9381/27/8/084021/meta |journal=Classical and Quantum Gravity |volume=27 |issue=8 |page=084021 |bibcode=2010CQGra..27h4021L |doi=10.1088/0264-9381/27/8/084021 |s2cid=123269358}} 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.{{cite journal |author1=The Virgo Collaboration |year=2011 |title=Status of the Virgo Project |url=https://hal.archives-ouvertes.fr/hal-00705154/file/PEER_stage2_10.1088%252F0264-9381%252F28%252F11%252F114002.pdf |journal=Classical and Quantum Gravity |volume=28 |issue=11 |pages=114002 |bibcode=2011CQGra..28k4002A |doi=10.1088/0264-9381/28/11/114002 |s2cid=59369141 |access-date=30 September 2019 |archive-date=30 September 2019 |archive-url=https://web.archive.org/web/20190930202732/https://hal.archives-ouvertes.fr/hal-00705154/file/PEER_stage2_10.1088%25252F0264-9381%25252F28%25252F11%25252F114002.pdf |url-status=live }}

File:GW170814 signal.png

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 {{convert|35|cm|in|abbr=on}} and a weight of {{convert|40|kg|lb|abbr=on}}, 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.{{Rp|page=75}}

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.{{cite news |last=Gibney |first=Elizabeth |author-link=Elizabeth Gibney |date=27 September 2017 |title=European Detector Spots Its First Gravitational Wave |url=https://www.nature.com/news/european-detector-spots-its-first-gravitational-wave-1.22690?WT.mc_id=TWT_NatureNews&sf117118315=1 |access-date=21 February 2024 |work=Nature |archive-date=12 July 2020 |archive-url=https://web.archive.org/web/20200712072050/https://www.nature.com/news/european-detector-spots-its-first-gravitational-wave-1.22690?WT.mc_id=TWT_NatureNews&sf117118315=1 |url-status=live }}

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.{{Cite journal |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=C. |last7=Adams |first7=T. |last8=Addesso |first8=P. |last9=Adhikari |first9=R. X. |last10=Adya |first10=V. B. |last11=Affeldt |first11=C. |last12=Afrough |first12=M. |last13=Agarwal |first13=B. |last14=Agathos |first14=M. |date=28 February 2018 |title=GW170817: Implications for the Stochastic Gravitational-Wave Background from Compact Binary Coalescences |url=https://link.aps.org/doi/10.1103/PhysRevLett.120.091101 |journal=Physical Review Letters |volume=120 |issue=9 |pages=091101 |arxiv=1710.05837 |bibcode=2018PhRvL.120i1101A |doi=10.1103/PhysRevLett.120.091101 |pmid=29547330 |s2cid=3889124}}

{{Gravitational waves observation periods graphical timeline}}

After further upgrades, Virgo began its third observation run (O3) in April 2019. Planned to last one year,{{Cite web |last=Bersanetti |first=Diego |date=13 July 2019 |title=Status of the Virgo Gravitational-wave Detector and the O3 Observing Run – EPS-HEP2019 |url=https://indico.cern.ch/event/577856/contributions/3422625/ |access-date=29 February 2024 |website=cern.ch}} the run ended early on 27 March 2020 due to the COVID-19 pandemic.{{Cite web |date=26 March 2020 |title=LIGO Suspends Third Observing Run (O3) |url=https://www.ligo.caltech.edu/news/ligo20200326 |access-date=16 April 2023 |website=ligo.caltech.edu |publisher=LIGO Lab {{!}} Caltech |archive-date=7 April 2023 |archive-url=https://web.archive.org/web/20230407154833/https://www.ligo.caltech.edu/news/ligo20200326 |url-status=live }}

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.

The fourth observation run (O4) was scheduled to begin in May 2023 and was planned to last for 20 months, including a commissioning break of up to two months.{{Cite web |title=IGWN {{!}} Observing Plans |url=https://observing.docs.ligo.org/plan/ |url-status=live |archive-url=https://web.archive.org/web/20220625063752/https://observing.docs.ligo.org/plan/ |archive-date=25 June 2022 |access-date=16 January 2024 |website=observing.docs.ligo.org}} On 11 May 2023, Virgo announced that it would not join the beginning of O4; the interferometer was not stable enough to reach the expected sensitivity and one mirror needed replacement, requiring several weeks of work.{{Cite web |date=11 May 2023 |title=Virgo Postpones Entry into O4 Observing Run – Virgo |url=https://www.virgo-gw.eu/news/virgo-postpones-entry-into-o4-observing-run/ |access-date=13 May 2023 |website=virgo-gw.eu |archive-date=13 May 2023 |archive-url=https://web.archive.org/web/20230513085601/https://www.virgo-gw.eu/news/virgo-postpones-entry-into-o4-observing-run/ |url-status=live }} Virgo did not join the O4 run during its first part (O4a, which ended on 16 January 2024), since it only reached a peak sensitivity of 45 Mpc instead of the 80 to 115 Mpc initially expected; it joined the second part of the run (O4b), which began on 10 April 2024, with a sensitivity of 50 to 55 Mpc. In June 2024, it was announced that the O4 run would last until 9 June 2025 to further prepare for the O5 upgrades. The schedule was further revised in January 2025, with an additional two-month break starting in April 2025, and an extension of the run until 7 October 2025 to accommodate for the missing time. In June 2025, the run was extended even further up to 18 November 2025, allowing observations overlapping with the first operations of the Vera Rubin Observatory. These three extensions to the run are designated as O4c (which officially started on 28 January 2025).{{Cite web |last=The Virgo Collaboration |date=3 February 2025 |title=LVK announces an update of the observing schedule |url=https://www.virgo-gw.eu/news/lvk-announces-an-update-of-the-observing-schedule/ |access-date=2025-02-21 |website=Virgo |language=en-GB}}

The detector will again be shut down for upgrades, including mirror-coating improvement, after the O4 run. A fifth observing run (O5) is planned to begin near the end of 2027. Virgo's target sensitivity, originally set at 150–260 Mpc, is being redefined in light of its performance during O4. Plans to enter the O5 run are expected to be known in the first half of 2025.

No official plans have been announced for the future of the Virgo installations after the O5 period, although projects for improving the detectors have been suggested. The collaboration's current plans are known as the Virgo_nEXT project.{{Cite web |last=Virgo researchers |date=7 June 2023 |title=Virgo_nEXT |url=https://atrium.in2p3.fr/nuxeo/nxfile/default/00e9c8e3-ba10-44da-8781-8dc167a452e2/blobholder:0/4-Doc-Virgo_nEXT-Barsuglia-Tournefier.pdf}}

{{Main|Ground-based interferometric gravitational-wave search#Principle}}File:Gravitational wave interferometer animation.ogg.{{Cite web |first= |title=What Is An Interferometer? |url=https://www.ligo.caltech.edu/page/what-is-interferometer |access-date=21 October 2024 |publisher=LIGO Lab {{!}} Caltech |website=ligo.caltech.edu |archive-date=27 September 2024 |archive-url=https://web.archive.org/web/20240927025526/https://www.ligo.caltech.edu/page/what-is-interferometer |url-status=live }}]]

In general relativity, a gravitational wave is a space-time perturbation which propagates at the speed of light. It slightly curves spacetime, changing the light path. This can be detected with a Michelson interferometer, in which a laser is divided into two beams travelling in orthogonal directions, bouncing on a mirror at the end of each arm. As the gravitational wave passes, it alters the path of the two beams differently; they are then recombined, and the resulting interferometric pattern is measured with a photodiode. Since the induced deformation is extremely small, precision in mirror position, laser stability, measurements, and isolation from outside noise are essential.{{cite book |author=Vinet |first1=Jean-Yves |url=https://artemis.oca.eu/images/Artemis/pdf_artemis/Vinet-Optics_and_related_topics-VPB_2020.pdf |title=The VIRGO Physics Book - Optics and Related Topics |last2=The Virgo Collaboration |year=2020 |pages=19 |access-date=16 April 2023 |archive-url=https://web.archive.org/web/20240327231110/https://artemis.oca.eu/images/Artemis/pdf_artemis/Vinet-Optics_and_related_topics-VPB_2020.pdf |archive-date=27 March 2024 |url-status=live}}

File:Virgo_Interferometer_O4_diagram.png |isbn=978-1-5106-3677-4 |editor-last1=Marshall |editor-first1=Heather K. |series=SPIE Conference Series |volume=11445 |pages=205–214 |chapter=Status and Plans of the Virgo Gravitational Wave Detector |bibcode=2020SPIE11445E..11F |doi=10.1117/12.2565418 |editor-last2=Spyromilio |editor-first2=Jason |editor-last3=Usuda |editor-first3=Tomonori |chapter-url=https://hal.science/hal-03107929/document |s2cid=230549331 |access-date=26 August 2023 |archive-date=28 May 2022 |archive-url=https://web.archive.org/web/20220528153836/https://hal.archives-ouvertes.fr/hal-03107929/file/1144511.pdf |url-status=live }}]]

The laser, the instrument's light source, must be powerful and stable in frequency and amplitude.{{cite journal |author1=F. Bondu |display-authors=etal |year=1996 |title=Ultrahigh-spectral-purity laser for the VIRGO experiment |journal=Optics Letters |volume=21 |issue=8 |pages=582–4 |bibcode=1996OptL...21..582B |doi=10.1364/OL.21.000582 |pmid=19876090}} To meet these specifications, the beam starts from a low-power, stable laser.{{cite journal |author1=F. Bondu |display-authors=etal |year=2002 |title=The VIRGO Injection System |url=http://people.na.infn.it/~garufi/Pubblicazioni/cqg19%282002%29_1829_1833.pdf |journal=Classical and Quantum Gravity |volume=19 |issue=7 |pages=1829–1833 |bibcode=2002CQGra..19.1829B |doi=10.1088/0264-9381/19/7/381 |s2cid=250902832 |access-date=16 December 2015 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304105744/http://people.na.infn.it/~garufi/Pubblicazioni/cqg19(2002)_1829_1833.pdf |url-status=live }} Light from the laser passes through several amplifiers, which enhance its power by a factor of 100. A 50 watt (W) output power was achieved for the last configuration of the initial Virgo detector (reaching 100 W during the O3 run after the Advanced Virgo upgrades), and is expected to be upgraded to 130 W at the beginning of the O4 run. The original Virgo detector had a master-slave laser system, where a "master" laser is used to stabilise a high-powered "slave" laser; the master laser was a Nd:YAG laser, and the slave laser was a Nd:YVO4 laser.{{Cite journal |last1=Acernese |first1=F. |last2=Amico |first2=P. |last3=Al-Shourbagy |first3=M. |last4=Aoudia |first4=S. |last5=Avino |first5=S. |display-authors=etal |date=August 2004 |title=The Status of VIRGO |url=https://hal.in2p3.fr/in2p3-00025837/ |journal=5th Rencontres du Vietnam Particle Physics and Astrophysics |location=Hanoi, Vietnam |pages=1–6 |via=HAL |access-date=16 April 2023 |archive-date=16 April 2023 |archive-url=https://web.archive.org/web/20230416094712/https://hal.in2p3.fr/in2p3-00025837/ |url-status=live }} The Advanced Virgo design uses a fibre laser, with an amplification stage also made of fibres, to improve the system's robustness; its final configuration is planned to combine the light of two lasers to reach the required power.{{cite book |author=The Virgo Collaboration |url=https://www.nikhef.nl/pub/departments/mt/projects/virgo/general/pub/TDR/AdV_TDR.pdf |title=Advanced Virgo Technical Design Report VIR–0128A–12 |date=13 April 2012 |access-date=3 October 2017 |archive-date=4 October 2017 |archive-url=https://web.archive.org/web/20171004035111/https://www.nikhef.nl/pub/departments/mt/projects/virgo/general/pub/TDR/AdV_TDR.pdf |url-status=live }}{{rp|87}}{{Cite thesis |last=Wei |first=Li-Wei |title=High-power Laser System for Advanced Virgo Gravitational Wave Detector : Coherently Combined Master Oscillator Fiber Power Amplifiers |date=3 December 2015 |degree=PhD |publisher=Université Nice Sophia Antipolis |url=https://theses.hal.science/tel-01284969 |language=en}} The laser's wavelength is 1064 nanometres in the original and Advanced Virgo configurations.

This laser beam is sent into the interferometer after passing through the injection system, which ensures its stability, adjusts its shape and power, and positions it correctly for entering the interferometer. The injection system includes the input mode cleaner, which is a {{convert|140|m|ft|adj=mid|-long}} cavity designed to improve beam quality by stabilising the frequency, removing unwanted light propagation and reducing the effect of laser misalignment. It also features a Faraday isolator preventing light from returning to the laser, and a mode-matching telescope which adapts the size and position of the beam before it enters the interferometer.{{rp|93–96}}

File:Initial Virgo mirror.jpg

The large mirrors in each arm are the interferometer's most critical optics. They include the two end mirrors at the ends of the {{convert|3|km|mi|abbr=on|adj=on}} interferometer arms and the two input mirrors near the beginning of the arms. These mirrors make a resonant optical cavity in each arm in which the light bounces thousands of times before returning to the beam splitter, maximising the signal's effect on the laser path{{Cite web |title=Optical Layout – Virgo |url=https://www.virgo-gw.eu/science/detector/optical-layout/ |access-date=5 March 2023 |publisher=The Virgo Collaboration |website=virgo-gw.eu |archive-date=5 March 2023 |archive-url=https://web.archive.org/web/20230305173927/https://www.virgo-gw.eu/science/detector/optical-layout/ |url-status=live }} and allowing the power of the light circulating in the arms to be increased. These mirrors (designed for Virgo) are cylinders {{convert|35|cm|in|abbr=on}} in diameter and {{convert|20|cm|in|abbr=on}} thick,{{rp|173}} made from extremely pure glass.{{cite journal |author1=J. Degallaix |year=2015 |title=Silicon, the Test Mass Substrate of Tomorrow? |url=http://www.kitpc.ac.cn/files/activities/PT20150406/report/Degallaix_KITPC_meeting.pdf |url-status=dead |journal=The Next Detectors for Gravitational Wave Astronomy |archive-url=https://web.archive.org/web/20151208125728/http://www.kitpc.ac.cn/files/activities/PT20150406/report/Degallaix_KITPC_meeting.pdf |archive-date=8 December 2015 |access-date=16 December 2015}} During the manufacturing process, the mirrors are polished to the atomic level to avoid diffusing (and losing) any light.{{cite thesis |author1=R. Bonnand |title=The Advanced Virgo Gravitational Wave Detector/ Study of the Optical Design and Development of the Mirrors |date=2012 |type=PhD |publisher=Université Claude Bernard – Lyon I |url=https://tel.archives-ouvertes.fr/tel-00797350 |language=French |access-date=16 December 2015 |archive-date=3 May 2016 |archive-url=https://web.archive.org/web/20160503065039/https://tel.archives-ouvertes.fr/tel-00797350 |url-status=live }} A reflective coating (a Bragg reflector made with ion-beam sputtering) is then added. The mirrors at the end of the arms reflect almost all incoming light, with less than 0.002 per cent lost at each reflection.{{cite journal |author1=R Flaminio |display-authors=etal |year=2010 |title=A Study of Coating Mechanical and Optical Losses in View of Reducing Mirror Thermal Noise in Gravitational Wave Detectors |url=https://hal.archives-ouvertes.fr/hal-00587621/file/PEER_stage2_10.1088%252F0264-9381%252F27%252F8%252F084030.pdf |journal=Classical and Quantum Gravity |volume=27 |issue=8 |pages=084030 |bibcode=2010CQGra..27h4030F |doi=10.1088/0264-9381/27/8/084030 |s2cid=122750664 |access-date=5 September 2020 |archive-date=30 September 2020 |archive-url=https://web.archive.org/web/20200930214616/https://hal.archives-ouvertes.fr/hal-00587621/file/PEER_stage2_10.1088%252F0264-9381%252F27%252F8%252F084030.pdf |url-status=live }}

Two other mirrors are also in the final design:

• The power-recycling mirror, between the laser and the beam splitter. Since most light is reflected toward the laser after returning to the beam splitter, this mirror re-injects the light into the main interferometer and increases power in the arms.
• {{Anchor|Signal recycling}}The signal-recycling mirror, at the interferometer output, re-injects part of the signal into the interferometer (transmission of this mirror is planned to be 40 per cent) and forms another cavity. With small adjustments to this mirror, quantum noise can be reduced in part of the frequency band and increased elsewhere; this makes it possible to tune the interferometer for certain frequencies. It is planned to use a wideband configuration, decreasing noise at high and low frequencies and increasing it at intermediate frequencies. Decreased noise at high frequencies is of particular interest for study of a signal right before and after a compact object merger.{{Cite journal |last1=Bersanetti |first1=Diego |last2=Patricelli |first2=Barbara |last3=Piccinni |first3=Ornella Juliana |last4=Piergiovanni |first4=Francesco |last5=Salemi |first5=Francesco |last6=Sequino |first6=Valeria |date=August 2021 |title=Advanced Virgo: Status of the Detector, Latest Results and Future Prospects |journal=Universe |language=en |volume=7 |issue=9 |pages=322 |bibcode=2021Univ....7..322B |doi=10.3390/universe7090322 |issn=2218-1997 |doi-access=free |hdl-access=free |hdl=11568/1161730}}

File:Virgo3 1.jpg.{{cite journal |last1=Accadia |first1=T. |last2=Acernese |first2=F. |last3=Alshourbagy |first3=M. |last4=Amico |first4=P. |last5=Antonucci |first5=F. |last6=Aoudia |first6=S. |last7=Arnaud |first7=N. |last8=Arnault |first8=C. |last9=Arun |first9=K. G. |last10=Astone |first10=P. |last11=Avino |first11=S. |last12=Babusci |first12=D. |last13=Ballardin |first13=G. |last14=Barone |first14=F. |last15=Barrand |first15=G. |display-authors=29 |date=29 March 2012 |title=Virgo: a Laser Interferometer to Detect Gravitational Waves |journal=Journal of Instrumentation |volume=7 |issue=3 |pages=03012 |bibcode=2012JInst...7.3012A |doi=10.1088/1748-0221/7/03/P03012 |doi-access=free |last16=Barsotti |first16=L. |author-link16=Lisa Barsotti |last17=Barsuglia |first17=M. |last18=Basti |first18=A. |last19=Bauer |first19=Th S. |last20=Beauville |first20=F. |last21=Bebronne |first21=M. |last22=Bejger |first22=M. |last23=Beker |first23=M. G. |last24=Bellachia |first24=F. |last25=Belletoile |first25=A. |last26=Beney |first26=J. L. |last27=Bernardini |first27=M. |last28=Bigotta |first28=S. |last29=Bilhaut |first29=R. |last30=Birindelli |first30=S.}} Seismic vibrations above 10 Hz are reduced by over 10¹² times,{{Cite journal |last=Boschi |first=Valerio |date=1 March 2019 |title=Seismic Isolation in Advanced Virgo Gravitational Wave Detector |url=https://doi.org/10.1121/1.5101119 |journal=Journal of the Acoustical Society of America |volume=145 |issue=3_Supplement |pages=1668 |bibcode=2019ASAJ..145.1668B |doi=10.1121/1.5101119 |issn=0001-4966 |s2cid=150337668|url-access=subscription }} and the mirror position is controlled.]]

To mitigate seismic noise which could propagate up to the mirrors, shaking them and obscuring potential gravitational-wave signals, the mirrors are suspended by a complex system. The main mirrors are suspended by four thin fibres made of silica{{cite journal |author1=M. Lorenzini & Virgo Collaboration |year=2010 |title=The Monolithic Suspension for the Virgo Interferometer |url=https://hal.archives-ouvertes.fr/hal-00587617/document |journal=Classical and Quantum Gravity |volume=27 |issue=8 |pages=084021 |bibcode=2010CQGra..27h4021L |doi=10.1088/0264-9381/27/8/084021 |s2cid=123269358 |access-date=16 December 2015 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304192619/https://hal.archives-ouvertes.fr/hal-00587617/document |url-status=live }} which are attached to a series of attenuators. This superattenuator, nearly {{convert|8|m|ft}} high, is in a vacuum.{{Cite journal |last1=Braccini |first1=S. |last2=Barsotti |first2=L. |last3=Bradaschia |first3=C. |last4=Cella |first4=G. |last5=Virgilio |first5=A. Di |last6=Ferrante |first6=I. |last7=Fidecaro |first7=F. |last8=Fiori |first8=I. |last9=Frasconi |first9=F. |last10=Gennai |first10=A. |last11=Giazotto |first11=A. |last12=Paoletti |first12=F. |last13=Passaquieti |first13=R. |last14=Passuello |first14=D. |last15=Poggiani |first15=R. |date=1 July 2005 |title=Measurement of the Seismic Attenuation Performance of the VIRGO Superattenuator |url=https://www.sciencedirect.com/science/article/pii/S092765050500068X |journal=Astroparticle Physics |language=en |volume=23 |issue=6 |pages=557–565 |bibcode=2005APh....23..557B |doi=10.1016/j.astropartphys.2005.04.002 |issn=0927-6505|url-access=subscription }} The superattenuators limit disturbances to the mirrors and allow mirror position and orientation to be precisely steered. The optical table with the injection optics used to shape the laser beam, such as the optical benches used for the light detection, are also suspended in a vacuum to limit seismic and acoustic noise. In the Advanced Virgo configuration, the instrumentation used to detect gravitational-wave signals and steer the interferometer (photodiodes, cameras, and associated electronics) is installed on several benches suspended in a vacuum.{{rp|477}}

Superattenuator design is based on passive attenuation of seismic noise achieved by chaining several pendula, each a harmonic oscillator. They have a resonant frequency (diminishing with pendulum length) above which noise will be dampened; chaining several pendula reduces noise by twelve orders of magnitude, introducing resonant frequencies which are higher than a single long pendulum.{{Cite journal |last1=Beker |first1=M. G. |last2=Blom |first2=M. |last3=van den Brand |first3=J. F. J. |last4=Bulten |first4=H. J. |last5=Hennes |first5=E. |last6=Rabeling |first6=D. S. |date=1 January 2012 |title=Seismic Attenuation Technology for the Advanced Virgo Gravitational Wave Detector |journal=Physics Procedia |series=Proceedings of the 2nd International Conference on Technology and Instrumentation in Particle Physics (TIPP 2011) |volume=37 |pages=1389–1397 |doi=10.1016/j.phpro.2012.03.741 |issn=1875-3892|doi-access=free |bibcode=2012PhPro..37.1389B }} The highest resonant frequency is around 2 Hz, providing meaningful noise reduction starting at 4 Hz{{rp|416}} and reaching the level needed to detect gravitational waves around 10 Hz. The system is limited in that noise in the resonant-frequency band (below 2 Hz) is not filtered and can generate large oscillations; this is mitigated by an active damping system, including sensors measuring seismic noise and actuators controlling the superattenuator to counteract the noise.

Part of the light in the arm cavities is sent towards the detection system by the beam splitter. The interferometer works near the "dark fringe", with very little light sent towards the output; most is sent back to the input, to be collected by the power-recycling mirror. A fraction of this light is reflected back by the signal-recycling mirror, and the rest is collected by the detection system. It first passes through the output mode cleaner, which filters the "high-order modes" (light propagating in an unwanted way, typically from small defects in the mirrors){{Cite journal |last1=Beauville |first1=F |last2=Buskulic |first2=D |last3=Derome |first3=L |last4=Dominjon |first4=A |last5=Flaminio |first5=R |last6=Hermel |first6=R |last7=Marion |first7=F |last8=Masserot |first8=A |last9=Massonnet |first9=L |last10=Mours |first10=B |last11=Moreau |first11=F |last12=Mugnier |first12=P |last13=Ramonet |first13=J |last14=Tournefier |first14=E |last15=Verkindt |first15=D |date=7 May 2006 |title=Improvement in the Shot Noise of a Laser Interferometer Gravitational Wave Detector by Means of an Output Mode-cleaner |url=https://iopscience.iop.org/article/10.1088/0264-9381/23/9/030 |journal=Classical and Quantum Gravity |volume=23 |issue=9 |pages=3235–3250 |bibcode=2006CQGra..23.3235B |doi=10.1088/0264-9381/23/9/030 |issn=0264-9381 |s2cid=123072147|url-access=subscription }} before reaching the photodiodes which measure the light intensity. The output mode cleaner and the photodiodes are suspended in a vacuum.{{cite journal |last1=Acernese |first1=F. |last2=Agathos |first2=M. |last3=Agatsuma |first3=K. |last4=Aisa |first4=D. |last5=Allemandou |first5=N. |last6=Allocca |first6=A. |last7=Amarni |first7=J. |last8=Astone |first8=P. |last9=Balestri |first9=G. |last10=Ballardin |first10=G. |last11=Barone |first11=F. |last12=Baronick |first12=J-P |last13=Barsuglia |first13=M. |last14=Basti |first14=A. |last15=Basti |first15=F. |display-authors=29 |year=2015 |title=Advanced Virgo: A Second-generation Interferometric Gravitational Wave Detector |journal=Classical and Quantum Gravity |volume=32 |issue=2 |pages=024001 |arxiv=1408.3978 |bibcode=2015CQGra..32b4001A |doi=10.1088/0264-9381/32/2/024001 |s2cid=20640558 |last16=Bauer |first16=Th S. |last17=Bavigadda |first17=V. |last18=Bejger |first18=M. |last19=Beker |first19=M. G. |last20=Belczynski |first20=C. |last21=Bersanetti |first21=D. |last22=Bertolini |first22=A. |last23=Bitossi |first23=M. |last24=Bizouard |first24=M. A. |last25=Bloemen |first25=S. |last26=Blom |first26=M. |last27=Boer |first27=M. |last28=Bogaert |first28=G. |last29=Bondi |first29=D. |last30=Bondu |first30=F.}}

File:VirgoDetectionBench2015.jpg

With the O3 run, a squeezed vacuum source was introduced to reduce the quantum noise which is one of the main limitations to sensitivity. When replacing the standard vacuum with a squeezed vacuum, the fluctuations of a quantity are decreased at the expense of increasing the fluctuations of the other quantity due to Heisenberg's uncertainty principle. In Virgo, the quantities are the amplitude and phase of the light. A squeezed vacuum was proposed in 1981 by Carlton Caves during the infancy of gravitational-wave detectors.{{Cite journal |last=Caves |first=Carlton M. |date=15 April 1981 |title=Quantum-mechanical noise in an interferometer |url=https://link.aps.org/doi/10.1103/PhysRevD.23.1693 |journal=Physical Review D |volume=23 |issue=8 |pages=1693–1708 |doi=10.1103/PhysRevD.23.1693|bibcode=1981PhRvD..23.1693C |url-access=subscription }} During the O3 run, frequency-independent squeezing was implemented; squeezing is identical at all frequencies, reducing shot noise (dominant at high frequencies) and increasing radiation pressure noise (dominant at low frequencies, and not limiting the instrument's sensitivity).{{Cite journal |last1=The Virgo Collaboration |last2=Acernese |first2=F. |last3=Agathos |first3=M. |last4=Aiello |first4=L. |last5=Ain |first5=A. |last6=Allocca |first6=A. |last7=Amato |first7=A. |last8=Ansoldi |first8=S. |last9=Antier |first9=S. |last10=Arène |first10=M. |last11=Arnaud |first11=N. |last12=Ascenzi |first12=S. |last13=Astone |first13=P. |last14=Aubin |first14=F. |last15=Babak |first15=S. |date=22 September 2020 |title=Quantum Backaction on kg-Scale Mirrors: Observation of Radiation Pressure Noise in the Advanced Virgo Detector |journal=Physical Review Letters |volume=125 |issue=13 |pages=131101 |doi=10.1103/PhysRevLett.125.131101|pmid=33034506 |bibcode=2020PhRvL.125m1101A |s2cid=222235425 |doi-access=free |hdl=11390/1193696 |hdl-access=free }} Due to the addition of the squeezed vacuum injection, quantum noise was reduced by 3.2 dB at high frequencies and the detector's range was increased by five to eight per cent.{{Cite journal |last1=Virgo Collaboration |last2=Acernese |first2=F. |last3=Agathos |first3=M. |last4=Aiello |first4=L. |last5=Allocca |first5=A. |last6=Amato |first6=A. |last7=Ansoldi |first7=S. |last8=Antier |first8=S. |last9=Arène |first9=M. |last10=Arnaud |first10=N. |last11=Ascenzi |first11=S. |last12=Astone |first12=P. |last13=Aubin |first13=F. |last14=Babak |first14=S. |last15=Bacon |first15=P. |date=5 December 2019 |title=Increasing the Astrophysical Reach of the Advanced Virgo Detector via the Application of Squeezed Vacuum States of Light |journal=Physical Review Letters |volume=123 |issue=23 |pages=231108 |bibcode=2019PhRvL.123w1108A |doi=10.1103/PhysRevLett.123.231108 |pmid=31868444 |s2cid=209446443|doi-access=free |hdl=11585/709335 |hdl-access=free }} More sophisticated squeezed states are produced{{Cite journal |last1=Virgo Collaboration |last2=Acernese |first2=F. |last3=Agathos |first3=M. |last4=Ain |first4=A. |last5=Albanesi |first5=S. |last6=Alléné |first6=C. |last7=Allocca |first7=A. |last8=Amato |first8=A. |last9=Amra |first9=C. |last10=Andia |first10=M. |last11=Andrade |first11=T. |last12=Andres |first12=N. |last13=Andrés-Carcasona |first13=M. |last14=Andrić |first14=T. |last15=Ansoldi |first15=S. |date=25 July 2023 |title=Frequency-Dependent Squeezed Vacuum Source for the Advanced Virgo Gravitational-Wave Detector |journal=Physical Review Letters |volume=131 |issue=4 |pages=041403 |doi=10.1103/PhysRevLett.131.041403|pmid=37566847 |bibcode=2023PhRvL.131d1403A |s2cid=260185660 |doi-access=free |hdl=11568/1196710 |hdl-access=free }} by combining the technology from O3 with a new 285-m-long (935 ft) filter cavity. This technology, known as frequency-dependent squeezing, helps to reduce shot noise at high frequencies (where radiation pressure noise is irrelevant) and reduce radiation-pressure noise at low frequencies (where shot noise is low).{{Cite journal |last1=Zhao |first1=Yuhang |last2=Aritomi |first2=Naoki |last3=Capocasa |first3=Eleonora |last4=Leonardi |first4=Matteo |last5=Eisenmann |first5=Marc |last6=Guo |first6=Yuefan |last7=Polini |first7=Eleonora |last8=Tomura |first8=Akihiro |last9=Arai |first9=Koji |last10=Aso |first10=Yoichi |last11=Huang |first11=Yao-Chin |last12=Lee |first12=Ray-Kuang |last13=Lück |first13=Harald |last14=Miyakawa |first14=Osamu |last15=Prat |first15=Pierre |date=28 April 2020 |title=Frequency-Dependent Squeezed Vacuum Source for Broadband Quantum Noise Reduction in Advanced Gravitational-Wave Detectors |url=https://link.aps.org/doi/10.1103/PhysRevLett.124.171101 |journal=Physical Review Letters |volume=124 |issue=17 |pages=171101 |doi=10.1103/PhysRevLett.124.171101|pmid=32412296 |arxiv=2003.10672 |bibcode=2020PhRvL.124q1101Z |s2cid=214623227 }}{{Cite journal |last=Polini |first=E |date=1 August 2021 |title=Broadband Quantum Noise Reduction via Frequency Dependent Squeezing for Advanced Virgo Plus |url=https://iopscience.iop.org/article/10.1088/1402-4896/abfef0 |journal=Physica Scripta |volume=96 |issue=8 |pages=084003 |bibcode=2021PhyS...96h4003P |doi=10.1088/1402-4896/abfef0 |issn=0031-8949 |s2cid=235285860|url-access=subscription }}

File:Virgo aerial view 01.jpg|alt=Aerial view, with several white buildings and two long blue tubes forming a right angle|2015 aerial view of the Virgo site, showing the west arm (top) and part of the north arm (right), along with the various buildings

File:Virgo Cascina panorama.jpg|alt=Panorama of the site, with mountains in the background|Panorama of the entrance of the Virgo site

File:IGP9215.JPG|alt=The north arm, with mountains in the background|The {{convert|3|km|mi|adj=mid|-long}} north arm

File:IGP9210.JPG|alt=Above-ground view of the site, with parked cars for scale|(Front) Detector control-room building and local computer center

File:Virgo Cascina Central building.jpg|alt=A square, white, windowless building with three flags in front|The central building, containing most of the instrument critical components

File:IGP9212.JPG|alt=Two blue tubes, one much longer than the other|The mode-cleaner cavity (left, which filters the laser beam) and the west arm

From the air, the Virgo detector has an "L" shape with its two {{convert|3|km|mi|adj=mid|-long}} perpendicular arms. At the intersection of the two arms, the central building is found, containing most of Virgo's key components including the laser, the beamsplitter and the input mirrors. Alongside the west arm, a shorter cavity and the associated building host the input mode-cleaner. The end mirrors are contained in a dedicated building at the end of each arm. South of the west arm, additional buildings contains offices, workshops, as well as the site computing center and the instrument control room.{{Cite book |last=The Virgo Collaboration |url=https://dcc.ligo.org/public/0028/T950145/000/T950145-00.pdf |title=VIRGO – Final Design |date=June 1995 |pages=2100.4–2100.10}}

The arm "tunnels" house pipes in which the laser beams travel in a vacuum. Virgo is one of Europe's largest ultra-high vacuum installation, with a volume of {{convert|6800|m3|U.S.gal}}. The two {{convert|3|km|mi|abbr=on|adj=on}} arms are made of a long steel pipe {{convert|1.2|m|ft|abbr=on}} in diameter, in which the target residual pressure is about one-thousandth of a billionth of an atmosphere (100 times thinner than in the original Virgo). The residual gas molecules, primarily hydrogen and water, have a limited impact on the laser beams' path.{{rp|525}} Large gate valves are at both ends of the arms so work can be done in the mirror-vacuum towers without breaking an arm's ultra-high vacuum. The towers containing the mirrors and attenuators are split into two sections, with different pressures.{{Cite web |last1=Pasqualetti |first1=A. |last2=EGO |title=VIRGO Vacuum System Overview |url=https://workarea.ego-gw.it/ego2/virgo/advanced-virgo/vac/varies/Virgo_Vacuum_system_Overview_r2.pdf |access-date=26 November 2024 |archive-date=22 March 2023 |archive-url=https://web.archive.org/web/20230322212141/https://workarea.ego-gw.it/ego2/virgo/advanced-virgo/vac/varies/Virgo_Vacuum_system_Overview_r2.pdf |url-status=live }} The tubes undergo a process, known as baking, in which they are heated to {{convert|150|C|F|abbr=on}} to remove unwanted particles from their surfaces; although the towers were also baked in the initial Virgo design, cryogenic traps are now used to prevent contamination.{{rp|526}}

Due to the interferometer's high power, its mirrors are susceptible to the effects of heating induced by the laser (despite extremely low absorption). These effects can cause deformation of the surface due to dilation or a change in refractive index of the substrate, resulting in power escaping from the interferometer and perturbations of the signal. These effects are accounted for by a thermal compensation system (TCS) which includes Hartmann wavefront sensors{{Cite journal |last1=Kelly |first1=Thu-Lan |last2=Veitch |first2=Peter J. |last3=Brooks |first3=Aidan F. |last4=Munch |first4=Jesper |date=20 February 2007 |title=Accurate and Precise Optical Testing with a Differential Hartmann Wavefront Sensor |url=https://opg.optica.org/ao/abstract.cfm?uri=ao-46-6-861 |journal=Applied Optics |language=EN |volume=46 |issue=6 |pages=861–866 |bibcode=2007ApOpt..46..861K |doi=10.1364/AO.46.000861 |issn=2155-3165 |pmid=17279130 |hdl-access=free |hdl=2440/43095 |access-date=7 March 2023 |archive-date=7 March 2023 |archive-url=https://web.archive.org/web/20230307221315/https://opg.optica.org/ao/abstract.cfm?uri=ao-46-6-861 |url-status=live }} to measure optical aberration through an auxiliary light source, and two actuators: CO₂ lasers (which heat parts of the mirror to correct the defects) and ring heaters, which adjust the mirror's radius of curvature. The system also corrects "cold defects": permanent defects introduced during mirror manufacture.{{Cite journal |last1=Rocchi |first1=A |last2=Coccia |first2=E |last3=Fafone |first3=V |last4=Malvezzi |first4=V |last5=Minenkov |first5=Y |last6=Sperandio |first6=L |date=1 June 2012 |title=Thermal Effects and their Compensation in Advanced Virgo |journal=Journal of Physics: Conference Series |volume=363 |issue=1 |pages=012016 |bibcode=2012JPhCS.363a2016R |doi=10.1088/1742-6596/363/1/012016 |issn=1742-6596 |s2cid=122763506 |doi-access=free}}{{rp|187–188}} During the O3 run, the TCS increased power inside the interferometer by 15 per cent and decreased power leaving the interferometer by a factor of two.{{Cite journal |last=Nardecchia |first=Ilaria |date=2022 |title=Detecting Gravitational Waves with Advanced Virgo |journal=Galaxies |language=en |volume=10 |issue=1 |pages=28 |doi=10.3390/galaxies10010028 |bibcode=2022Galax..10...28N |issn=2075-4434|doi-access=free }}

File:Newtonian Calibrator Virgo.png

Another important component is the system for controlling stray light (any light leaving the interferometer's designated path, by scattering on a surface or from unwanted reflection). Recombination of stray light with the interferometer's main beam can be a significant noise source, often difficult to track and model. Most efforts to mitigate stray light are based on absorbing plates (known as baffles) placed near the optics and within the tubes; additional precautions are taken to prevent the baffles from affecting interferometer operation.{{Cite journal |last1=Vinet |first1=Jean-Yves |last2=Brisson |first2=Violette |last3=Braccini |first3=Stefano |last4=Ferrante |first4=Isidoro |last5=Pinard |first5=Laurent |last6=Bondu |first6=François |last7=Tournié |first7=Eric |date=15 November 1997 |title=Scattered Light Noise in Gravitational Wave Interferometric Detectors: A Statistical Approach |url=https://link.aps.org/doi/10.1103/PhysRevD.56.6085 |journal=Physical Review D |volume=56 |issue=10 |pages=6085–6095 |bibcode=1997PhRvD..56.6085V |doi=10.1103/PhysRevD.56.6085|url-access=subscription }}{{Cite journal |last1=Vinet |first1=Jean-Yves |last2=Brisson |first2=Violette |last3=Braccini |first3=Stefano |date=15 July 1996 |title=Scattered light noise in Gravitational Wave Interferometric Detectors: Coherent Effects |url=https://link.aps.org/doi/10.1103/PhysRevD.54.1276 |journal=Physical Review D |volume=54 |issue=2 |pages=1276–1286 |bibcode=1996PhRvD..54.1276V |doi=10.1103/PhysRevD.54.1276 |pmid=10020804|url-access=subscription }}

Calibration is required to estimate the detector's response to gravitational waves and correctly reconstruct the signal. It involves moving the mirrors in a controlled way and measuring the result. During the initial Virgo era, this was primarily achieved by agitating a pendulum on which the mirror is suspended with coils to generate a magnetic field interacting with magnets fixed to the pendulum.{{Cite journal |last1=Accadia |first1=T |last2=Acernese |first2=F |last3=Antonucci |first3=F |last4=Astone |first4=P |last5=Ballardin |first5=G |last6=Barone |first6=F |last7=Barsuglia |first7=M |last8=Basti |first8=A |last9=Bauer |first9=Th S |last10=Beker |first10=M G |last11=Belletoile |first11=A |last12=Birindelli |first12=S |last13=Bitossi |first13=M |last14=Bizouard |first14=M A |last15=Blom |first15=M |date=21 January 2011 |title=Calibration and Sensitivity of the Virgo Detector During Its Second Science Run |url=https://iopscience.iop.org/article/10.1088/0264-9381/28/2/025005 |journal=Classical and Quantum Gravity |volume=28 |issue=2 |pages=025005 |arxiv=1009.5190 |bibcode=2011CQGra..28b5005A |doi=10.1088/0264-9381/28/2/025005 |issn=0264-9381 |s2cid=118586058}} This technique was used until O2. For O3, the primary calibration method was photon calibration (PCal); it had been a secondary method to validate the results, using an auxiliary laser to displace the mirror with radiation pressure.{{Cite journal |last1=Estevez |first1=D |last2=Lagabbe |first2=P |last3=Masserot |first3=A |last4=Rolland |first4=L |last5=Seglar-Arroyo |first5=M |last6=Verkindt |first6=D |date=25 February 2021 |title=The Advanced Virgo Photon Calibrators |url=https://doi.org/10.1088/1361-6382/abe2db |journal=Classical and Quantum Gravity |volume=38 |issue=7 |pages=075007 |arxiv=2009.08103 |bibcode=2021CQGra..38g5007E |doi=10.1088/1361-6382/abe2db |issn=0264-9381 |s2cid=221761337}}{{Cite journal |last1=Acernese |first1=F |last2=Agathos |first2=M |last3=Ain |first3=A |last4=Albanesi |first4=S |last5=Allocca |first5=A |last6=Amato |first6=A |last7=Andrade |first7=T |last8=Andres |first8=N |last9=Andrić |first9=T |last10=Ansoldi |first10=S |last11=Antier |first11=S |last12=Arène |first12=M |last13=Arnaud |first13=N |last14=Assiduo |first14=M |last15=Astone |first15=P |date=21 January 2022 |title=Calibration of Advanced Virgo and Reconstruction of the Detector Strain h(t) During the Observing Run O3 |url=https://doi.org/10.1088/1361-6382/ac3c8e |journal=Classical and Quantum Gravity |volume=39 |issue=4 |pages=045006 |arxiv=2107.03294 |bibcode=2022CQGra..39d5006A |doi=10.1088/1361-6382/ac3c8e |issn=0264-9381 |s2cid=238634092 |hdl=11368/3006794}} A method known as Newtonian calibration (NCal) was introduced at the end of O2 to validate the PCal results; it relies on gravity to move the mirror, placing a rotating mass at a specific distance from it.{{Cite journal |last1=Estevez |first1=D |last2=Lieunard |first2=B |last3=Marion |first3=F |last4=Mours |first4=B |last5=Rolland |first5=L |last6=Verkindt |first6=D |date=9 November 2018 |title=First Tests of a Newtonian Calibrator on an Interferometric Gravitational Wave Detector |url=https://doi.org/10.1088/1361-6382/aae95f |journal=Classical and Quantum Gravity |volume=35 |issue=23 |pages=235009 |arxiv=1806.06572 |bibcode=2018CQGra..35w5009E |doi=10.1088/1361-6382/aae95f |issn=0264-9381 |s2cid=119192600}} At the beginning of the second part of O4, Ncal became the main calibration method because it performed better than PCal; PCal is still used to validate NCal results and probe higher frequencies which are inaccessible to the NCal.{{cite journal |last1=Aubin |first1=Florian |last2=Dangelser |first2=Eddy |last3=Estevez |first3=Dimitri |last4=Masserot |first4=Alain |last5=Mours |first5=Benoît |last6=Pradier |first6=Thierry |last7=Syx |first7=Antoine |last8=Van Hove |first8=Pierre |date=6 September 2024 |title=The Virgo Newtonian Calibration System for the O4 Observing Run |journal=Classical and Quantum Gravity |volume=41 |issue=23 |arxiv=2406.10028 |bibcode=2024CQGra..41w5003A |doi=10.1088/1361-6382/ad869c}}

The instrument requires an efficient data-acquisition system which manages data measured at the interferometer's output and from sensors on the site, writing it in files and distributing the files for data analysis. Dedicated electronic hardware and software have been developed for this purpose.{{Cite book |last1=Acernese |first1=F. |last2=Amico |first2=P. |last3=Alshourbagy |first3=M. |last4=Antonucci |first4=F. |last5=Aoudia |first5=S. |last6=Astone |first6=P. |last7=Avino |first7=S. |last8=Babusci |first8=D. |last9=Ballardin |first9=G. |last10=Barone |first10=F. |last11=Barsotti |first11=L. |last12=Barsuglia |first12=M. |last13=Bauer |first13=Th. S. |last14=Beauville |first14=F. |last15=Bigotta |first15=S. |title=2007 15th IEEE-NPSS Real-Time Conference |chapter=Data Acquisition System of the Virgo Gravitational Waves Interferometric Detector |date=April 2007 |chapter-url=https://ieeexplore.ieee.org/document/4382842 |pages=1–8 |doi=10.1109/RTC.2007.4382842 |isbn=978-1-4244-0866-5 |s2cid=140107498 |access-date=29 March 2023 |archive-date=29 March 2023 |archive-url=https://web.archive.org/web/20230329204109/https://ieeexplore.ieee.org/document/4382842/ |url-status=live }}

File:Gravitational_wave_"Koi_fish"_glitch.png

The Virgo detector is sensitive to several noise sources which limit its ability to detect gravitational-wave signals. Some have large frequency ranges and limit the overall sensitivity of the detector, such as:{{Cite thesis |last=Vajente |first=Gabriele |title=Analysis of Sensitivity and Noise Sources for the Virgo Gravitational Wave Interferometer |date=2008 |access-date=26 November 2024 |degree=PhD |publisher=Scuola Normale Superiore |url=https://gwic.ligo.org/assets/docs/theses/Vajente_Thesis.pdf |archive-date=27 March 2024 |archive-url=https://web.archive.org/web/20240327230833/https://gwic.ligo.org/assets/docs/theses/Vajente_Thesis.pdf |url-status=live }}{{Cite web |title=Fighting Noises – Virgo |url=https://www.virgo-gw.eu/science/detector/fighting-noises/ |access-date=21 February 2023 |publisher=The Virgo Collaboration |website=virgo-gw.eu |archive-date=21 February 2023 |archive-url=https://web.archive.org/web/20230221171840/https://www.virgo-gw.eu/science/detector/fighting-noises/ |url-status=live }}

• seismic noise (any ground motion from sources such as waves in the Mediterranean Sea, wind, or human activity), generally in low frequencies up to about 10 Hertz (Hz)
• thermal noise of the mirrors and their suspension wires corresponding to the agitation of the mirror or suspension from its own temperature, from a few tens to a few hundred Hz
• quantum noise, which includes laser shot noise corresponding to fluctuation in power received by the photodiodes and relevant above a few hundred Hz, and radiation pressure noise corresponding to pressure by the laser on the mirror (relevant at low frequency)
• Newtonian noise, caused by tiny fluctuations in the Earth's gravitational field which affect the position of the mirror; relevant below 20 Hz

In addition to these broad noise sources, others may affect specific frequencies. These include a source at 50 Hz (and harmonics at 100, 150, and 200 Hz), corresponding to the frequency of the European power grid; "violin modes" at 300 Hz (and several harmonics), corresponding to the resonant frequency of the suspension fibres (which can vibrate at a specific frequency, as the strings of a violin do); and calibration lines, appearing when mirrors are moved for calibration.{{Cite web |title=O2 Instrumental Lines |url=https://www.gw-openscience.org/o2speclines/ |access-date=24 March 2023 |website=gw-openscience.org |language=en-us |archive-date=24 March 2023 |archive-url=https://web.archive.org/web/20230324223110/https://www.gw-openscience.org/o2speclines/ |url-status=live }}{{Cite web |title=Virgo Logbook – Detector Characterisation (Spectral lines) |url=https://logbook.virgo-gw.eu/virgo/?r=40306 |access-date=24 March 2023 |website=logbook.virgo-gw.eu |archive-date=24 March 2023 |archive-url=https://web.archive.org/web/20230324223111/https://logbook.virgo-gw.eu/virgo/?r=40306 |url-status=live }}

Additional noise sources may have a short-term impact; bad weather or earthquakes may temporarily increase the noise level. Short-lived artefacts may appear in the data due to many possible instrumental issues, and are usually referred to as "glitches". It is estimated that about 20 per cent of detected events are impacted by glitches, requiring specific data-processing methods to mitigate their impact.{{Cite journal |last1=Davis |first1=D |last2=Littenberg |first2=T B |last3=Romero-Shaw |first3=I M |last4=Millhouse |first4=M |last5=McIver |first5=J |last6=Di Renzo |first6=F |last7=Ashton |first7=G |date=15 December 2022 |title=Subtracting Glitches from Gravitational-wave Detector Data during the Third LIGO-Virgo Observing Run |url=https://iopscience.iop.org/article/10.1088/1361-6382/aca238 |journal=Classical and Quantum Gravity |volume=39 |issue=24 |pages=245013 |arxiv=2207.03429 |bibcode=2022CQGra..39x5013D |doi=10.1088/1361-6382/aca238 |issn=0264-9381 |s2cid=250334515}}

File:BestVirgoSensitivityCurveVSR4.pngs and instrumental noises, including the 50-Hz utility frequency and its harmonics.]]

Sensitivity depends on frequency, and is usually represented as a curve corresponding to the noise power spectrum (or amplitude spectrum, the square root of the power spectrum); the lower the curve, the greater the sensitivity. Virgo is a wide-band detector whose sensitivity ranges from a few Hz to 10 kHz; a 2011 Virgo sensitivity curve is plotted with a log-log scale.{{Cite web |title=Sensitivity |url=https://www.virgo-gw.eu/science/detector/sensitivity/ |access-date=21 October 2024 |website=virgo-gw.eu |publisher=The Virgo Collaboration |language=en-GB |archive-date=24 July 2024 |archive-url=https://web.archive.org/web/20240724130646/https://www.virgo-gw.eu/science/detector/sensitivity/ |url-status=live }}

The most common measure of gravitational-wave-detector sensitivity is the range distance, defined as the distance at which a reference target produces a signal-to-noise ratio of 8 in the detector. The reference is usually a binary neutron star with both components having a mass of 1.4 solar masses; the distance is generally expressed in megaparsecs. The range for Virgo during the O3 run was between 40 and 50 Mpc. This range is an indicator, not a maximal range for the detector; signals from more massive sources will have a larger amplitude, and can be detected from further away.{{Cite journal |last1=Chen |first1=Hsin-Yu |last2=Holz |first2=Daniel E. |last3=Miller |first3=John |last4=Evans |first4=Matthew |last5=Vitale |first5=Salvatore |last6=Creighton |first6=Jolien |date=4 March 2021 |title=Distance Measures in Gravitational-wave Astrophysics and Cosmology |journal=Classical and Quantum Gravity |volume=38 |issue=5 |pages=055010 |arxiv=1709.08079 |bibcode=2021CQGra..38e5010C |doi=10.1088/1361-6382/abd594 |issn=0264-9381}}

Calculations indicate that the detector sensitivity roughly scales as $\backslash frac\{1\}\{L\backslash times\backslash sqrt\{P\}\}$, where $L$ is the arm-cavity length and $P$ the laser power on the beam splitter. To improve it, these quantities must be increased. This is achieved with long arms, optical cavities inside the arm to maximise exposure to the signal, and power recycling to increase power in the arms.{{Cite report |url=https://inis.iaea.org/collection/NCLCollectionStore/_Public/30/020/30020475.pdf |title=Détection des Ondes Gravitationnelles – Ecole Joliot Curie |trans-title=Detection of Gravitational Waves – Joliot Curie School |last=Hello |first=Patrice |date=1997 |language=fr |access-date=20 April 2023 |archive-date=27 March 2024 |archive-url=https://web.archive.org/web/20240327230847/https://inis.iaea.org/collection/NCLCollectionStore/_Public/30/020/30020475.pdf |url-status=live }}

{{Main|Ground-based interferometric gravitational-wave search#Data analysis}}

An important part of Virgo collaboration resources is dedicated to the development and deployment of data-analysis software designed to process the detector's output. Apart from the data-acquisition software and tools for distributing the data, the effort is shared with members of the LIGO and KAGRA collaborations as part of the LIGO-Virgo-KAGRA (LVK) collaboration.{{Cite web |title=Our Collaborations |url=https://www.ligo.caltech.edu/page/ligo-scientific-collaboration |access-date=26 February 2023 |publisher=LIGO Lab {{!}} Caltech |website=ligo.caltech.edu |archive-date=26 February 2023 |archive-url=https://web.archive.org/web/20230226214136/https://www.ligo.caltech.edu/page/ligo-scientific-collaboration |url-status=live }}

Data from the detector is initially only available to LVK members. Segments of data surrounding detected events are released at the publication of the related paper, and the full data is released after a proprietary period (currently 18 months). During the third observing run (O3), this resulted in two separate data releases (O3a and O3b) corresponding to the first and last six months of the run.{{Cite web |date=7 October 2022 |title=LIGO-M1000066-v27: LIGO Data Management Plan |url=https://dcc.ligo.org/LIGO-M1000066/public |access-date=26 February 2023 |website=dcc.ligo.org |archive-date=26 February 2023 |archive-url=https://web.archive.org/web/20230226214138/https://dcc.ligo.org/LIGO-M1000066/public |url-status=live }} The data is then generally available on the Gravitational Wave Open Science Center (GWOSC) platform.{{Cite web |title=GWOSC |url=https://www.gw-openscience.org/ |access-date=5 March 2023 |website=gw-openscience.org |archive-date=5 March 2023 |archive-url=https://web.archive.org/web/20230305215153/https://www.gw-openscience.org/ |url-status=live }}{{Cite journal |last1=The LIGO Scientific Collaboration |last2=the Virgo Collaboration |last3=the KAGRA Collaboration |last4=Abbott |first4=R. |last5=Abe |first5=H. |last6=Acernese |first6=F. |last7=Ackley |first7=K. |last8=Adhicary |first8=S. |last9=Adhikari |first9=N. |last10=Adhikari |first10=R. X. |last11=Adkins |first11=V. K. |last12=Adya |first12=V. B. |last13=Affeldt |first13=C. |last14=Agarwal |first14=D. |last15=Agathos |first15=M. |date=7 February 2023 |title=Open Data from the Third Observing Run of LIGO, Virgo, KAGRA, and GEO |journal=The Astrophysical Journal Supplement Series |volume=267 |issue=2 |page=29 |doi=10.3847/1538-4365/acdc9f |arxiv=2302.03676|bibcode=2023ApJS..267...29A |s2cid=256627681 |doi-access=free }}

Analysis of the data requires a variety of techniques targeting different types of sources. Most of the effort is dedicated to the detection and analysis of mergers of compact objects, the only type of source detected until now. Analysis software is running the data in search of this type of event, and a dedicated infrastructure is used to alert the online community.{{Cite journal |last1=The LIGO Scientific Collaboration |last2=the Virgo Collaboration |last3=the KAGRA Collaboration |last4=Abbott |first4=R. |last5=Abbott |first5=T. D. |last6=Acernese |first6=F. |last7=Ackley |first7=K. |last8=Adams |first8=C. |last9=Adhikari |first9=N. |last10=Adhikari |first10=R. X. |last11=Adya |first11=V. B. |last12=Affeldt |first12=C. |last13=Agarwal |first13=D. |last14=Agathos |first14=M. |last15=Agatsuma |first15=K. |date=2023 |title=GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo during the Second Part of the Third Observing Run |journal=Physical Review X |volume=13 |issue=4 |page=041039 |arxiv=2111.03606 |bibcode=2023PhRvX..13d1039A |doi=10.1103/PhysRevX.13.041039}} Other efforts are carried out after the data-acquisition period (offline), including searches for continuous sources,{{Cite journal |last=Riles |first=Keith |year=2023 |title=Searches for Continuous-wave Gravitational Radiation |journal=Living Reviews in Relativity |volume=26 |issue=1 |page=3 |arxiv=2206.06447 |bibcode=2023LRR....26....3R |doi=10.1007/s41114-023-00044-3 |s2cid=249642127}} a stochastic background,{{Cite journal |last=Christensen |first=Nelson |date=1 January 2019 |title=Stochastic Gravitational Wave Backgrounds |url=https://iopscience.iop.org/article/10.1088/1361-6633/aae6b5 |journal=Reports on Progress in Physics |volume=82 |issue=1 |pages=016903 |arxiv=1811.08797 |bibcode=2019RPPh...82a6903C |doi=10.1088/1361-6633/aae6b5 |issn=0034-4885 |pmid=30462612 |s2cid=53712558}} or deeper analysis of detected events.

File:GW170814.png

{{Further|List of gravitational wave observations}}

Virgo first detected a gravitational signal during the second observation run (O2) of the "advanced" era; only the LIGO detectors were operating during the first observation run. The event, named GW170814, was a coalescence between two black holes. It was the first event detected by three different detectors, allowing for greatly-improved localisation compared to events from the first observation run. It also allowed for the first conclusive measure of gravitational-wave polarisation, providing evidence against polarisations other than those predicted by general relativity.{{Cite journal |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=C. |last7=Adams |first7=T. |last8=Addesso |first8=P. |last9=Adhikari |first9=R. X. |last10=Adya |first10=V. B. |last11=Affeldt |first11=C. |last12=Afrough |first12=M. |last13=Agarwal |first13=B. |last14=Agathos |first14=M. |last15=Agatsuma |first15=K. |date=6 October 2017 |title=GW170814: A Three-Detector Observation of Gravitational Waves from a Binary Black Hole Coalescence |url=https://link.aps.org/doi/10.1103/PhysRevLett.119.141101 |journal=Physical Review Letters |language=en |volume=119 |issue=14 |pages=141101 |doi=10.1103/PhysRevLett.119.141101 |pmid=29053306 |arxiv=1709.09660 |bibcode=2017PhRvL.119n1101A |s2cid=46829350 |issn=0031-9007}}

It was soon followed by the better-known GW170817, the first merger of two neutron stars detected by the gravitational-wave network and ({{as of|2025|02|lc=y}}) the only event with a confirmed detection of an electromagnetic counterpart in gamma rays, optical telescopes, radio and x-ray domains. No signal was observed in Virgo, but this absence was crucial to more tightly constrain the event's localisation, as it allows to exclude regions of the sky where the signal would have been visible in Virgo data.{{Cite journal |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=C. |last7=Adams |first7=T. |last8=Addesso |first8=P. |last9=Adhikari |first9=R. X. |last10=Adya |first10=V. B. |last11=Affeldt |first11=C. |last12=Afrough |first12=M. |last13=Agarwal |first13=B. |last14=Agathos |first14=M. |last15=Agatsuma |first15=K. |date=16 October 2017 |title=Multi-messenger Observations of a Binary Neutron Star Merger |journal=The Astrophysical Journal |volume=848 |issue=2 |pages=L12 |arxiv=1710.05833 |bibcode=2017ApJ...848L..12A |doi=10.3847/2041-8213/aa91c9 |issn=2041-8213 |s2cid=217162243 |doi-access=free}} This event, involving over 4,000 astronomers,{{Cite news |date=16 October 2017 |title=Astronomers Catch Gravitational Waves from Colliding Neutron Stars |language=en-US |work=Sky & Telescope |url=https://skyandtelescope.org/astronomy-news/astronomers-catch-gravitational-waves-from-colliding-neutron-stars/ |access-date=20 February 2023 |archive-date=20 February 2023 |archive-url=https://web.archive.org/web/20230220205147/https://skyandtelescope.org/astronomy-news/astronomers-catch-gravitational-waves-from-colliding-neutron-stars/ |url-status=live }} improved the understanding of neutron-star mergers{{Cite journal |last1=Watson |first1=Darach |last2=Hansen |first2=Camilla J. |last3=Selsing |first3=Jonatan |last4=Koch |first4=Andreas |last5=Malesani |first5=Daniele B. |last6=Andersen |first6=Anja C. |last7=Fynbo |first7=Johan P. U. |last8=Arcones |first8=Almudena |last9=Bauswein |first9=Andreas |last10=Covino |first10=Stefano |last11=Grado |first11=Aniello |last12=Heintz |first12=Kasper E. |last13=Hunt |first13=Leslie |last14=Kouveliotou |first14=Chryssa |last15=Leloudas |first15=Giorgos |date=October 2019 |title=Identification of Strontium in the Merger of Two Neutron Stars |url=https://www.nature.com/articles/s41586-019-1676-3 |journal=Nature |language=en |volume=574 |issue=7779 |pages=497–500 |arxiv=1910.10510 |bibcode=2019Natur.574..497W |doi=10.1038/s41586-019-1676-3 |issn=1476-4687 |pmid=31645733 |s2cid=204837882 |access-date=5 March 2023 |archive-date=18 February 2023 |archive-url=https://web.archive.org/web/20230218114617/https://www.nature.com/articles/s41586-019-1676-3 |url-status=live }} and put tight constraints on the speed of gravity.{{Cite journal |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=C. |last7=Adams |first7=T. |last8=Addesso |first8=P. |last9=Adhikari |first9=R. X. |last10=Adya |first10=V. B. |last11=Affeldt |first11=C. |last12=Afrough |first12=M. |last13=Agarwal |first13=B. |last14=Agathos |first14=M. |last15=Agatsuma |first15=K. |date=16 October 2017 |title=Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A |journal=The Astrophysical Journal |volume=848 |issue=2 |pages=L13 |doi=10.3847/2041-8213/aa920c |arxiv=1710.05834 |bibcode=2017ApJ...848L..13A |s2cid=126310483 |issn=2041-8213 |doi-access=free }}

Several searches for continuous gravitational waves have been performed on data from past runs. O3-run searches include an all-sky search,{{Cite journal |last1=LIGO Scientific Collaboration |first1=Virgo Collaboration, and KAGRA Collaboration |last2=Abbott |first2=R. |last3=Abe |first3=H. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adhikari |first6=N. |last7=Adhikari |first7=R. X. |last8=Adkins |first8=V. K. |last9=Adya |first9=V. B. |last10=Affeldt |first10=C. |last11=Agarwal |first11=D. |last12=Agathos |first12=M. |last13=Agatsuma |first13=K. |last14=Aggarwal |first14=N. |last15=Aguiar |first15=O. D. |date=28 November 2022 |title=All-sky Search for Continuous Gravitational Waves from Isolated Neutron Stars Using Advanced LIGO and Advanced Virgo O3 Data |url=https://link.aps.org/doi/10.1103/PhysRevD.106.102008 |journal=Physical Review D |volume=106 |issue=10 |pages=102008 |arxiv=2201.00697 |bibcode=2022PhRvD.106j2008A |doi=10.1103/PhysRevD.106.102008 |s2cid=245650351 |hdl=1854/LU-01GXN8M856WCY1YG62A5ACCPTN}} targeted searches toward Scorpius X-1{{Cite journal |last1=Whelan |first1=John T. |last2=Sundaresan |first2=Santosh |last3=Zhang |first3=Yuanhao |last4=Peiris |first4=Prabath |date=20 May 2015 |title=Model-based Cross-correlation Search for Gravitational Waves from Scorpius X-1 |url=https://link.aps.org/doi/10.1103/PhysRevD.91.102005 |journal=Physical Review D |volume=91 |issue=10 |pages=102005 |arxiv=1504.05890 |bibcode=2015PhRvD..91j2005W |doi=10.1103/PhysRevD.91.102005 |s2cid=59360101}} and several known pulsars (including the Crab and Vela Pulsars),{{Cite journal |last1=Abbott |first1=R. |last2=Abe |first2=H. |last3=Acernese |first3=F. |last4=Ackley |first4=K. |last5=Adhikari |first5=N. |last6=Adhikari |first6=R. X. |last7=Adkins |first7=V. K. |last8=Adya |first8=V. B. |last9=Affeldt |first9=C. |last10=Agarwal |first10=D. |last11=Agathos |first11=M. |last12=Agatsuma |first12=K. |last13=Aggarwal |first13=N. |last14=Aguiar |first14=O. D. |last15=Aiello |first15=L. |date=25 May 2022 |title=Searches for Gravitational Waves from Known Pulsars at Two Harmonics in the Second and Third LIGO-Virgo Observing Runs |journal=The Astrophysical Journal |volume=935 |issue=1 |pages=1 |doi=10.3847/1538-4357/ac6acf |arxiv=2111.13106 |bibcode=2022ApJ...935....1A |s2cid=244709285 |issn=0004-637X |doi-access=free }}{{Cite web |date=21 December 2021 |title=Narrow-band Searches for Continuous and Long-duration Transient Gravitational Waves from Known Pulsars in the Third LIGO-Virgo Observing Run |url=https://www.ligo.org/science/Publication-O3NarrowbandCW/ |access-date=29 March 2023 |website=ligo.org |publisher=LIGO Lab {{!}} Caltech |archive-date=29 March 2023 |archive-url=https://web.archive.org/web/20230329201735/https://www.ligo.org/science/Publication-O3NarrowbandCW/ |url-status=live }} and a directed search towards the supernova remnants Cassiopeia A and Vela Jr.{{Cite journal |last1=LIGO Scientific Collaboration and Virgo Collaboration |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=C. |last7=Adhikari |first7=N. |last8=Adhikari |first8=R. X. |last9=Adya |first9=V. B. |last10=Affeldt |first10=C. |last11=Agarwal |first11=D. |last12=Agathos |first12=M. |last13=Agatsuma |first13=K. |last14=Aggarwal |first14=N. |last15=Aguiar |first15=O. D. |date=28 April 2022 |title=Search of the Early O3 LIGO Data for Continuous Gravitational Waves from the Cassiopeia A and Vela Jr. Supernova Remnants |url=https://link.aps.org/doi/10.1103/PhysRevD.105.082005 |journal=Physical Review D |volume=105 |issue=8 |pages=082005 |arxiv=2111.15116 |bibcode=2022PhRvD.105h2005A |doi=10.1103/PhysRevD.105.082005 |s2cid=244729269}} and the Galactic Center.{{Cite journal |last1=LIGO Scientific Collaboration |first1=Virgo Collaboration, and KAGRA Collaboration |last2=Abbott |first2=R. |last3=Abe |first3=H. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adhikari |first6=N. |last7=Adhikari |first7=R. X. |last8=Adkins |first8=V. K. |last9=Adya |first9=V. B. |last10=Affeldt |first10=C. |last11=Agarwal |first11=D. |last12=Agathos |first12=M. |last13=Agatsuma |first13=K. |last14=Aggarwal |first14=N. |last15=Aguiar |first15=O. D. |date=9 August 2022 |title=Search for Continuous Gravitational Wave Emission from the Milky Way Center in O3 LIGO-Virgo Data |url=https://link.aps.org/doi/10.1103/PhysRevD.106.042003 |journal=Physical Review D |volume=106 |issue=4 |pages=042003 |arxiv=2204.04523 |bibcode=2022PhRvD.106d2003A |doi=10.1103/PhysRevD.106.042003 |s2cid=248085352}} Although none of the searches identified a signal, this enabled upper limits to be set on some parameters; in particular, it was found that the deviation from perfect spinning spheres for close known pulsars is at most {{convert|1|mm|in|abbr=on}}.

Virgo was included in the latest search for a gravitational-wave background with LIGO, combining the results of O3 with the O1 and O2 runs (which only used LIGO data). No stochastic background was observed, improving previous constraints on the energy of the background by an order of magnitude.{{Cite journal |last1=Abbott |first1=R. |last2=Abbott |first2=T. D. |last3=Abraham |first3=S. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=A. |last7=Adams |first7=C. |last8=Adhikari |first8=R. X. |last9=Adya |first9=V. B. |last10=Affeldt |first10=C. |last11=Agarwal |first11=D. |last12=Agathos |first12=M. |last13=Agatsuma |first13=K. |last14=Aggarwal |first14=N. |last15=Aguiar |first15=O. D. |date=23 July 2021 |title=Upper Limits on the Isotropic Gravitational-wave Background from Advanced LIGO and Advanced Virgo's Third Observing Run |url=https://link.aps.org/doi/10.1103/PhysRevD.104.022004 |journal=Physical Review D |language=en |volume=104 |issue=2 |pages=022004 |arxiv=2101.12130 |bibcode=2021PhRvD.104b2004A |doi=10.1103/PhysRevD.104.022004 |issn=2470-0010 |s2cid=231719405}}

Broad estimates of the Hubble constant have also been obtained; the current best estimate is 68{{Su|p=+12|b=-8}} km s⁻¹ Mpc⁻¹, combining results from binary black holes and the GW170817 event. This result is consistent with other estimates of the constant, but not precise enough to solve the current debates about its exact value.

The Virgo Collaboration participates in several activities promoting communication and education about gravitational waves for the general public.{{Cite news |title=Outreach – Virgo |url=https://www.virgo-gw.eu/about/impact-on-society/outreach/ |access-date=8 May 2023 |newspaper=Virgo |archive-date=8 May 2023 |archive-url=https://web.archive.org/web/20230508153044/https://www.virgo-gw.eu/about/impact-on-society/outreach/ |url-status=live }} One example of an activity is guided tours of the Virgo facilities for schools, universities, and the public;{{Cite web |title=Guided Tour |url=https://www.ego-gw.it/visit-us/guided-tour/ |access-date=26 February 2023 |publisher=European Gravitational Observatory |website=ego-gw.it |language=en-GB |archive-date=26 February 2023 |archive-url=https://web.archive.org/web/20230226214141/https://www.ego-gw.it/visit-us/guided-tour/ |url-status=live }} however, many of outreach activities take place outside the Virgo site. This includes public lectures and courses about Virgo activities and participation in science festivals,{{Cite web |date=18 April 2024 |title=Le Mappe del Cosmo. Storie che Hanno Cambiato l'Universo |url=https://www.auditorium.com/it/event/le-mappe-del-cosmo-storie-che-hanno-cambiato-luniverso/ |access-date=6 June 2024 |website=Auditorium Parco della Musica |language=it-IT |archive-date=6 June 2024 |archive-url=https://web.archive.org/web/20240606082118/https://www.auditorium.com/it/event/le-mappe-del-cosmo-storie-che-hanno-cambiato-luniverso/ |url-status=live }}{{Cite web |date=April 2024 |title=The Sounds of the Cosmos |url=https://www.athens-science-festival.gr/en/exhibitions/the-sounds-of-the-cosmos/ |access-date=6 June 2024 |website=Athens Science Festival |language=en-US |archive-date=15 June 2024 |archive-url=https://web.archive.org/web/20240615023850/https://www.athens-science-festival.gr/en/exhibitions/the-sounds-of-the-cosmos/ |url-status=live }}{{Cite web |last=Rossi |first=Giada |date=23 November 2022 |title=Black Hole: a New Interactive Installation by EGO and INFN at Città della Scienza in Naples |url=https://www.ego-gw.it/blog/2022/11/23/black-hole-a-new-interactive-installation-by-ego-and-infn-at-citta-della-scienza-in-naples/ |access-date=8 May 2023 |publisher=European Gravitational Observatory |website=ego-gw.it |language=en-GB |archive-date=8 May 2023 |archive-url=https://web.archive.org/web/20230508153046/https://www.ego-gw.it/blog/2022/11/23/black-hole-a-new-interactive-installation-by-ego-and-infn-at-citta-della-scienza-in-naples/ |url-status=live }} and developing methods and devices for the public understanding of gravitational waves and related topics. The Collaboration is involved in several artistic projects, ranging from visual projects such as "The Rhythm of Space" at the Museo della Grafica in Pisa{{Cite web |title=Home page |url=https://sites.ego-gw.eu/ilritmodellospazio/ |access-date=26 February 2023 |website=Il Ritmo Dello Spazio (The Rhythm of Space) |language=en-US |archive-date=26 February 2023 |archive-url=https://web.archive.org/web/20230226214141/https://sites.ego-gw.eu/ilritmodellospazio/ |url-status=live }} and "On Air" at the Palais de Tokyo{{Cite web |date=13 October 2018 |title=On Air |url=https://studiotomassaraceno.org/on-air/ |access-date=26 February 2023 |website=Studio Tomás Sarceno |language=en-US |archive-date=26 February 2023 |archive-url=https://web.archive.org/web/20230226214141/https://studiotomassaraceno.org/on-air/ |url-status=live }} to concerts.{{Cite web |last=Rossi |first=Giada |date=23 December 2023 |title='Cosmic' Concert at Teatro Verdi in Pisa to Celebrate 20 Years of Virgo |url=https://www.ego-gw.it/blog/2023/12/23/cosmic-concert-at-teatro-verdi-in-pisa-to-celebrate-20-years-of-virgo/ |access-date=6 June 2024 |publisher = European Gravitational Observatory |website=ego-gw.it |language=en-GB |archive-date=6 June 2024 |archive-url=https://web.archive.org/web/20240606082118/https://www.ego-gw.it/blog/2023/12/23/cosmic-concert-at-teatro-verdi-in-pisa-to-celebrate-20-years-of-virgo/ |url-status=live }} It includes activities promoting gender equality in science, highlighting women working in Virgo in communications to the general public.{{Cite web |date=11 February 2023 |title=International Day of Women and Girls in Science 2023 – Virgo |url=https://www.virgo-gw.eu/news/international-day-of-women-and-girls-in-science-2023/ |access-date=26 February 2023 |website=virgo-gw.eu |publisher=The Virgo Collaboration |archive-date=26 February 2023 |archive-url=https://web.archive.org/web/20230226214138/https://www.virgo-gw.eu/news/international-day-of-women-and-girls-in-science-2023/ |url-status=live }}

{{Reflist|30em}}

• [http://www.ego-gw.it/virgodescription/pag_4.html Description on EGO's website] {{Webarchive|url=https://web.archive.org/web/20060713230620/http://www.ego-gw.it/virgodescription/pag_4.html |date=13 July 2006 }}
• [https://web.archive.org/web/20071014004427/http://www.virgo.infn.it/ Virgo's homepage]
• [https://archive.today/20130413031159/https://tds.ego-gw.it/ql/?c=8940 Advanced Virgo Technical Design Report]

{{Gravitational waves}}

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