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The Virgo experiment
Formation1993
TypeInternational scientific collaboration
PurposeGravitational wave detection
HeadquartersEuropean Gravitational Observatory
Location
Coordinates43°37′53″N 10°30′16″E / 43.6313°N 10.5045°E / 43.6313; 10.5045
Region
Italy
FieldsBasic research
Spokesperson
Gianluca Gemme
AffiliationsLVK (LIGO-Virgo-KAGRA collaboration)
Budget
About ten million euros per year
Staff
Around 850 people participate in the Virgo Collaboration
Websitewww.virgo-gw.eu

The Virgo interferometer is a large Michelson interferometer designed to detect the gravitational waves predicted by general relativity. It is in Santo Stefano a Macerata, near the city of Pisa, Italy. The instrument's two arms are three kilometres long and contain its mirrors and instrumentation inside an ultra-high vacuum.

Virgo is hosted by the European Gravitational Observatory (EGO), a consortium founded by the French Centre National de la Recherche Scientifique (CNRS) and Italian Istituto Nazionale di Fisica Nucleare (INFN).[1] The Virgo Collaboration operates the detector and defines the strategy and policy for its use and upgrades. It is composed of several hundreds of members across 16 different countries.[2] These operations are carried out jointly with other similar detectors, including the two LIGO interferometers in the United States (at the Hanford Site and in Livingston, Louisiana) and the Japanese interferometer KAGRA (in the Kamioka mine). Cooperation between several detectors is crucial for detecting gravitational waves and pinpointing their origin, which is why the LIGO and Virgo collaborations have been sharing their data since 2007, with KAGRA joining in 2019 to form the LIGO-Virgo-KAGRA (LVK) collaboration.[3]

The interferometer is named after the Virgo Cluster, a cluster of about 1,500 galaxies in the Virgo constellation, about 50 million light-years from Earth. Founded at a time when gravitational waves were only a prediction by general relativity, it has now participated in detecting multiple gravitational wave events, making its first detection in 2017 (jointly with the two LIGO detectors), quickly followed by the GW170817 event, the only one to have also been observed with classical methods (optical, gamma-ray, X-ray and radio telescopes) as of 2024.[4] The detector currently participates in joint observing runs with the other detectors, separated by commissioning periods during which the detector is upgraded to increase its sensitivity and scientific output.[5]

Organization

The Virgo experiment is managed by the European Gravitational Observatory (EGO) consortium, created in December 2000 by the CNRS and INFN.[6] The Dutch Institute for Nuclear and High-Energy Physics, Nikhef, later joined as an observer and eventually became a full member. EGO is responsible for the Virgo site, in charge of the construction, maintenance, and operation of the detector, as well as its upgrades. One of the goals of EGO is also to promote research on and studies of gravitation in Europe.[1]

The Virgo Collaboration consolidates all the researchers working on various aspects of the detector. As of May 2023, around 850 members, representing 142 institutions in 16 different countries, are part of the collaboration.[2][7] This includes institutions from France, Italy, the Netherlands, Poland, Spain, Belgium, Germany, Hungary, Portugal, Greece, Czechia, Denmark, Ireland, Monaco, China, and Japan.[8]

The Virgo Collaboration is part of the larger LIGO-Virgo-KAGRA (LVK) Collaboration, which gathers scientists from the other major gravitational waves experiment, for the purpose of carrying out joint analysis of the data which is crucial for gravitational wave detections.[9] LVK first started in 2007[3] as the LIGO-Virgo Collaboration, and was expanded when KAGRA joined in 2019.[10][11]

History

The Virgo project was approved in 1992 by the French CNRS and in 1993 by the Italian INFN, the two institutes at the origin of the experiment. The construction of the detector started in 1996 at the Cascina site near Pisa, Italy, and was completed in 2003. After several observation runs without detection, the interferometer was shut down in 2011 to allow for significant upgrades as part of the Advanced Virgo project. It started making observations again in 2017, quickly making its first detections along with the LIGO detectors.

Conception

Although the concept of gravitational waves is more than 100 years old, having been predicted by Einstein in 1916,[12] it was not before the 1970s that serious projects for detecting them started to appear. The first were the Weber bars, invented by Joseph Weber;[13] while they could in principle detect gravitational waves, none of the experiments succeeded. They did however spark the creation of many research groups dedicated to gravitational waves.[14]

The idea of a large interferometric detector began to gain credibility in the early 1980s, and in 1985, the Virgo project was conceptualized by the Italian researcher Adalberto Giazotto and the French researcher Alain Brillet after they met in Rome. One of the key ideas that set Virgo apart from other projects was targeting low frequencies (around 10 Hz), whereas most projects focused on higher frequencies (around 500 Hz); many believed at the time that this was not doable, and only France and Italy started working on the project,[15] which was first presented in 1987.[16] After being approved by the CNRS and the INFN, the construction of the interferometer began in 1996, with the aim of beginning observations by 2000.[17]

The first goal of Virgo was to directly observe gravitational waves. The study of the binary pulsar 1913+16 over three decades, whose discoverers were awarded the 1993 Nobel Prize in Physics, had already led to indirect evidence of their existence. 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.[18]

Initial Virgo detector

In the 2000s, the Virgo detector was first built, commissioned, and operated. The instrument successfully reached its planned design sensitivity. This initial endeavor was used to validate the Virgo technical design choices; it also demonstrated that giant interferometers were promising devices for detecting gravitational waves in a wide frequency band.[19][20] This phase is generally named the "initial Virgo" or "original Virgo".

The construction of the initial Virgo detector was completed in June 2003,[21] and several data collection periods ("science runs") followed between 2007 and 2011.[22][23] Some of these runs were done simultaneously with the two LIGO detectors. There was a shut-down of a few months in 2010 to allow for a major upgrade of the Virgo suspension system: the original steel suspension wires were replaced by glass fibers to reduce the thermal noise.[24]

However, the initial Virgo detector was not sensitive enough to detect gravitational waves. After several months of data collection with the upgraded suspension system, the initial Virgo detector was shut down in September 2011 to begin the installation of Advanced Virgo.[25]

Advanced Virgo detector

First direct detection of a gravitational wave by Virgo, on 14 August 2017 (GW170814)

The Advanced Virgo detector aimed to increase the sensitivity (and thus the distance at which a signal can be detected) by a factor of 10, allowing it to probe a volume of the Universe 1,000 times larger, making detection of gravitational waves more likely.[15][26] 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 remainder of the interferometer was significantly 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 (350 mm in diameter, with a weight of 40 kg), and their optical performance was improved. The critical optical elements used to control the interferometer are under vacuum on suspended benches. A system of adaptive optics was to be 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.[27]

Advanced Virgo started the commissioning process in 2016, joining the two advanced LIGO detectors ("aLIGO") on 1 August 2017, during the "O2" observation period. On 14 August 2017, LIGO and Virgo detected a signal, GW170814, which was reported on 27 September 2017. It was the first binary black hole merger detected by both LIGO and Virgo (and the first one for Virgo).[28][29]

Just a few days later, GW170817 was detected by LIGO and Virgo on 17 August 2017. The signal was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and represents both the first binary neutron star merger observed and the first gravitational wave observation which was confirmed by non-gravitational means. Indeed, the resulting gamma-ray burst was also detected, and optical telescopes later discovered a kilonova corresponding to the merger.[4][30]

After further upgrades, Virgo started the third observation run ("O3") in April 2019, planned to last one year.[31] However, the run ended earlier that expected on 27 March 2020, due to the COVID-19 pandemic.[32]

The upgrades following O3 are part of the "Advanced Virgo +" program, divided in two phases, the first one preceding the O4 run and the second one preceding the O5 run. The first phase focuses on the reduction of quantum noise by introducing a more powerful laser, improving the squeezing introduced in O3, and implementing a new technique called signal recycling; seismic sensors are also installed around the mirrors. The second phase will then try 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 by improving the coating of the mirrors; the end mirrors will also be significantly larger, requiring improvements to the suspension. Further improvements for quantum noise reduction are also expected in the second phase, building upon the changes from the first.[33]

The fourth observation run ("O4") was scheduled to start in May 2023, and was planned to last for a total of 20 months, including a commissioning break of up to two months.[5] However, on 11 May 2023, Virgo announced that it would not join at the beginning of O4, as the interferometer was not stable enough to reach the expected sensitivity and needed to undergo the replacement of one of the mirrors, requiring several weeks of work.[34] Virgo has not joined the O4 run during the first part of the run ("O4a"), which ended on 16 January 2024, as it only managed to reach 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 get more preparation for the O5 upgrades.[5]

Future

Following the O4 run, the detector will once again be shut down to undergo upgrades, including an improvement in the coating of the mirrors. A fifth observing run (O5) is currently planned for beginning around June 2027; the target sensitivity for Virgo, which was originally set to be 150–260 Mpc, is currently being redefined in light of the performance during O4; plans to enter the O5 run are expected to be known before the end of 2024.[5]

No official plans have been announced for the future of the Virgo installations following the O5 period, although projects for further improving the detectors have been suggested; the current plans of the collaboration are referred to as the Virgo_nEXT project.[35]

Science case

Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes.

Virgo is designed to look for gravitational waves emitted by astrophysical sources across the universe, which can be broadly classified into three types:[36]

Typical "chirp" of a gravitational wave signal, from the GW170817 event. The x-axis represents the time, and y-axis is the frequency; the way the frequency rises with the time is typical of gravitational waves from compact object binaries, and its exact shape is mainly determined by the masses of the objects.
  • Transient sources, representing objects only detectable for a short period. The main sources in this category are the compact binary coalescenses (CBC), corresponding to binary black holes (or neutron stars) merging together, emitting a rapidly growing signal as they get closer to each other, which only becomes detectable in the last seconds before the merger. Other possible sources of short-lived gravitational waves are supernovas, instabilities in compact systems, or more exotic sources such as cosmic strings.
  • Continuous sources, emitting a signal observable on long timescales. The prime candidates are rapidly-spinning neutron stars (pulsars), which may emit gravitational waves if they are not perfectly spheric (e.g. if there are tiny "mountains" on the surface).
  • Stochastic backgrounds, a specific type of (generally) continuous signal where the signal is diffused across large regions of the sky rather than a single source. It could be constituted of a large number of indistiguishable sources from the above categories, or originate from the early instants of the universe.

The detection of these sources gives a new way to observe them (often carrying different information than more classical ways, e.g. using telescopes), and allows to probe fundamental properties of gravity, such as the polarization of gravitational waves,[37] possible gravitational lensing,[38] or more generally whether the observed signals are correctly described by general relativity.[39] It also provides a way to measure the Hubble constant.[40]

Instrument

Principle

Aerial view of the site of the Virgo experiment showing the central building, the Mode-Cleaner building, the full 3 km-long west arm and the beginning of the north arm (on the right). The other buildings include offices, workshops, the local computing center and the interferometer control room. When this picture was shot, the building hosting the project management and the canteen had not been built yet.

In general relativity, a gravitational wave is a space-time perturbation which propagates at the speed of light. It slightly curves space-time, which locally changes the light path. Concretely, it can be detected using a Michelson interferometer design, where a laser is divided in two beams travelling in orthogonal directions, bouncing on a mirror located at the end of each arm. As the gravitational wave passes, it alters the path of the two beams in a different manner; they are then recombined, and the resulting interferometric pattern is measured using a photodiode. As the induced deformation is extremely small, the design requires an excellent precision in the position of the mirrors, the stability of the laser, the measurements, and a very good isolation from the outside world to reduce the amount of noise.[41]

Animation displaying the principle of gravitational wave detection with an interferometer such as Virgo. Mirror displacements and phase difference are widely exaggerated ; time is also slowed down by more than a factor 10.

Laser and injection system

Layout of the Virgo interferometer during the O4 run (2023-2024). It includes the signal recycling mirror and the filter cavity, not present during the previous run. All laser power estimates are indicative as they can fluctuate rapidly.

The laser is the light source of the experiment. It must be powerful, while extremely stable in frequency and amplitude.[42] To meet all these (somewhat opposing) specifications, the beam starts from a very low power, yet very stable, laser.[43] The light from this laser passes through several amplifiers which enhance its power by a factor of 100. A 50 W output power was achieved for the last configuration of the initial Virgo detector, and later reached 100 W during the O3 run, following the Advanced Virgo upgrades; it is expected to be upgraded to 130 W at the beginning of the O4 run.[33] The original Virgo detector used a master-slave laser system, where a "master" laser is used to stabilize a high-powered "slave" laser; the master laser was a Nd:YAG laser, and the slave laser a Nd:YVO4 laser.[21] The retained solution for Advanced Virgo is to have a fiber laser with an amplification stage made of fibers as well, to improve the robustness of the system; in its final configuration, it is planned to coherently combine the light of two lasers in order to achieve the required power.[27][44] The wavelength of the laser is 1064 nanometres, in both the original and Advanced Virgo configurations.[33]

This laser is sent into the interferometer after passing through the injection system, which further ensures the stability of the beam, adjusts its shape and power, and positions it correctly for entering the interferometer. Key components of the injection system include the input mode cleaner (a 140-metre-long cavity made for improving the beam quality, by stabilizing the frequency, removing light propagating in an unwanted way and reducing the effect of misalignment of the laser), a Faraday isolator preventing any light from returning to the laser, and a mode-matching telescope, which adapts the size and position of the beam right before it enters the interferometer.[27]

Mirrors

The large mirrors located in each arm are the most critical optics of the interferometer. They include the two end mirrors, located at the ends of the 3-km interferometer arms, and the two input mirrors, located near the beginning of the arms. Together, these mirrors make a resonant optical cavity in each arm, where the light bounces thousands of times before returning to the beam splitter, maximizing the effect of the signal on the laser path.[45] It also allows to increase the power of the light circulating in the arms. These mirrors have been specifically designed for Virgo and are made from state-of-the-art technologies. They are cylinders 35 cm in diameter and 20 cm thick,[27] made from the purest glass in the world.[46] The mirrors are polished to the atomic level to avoid diffusing (and hence losing) any light.[47] Finally, a reflective coating (a Bragg reflector made with ion beam sputtering) is added. The mirrors located at the end of the arms reflect almost all incoming light; less than 0.002% of the light is lost at each reflection.[48]

One of the mirrors from the initial Virgo detector, now used as an exposition model at the Virgo site.

In addition, two other mirrors are present in the final design:

  • The power recycling mirror, placed between the laser and the beam splitter. As most light is reflected toward the laser after returning to the beam splitter, this mirror re-injects this light back into the main interferometer, increasing the power in the arms.
  • The signal recycling mirror, located at the output of the interferometer, re-injects part of the signal within the interferometer (currently, the transmission of this mirror is planned to be 40%), effectively forming another cavity. By making small adjustments to this signal recycling mirror, quantum noise can be reduced in part of the frequency band, while increasing it elsewhere, making it possible to tune the interferometer for certain frequencies. It is currently planned to use the "wideband" configuration, decreasing the noise at high and low frequencies but increasing it at intermediate frequencies. The decreased noise at high frequencies is of particular interest to study the signal from moments right before and after a compact object merger.[33][14]

Superattenuators

Any Virgo mirror is supported, under vacuum, by a mechanical structure enormously damping seismic vibrations. A "superattenuator" consists of a chain of pendula, hanging from an upper platform, supported by three long flexible legs clamped to ground, forming an inverted pendulum. In this way seismic vibrations above 10 Hz are reduced by more than 1012 times[49] and the position of the mirror is very carefully controlled.

To mitigate the seismic noise which could propagate up to the mirrors, shaking them and hence obscuring potential gravitational wave signals, the large mirrors are suspended by a complex system. All of the main mirrors are suspended by four thin fibers made of silica[50] which are attached to a series of attenuators. This chain of suspension, called the "superattenuator", is close to 8 meters high and is also under vacuum.[51] The superattenuators do not only limit the disturbances on the mirrors, they also allow the mirror position and orientation to be precisely steered. The optical table where the injection optics used to shape the laser beam are located, such as the benches used for the light detection, are also suspended and under vacuum, to limit the seismic and acoustic noises. In the Advanced Virgo configuration, the whole instrumentation used to detect gravitational waves signals and to steer the interferometer (photodiodes, cameras, and the associated electronics) is also installed on several suspended benches, and under vacuum.[27]

The design of the superattenuators is mainly based on the passive attenuation of the seismic noise, which is achieved by chaining several pendula, each acting as an harmonic oscillator. They are characterized by a resonance frequency (which diminishes with the length of the pendulum) above which the noise will be dampened; chaining several pendula allows to reduce the noise by twelve orders of magnitude, at the cost of introducing multiple, collective resonance frequencies, which are at a higher frequency than a single long pendulum.[52] In the current design, the highest resonance frequency is around 2 Hz, providing a meaningful noise reduction starting at 4 Hz,[27] and reaching the level needed for detecting gravitational waves around 10 Hz. A limit of the system is that the noise in the resonance frequency band (below 2 Hz) is not filtered and can generate large oscillations; this is mitigated by an active damping system, including sensors measuring the seismic noise and actuators controlling the superattenuator to counteract the noise.[52]

Detection system

Part of the light circulating in the arm cavities is sent towards the detection system by the beam splitter. In its optimal configuration, the interferometer works close to the "dark fringe", meaning that very little light is sent towards the output (most of it 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 allows to filter the so-called "high-order modes" (light propagating in an unwanted way, typically introduced by small defects in the mirrors, and susceptible to degrade the measurement[53]), before reaching the photodiodes, which measure the light intensity. Both the output mode cleaner and the photodiodes are suspended and under vacuum.[26]

Detection bench of the Virgo interferometer before being installed in April 2015. It is 88 cm wide and hosts the output mode cleaner; the photodiode measuring the signal is placed on another bench.

Starting 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 by 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 the case of Virgo, the two quantities are the amplitude and the phase of the light. The idea of using squeezed vacuum was first proposed in 1981 by Carlton Caves, during the infancy of gravitational wave detectors.[54] During the O3 run, frequency-independent squeezing was implemented, meaning that the squeezing is identical at all frequencies; it was used to reduce the shot noise (dominant at high frequencies) and increase the radiation pressure noise (dominant at low frequencies), as the latter was not limiting the instrument's sensitivity.[55] Due to the addition of the squeezed vacuum injection, the quantum noise was reduced by 3.2 dB at high frequencies, resulting in an increase of the range of the detector by 5–8%.[56] Currently, more sophisticated squeezed states are produced[57] by combining the technology from O3 with a new 285 m long cavity, known as the filter cavity. This technology is known as frequency-dependent squeezing, and helps reduce the shot noise at high frequencies (where radiation pressure noise is not relevant), and reduce the radiation pressure noise at low frequencies (where shot noise is low).[58][59]

Infrastructure

Seen from the air, the Virgo detector has a characteristic "L" shape with its two 3-km-long perpendicular arms. The arm "tunnels" house vacuum pipes in which the laser beams are travelling.

Virgo is the largest ultra-high vacuum installation in Europe, with a total volume of 6,800 cubic meters.[60] The two 3-km arms are made of a long steel pipe 1.2m in diameter in which the target residual pressure is about 1 thousandth of a billionth of an atmosphere (improving by a factor of 100 from the original Virgo level). Thus, the residual gas molecules (mainly hydrogen and water) have a limited impact on the path of the laser beams.[27] Large gate valves are located at both ends of the arms so that 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 themselves split in two sections with different pressures.[61] The tubes undergo a process called baking, where they are heated at 150°C to remove unwanted particles stuck on the surfaces; while the towers were also baked-out in the initial Virgo design, cryogenic traps are now used to prevent contamination.[27]

Due to the high power in the interferometer, the mirrors are susceptible to thermal effects due to the heating induced by the laser (despite having an extremely low absorption). These thermal effects can take the shape of a deformation of the surface due to dilation, or a change in the refractive index of the substrate; this results in power escaping from the interferometer and in perturbations of the signal. These two effects are accounted for by the thermal compensation system (TCS), which includes sensors called Hartmann wavefront sensors[62] (HWS), used to measure the optical aberration through an auxiliary light source, and two actuators: CO2 lasers, which selectively heat parts of the mirror to correct the defects, and ring heaters, which precisely adjust the radius of curvature of the mirror. The system also corrects the "cold defects", which are permanent defects introduced during the mirror manufacturing.[63][27] During the O3 run, the TCS was able to increase the power circulating inside the interferometer by 15%, and decrease the power leaving the interferometer by a factor of 2.[64]

One of the Newtonian calibrators ("NCal") before it was installed at the detector. Several of them are installed near one of the end mirrors ; the movement of the rotor generates a varying gravitational force on the mirror, allowing to move it in a controlled manner.

Another important component is the system for controlling stray light, which refers to any light leaving the designated path of the interferometer, either by scattering on a surface or from unwanted reflection. The recombination of this stray light with the main beam of the interferometer can be a significant source of noise, and is often hard to track and to model. Most of the efforts to mitigate stray light are based on absorbing plates called "baffles", placed near the optics as well as within the tubes; additional precautions are needed to prevent the baffles from having an effect on the interferometer operation.[65][66][60]

To estimate properly the response of the detector to gravitational waves and thus correctly reconstruct the signal, a calibration step is required, which involves moving the mirrors in a controlled way and measuring the result. During the initial Virgo era, this was primarily achieved by agitating one of the pendulum to which the mirror is suspended using coils to generate a magnetic field interacting with magnets fixed to the pendulum.[67] This technique was employed until O2. For O3, the main calibration method became the photon calibration ("PCal") which had until then been used as a secondary method to validate the results; it uses an auxiliary laser to displace the mirror via radiation pressure.[68][69] In addition, a new method called Newtonian calibration ("NCal") has been introduced at the end of O2 and is now used to validate the PCal; it relies on gravity to move the mirror, by placing a rotating mass at a specific distance of the mirror.[70][69]

Finally, the instrument requires an efficient data acquisition system. This system is in charge of managing the data measured at the output of the interferometer and from the many sensors present on the site, writing it in files, and distributing the files for data analysis. To this end, dedicated hardware and software have been developed to accommodate the specific needs of Virgo.[71]

Noise and sensitivity

Noise sources

Visualization of a gravitational wave "koi fish" glitch, from LIGO Hanford data taken in 2015. The top part represents the output of the detector ("strain") as a function of time, while the bottom part displays the frequency distribution of the power as a function of time. This type of glitch is of unknown origin, and covers a broad frequency range, with characteristic "fins" at lower frequencies in the time-frequency plot.[72]

Due to the precision required in the measurement, the Virgo detector is sensitive to several sources of noise which limit its ability to detect gravitational wave signals. Some of these sources correspond to large frequency ranges and limit the overall sensitivity of the detector,[73][60] such as:

  • seismic noise (any ground motion from numerous sources, such as waves in the Mediterranean Sea, wind, or human activity like traffic), generally in the low frequencies up to about 10 Hertz (Hz)
  • thermal noise of the mirrors and their suspension wires, corresponding to the agitation of the mirror/suspension from its own temperature, from a few tens to a few hundreds of Hz
  • quantum noise, which includes the laser shot noise, corresponding to the fluctuation of the power received by the photodiodes and relevant above a few hundreds of Hz, and the radiation pressure noise, corresponding to the pressure applied by the laser on the mirror, which is relevant at low frequency
  • Newtonian noise, caused by the variation of the gravity field which affects the position of the mirror, relevant below 20 Hz

In addition to these broad noise sources, some other ones may affect specific frequencies. These notably include a source at 50 Hz (as well as harmonics at 100, 150, and 200 Hz), corresponding to the frequency of the European power grid; so-called "violin modes" at 300 Hz (and several harmonics), corresponding to the resonance frequency of the suspension fibers (which can vibrate at a specific frequency just as the strings of a violin do); and calibration lines, appearing when mirrors are moved for calibration.[74][75]

Additional noise sources may also have a short-term impact—bad weather or earthquakes may temporarily increase the noise level.[60]

Finally, several short-lived artifacts may appear in the data due to many possible instrumental issues; these are usually referred to as "glitches". It is estimated that about 20% of the detected events are impacted by glitches, requiring specific data processing methods to mitigate their impact.[76]

Detector sensitivity

A sensitivity curve from the Virgo detector in the frequency band [10 Hz; 10 kHz], computed in August 2011.[77] Its shape is typical: the thermal noise of the mirror suspension pendulum dominates at low frequency while the increase at high frequency is due to the laser shot noise. In between, one can see resonances (for instance, the suspension wire violin modes) and contributions from various instrumental noises (among which the 50 Hz frequency from the power grid and its harmonics) .

A detector like Virgo is characterized by its sensitivity, which provides information about the tiniest signal the instrument could detect. As the sensitivity depends on the frequency, it is usually represented as a curve corresponding to the noise power spectrum (or often amplitude spectrum, which is the square root of the power spectrum); the lower the curve, the better the sensitivity. Virgo is a wide band detector whose sensitivity ranges from a few Hz up to 10 kHz; the image attached shows an example of a Virgo sensitivity curve from 2011, plotted using a log-log scale.

The most common measure for the sensitivity of a gravitational wave detector is the "horizon distance", defined as the distance at which a binary neutron star with masses 1.4 M–1.4 M (where M is the solar mass) produces a signal-to-noise ratio of 8 in the detector. It is generally expressed in megaparsecs.[78] For instance, the range for Virgo during the O3 run was between 40 and 50 Mpc.[5] This range is only an indicator and does not represent a maximal range for the detector; signals from more massive sources will have a larger amplitude, and can thus be detected from further away.

Calculations show that the detector sensitivity roughly scales as , where is the arm cavity length and the laser power on the beam splitter. To improve it, these two quantities must be increased. This is achieved by having long arms, using optical cavities inside the arm to maximize the exposition to the signal, and implementing power recycling to increase the power in the arms.[73][79]

Data analysis

An important part of the Virgo collaboration resources is dedicated to the development and deployment of data analysis software designed to process the output of the detector. Apart from the data acquisition software and the tools for distributing the data, this effort is mostly shared with members of the LIGO and KAGRA collaborations, as part of the LIGO-Virgo-KAGRA (LVK) collaboration.[80]

The data from the detector is initially only available to LVK members; segments of data around detected events are released at the time of publication of the related paper, and the full data is released after a proprietary period, currently lasting 18 months. During the third observing run (O3), this resulted in two separated data releases (O3a and O3b), corresponding to the first six months and last six months of the run respectively.[81] The data is then available for anyone on the Gravitational Wave Open Science Center (GWOSC) platform.[82][83]

The analysis of the data requires a variety of different techniques, targetting the different type of sources. The major part of the effort is dedicated to the detection and analysis of mergers of compact objects, the only type of source detected up until now. Several different analysis software are running on the data searching for this type of event, and a dedicated infrastructure is used to emit alerts to the online community. Other efforts are carried out after the data taking period ("offline"), including searches for continuous sources or for a stochastic background, as well as deeper analysis of the detected events.

Scientific results

Map of the entire sky using the Mollweide projection, showing two areas corresponding to the localization of an event using only the 2 LIGO detectors, and using both LIGO and Virgo. The area with the 3 detectors is smaller by a factor 20.
Sky localization of the GW170814 event, both with the two LIGO detectors and with the full network. The addition of Virgo allows for a much more precise localization.

The first detection of a gravitational signal by Virgo took place at during the second observing run (O2) of the "Advanced" era, as only the LIGO detectors were operating during the first observing run. The event, named GW170814, was a coalescence between two black holes, and also the first event to be detected by three different detectors, allowing for its localization to be greatly improved compared to the events from the first observing run. It also allowed for the first conclusive measure of gravitational wave polarizations, providing evidence against the existence of polarizations other than the ones predicted by general relativity.[28]

It was soon followed by the more famous GW170817, first merger of two neutron stars detected by the gravitational wave network, and as of January 2023 the only event with a confirmed detection of an electromagnetic counterpart, both in gamma rays and in optical telescopes, and later in the radio and x-ray domains. While no signal was observed in Virgo, this absence was crucial to put tighter constraints on the localization of the event.[4] This event had tremendous repercussions in the astronomical community, involving more than 4000 astronomers,[84] improving the understanding of neutron star mergers,[85] and putting very tight constraints on the speed of gravity.[86]

Several searches for continuous gravitational waves have been performed on data from the past runs. On the O3 run, these include an all-sky search,[87] targeted searches toward Scorpius X-1[88] and several known pulsars (including the Crab and Vela pulsars),[89][90] and directed search towards the supernova remnants Cassiopeia A and Vela Jr.[91] and the Galactic Center.[92] While none of the searches managed to identify a signal, this allowed upper limits to be set on some parameters; in particular, it was found that the deviation from perfect spinning balls for close known pulsars is at most of the order of 1 mm.[87]

Virgo was included in the latest search for a gravitational wave background along with LIGO, combining the results of O3 with the ones from 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.[93]

Constraints on the Hubble constant have also been obtained; the current best estimate is 68+12
-8
km s−1 Mpc−1, combining results from binary black holes and from the GW170817 event. This result is coherent with other estimates of the constant, but not precise enough to resolve the tension regarding its exact value.[94]

Outreach

The Virgo collaboration participates in several activities promoting communication and education on gravitational waves for the general public.[95] One of the more forefront activities is the organization of guided tours of the Virgo facilities for schools, universities, and the general public;[96] however, many of the outreach activities take place outside the Virgo site. This includes educational activities such as public lectures and courses about Virgo activities, including toward [vague] school classes,[95] but also participating in several science festivals,[97][98][99] which involves the development of methods and devices for the vulgarization[vague] of gravitational waves (and related topics). The collaboration is also involved in several artistic projects, ranging from visual projects such as "The Rhythm of Space" at the Museo della Grafica in Pisa,[100] or "On Air" at the Palais de Tokyo,[101] to musical ones with different concerts.[102] It also includes activities promoting gender equality in science, for instance highlighting the women working in Virgo in communications to the general public.[103]

Gallery

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