Accretion – Accretion is a process where the gravitational potential of a massive object pulls in matter from another object that is close enough in proximity. The infalling matter usually forms an accretion disk due to conservation of angular momentum. In this disk, gas spirals towards the centre, becoming hotter as gravitational potential is converted into thermal energy. The energy can be radiated away as infrared, visible, ultraviolet, and/or x-ray light. Accretion disks around white dwarfs, neutron stars, and black hole candidates that are in binary systems can radiate massive amounts of energy at the ultraviolet and x-ray frequencies; some of these binary systems are among the brightest x-ray sources in the sky. (Wikipedia)

Accretion disk – Mass of material (in the form of a disk) in orbital motion that slowly inspirals toward a massive central body. (Wikipedia)

Active galactic nucleus – Active galactic nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies. (Wikipedia)

Ad-hoc models – Non-physical waveforms used solely to characterize pipeline capabilities and sensitivity. (Wikipedia)

Advanced LIGO/Virgo – The second generation of LIGO/Virgo, using upgraded detectors which are (to date) about three times more sensitive than the initial ones, with further improvements in sensitivity planned. (Advanced LIGO, Advanced Virgo)

All-sky searches – Those are the standard searches. For the O3GK data, LVK collaboration has performed a matched-filter search for the BNS coalescences and generic unmodeled bursts.

Amplitude and phase – The two variables describing an electro-magnetic wave: “amplitude” describes the height of the peak of the wave, while the “phase” tells you when the peaks arrive at a certain location. (Mathisfun)

Amplitude spectral density – The amplitude spectrum (or amplitude spectral density) is a measure of the frequency content of a signal. (Wikipedia)

Astrophysical background – A stochastic background of gravitational waves produced by sources such as neutron stars and black holes. (Wikipedia)

Asymmetry – Continuous gravitational waves are not produced by spherically symmetric objects. An asymmetry is some deviation from spherical symmetry and is required in order to produce continuous gravitational waves. Examples include mountains on the surface of an otherwise perfect sphere.

Auxiliary channels – Used to monitor the environmental behaviour around the detector, they register whatever can contribute to identify any disturbances. Some examples are magnetometer or seismometer readings, or monitors recording human activities or weather conditions.

Axion – A hypothesized lightweight elementary particle proposed in 1977. (Wikipedia)

Axion black hole superradiance – A process in which axions extract energy from a black hole, and a black hole emits gravitational waves in return.

Band-limiting – The limiting of a signal’s spread in the frequency domain. Compact binary coalescence: The merger of two dense, compact objects like neutron stars and black holes orbiting around each other. (Wikipedia)

Baryon minus lepton number (B-L) – Baryons and leptons are classes of particles. Baryon minus lepton number, or B-L, equals the number of baryons minus the number of leptons. Protons and neutrons are baryons, and electrons are leptons. Since the number of protons and electrons are the same for neutral atoms, B-L equals the number of neutrons. Just like electric charge is conserved, B-L is also conserved in the standard model of particle physics. So, baryon minus lepton number can be a new type of charge for a new force we don’t know yet. (Wikipedia)

Bayes factor – The ratio of likelihoods of two different hypotheses. It can be used to determine if the data favor one hypothesis over another. (Wikipedia)

Bayesian inference – Method that allows us to combine new data with some knowledge that we already have (commonly known as prior information), expressed as probability. The combination is used to update our current knowledge and is also expressed as probability (the posterior probability). More information can be found here. (Wikipedia)

Big Bang theory – Explanation for the origin and evolution of the observable universe which describes how the universe began about 14 billion years ago and has expanded from an initially very hot and dense state. The Big Bang theory is widely accepted as explaining many of the observed properties of the universe, including the abundance of the lightest chemical elements and the existence of the cosmic microwave background radiation. (Wikipedia)

Binary black hole – A system consisting of two black holes in close orbit around each other. (Wikipedia)

Binary neutron star – A system consisting of two neutron stars in close orbit around each other. (Wikipedia)

Binary neutron star observable range – A standard measure of interferometer sensitivity, which is the average distance at which the inspiral of a binary system consisting of double neutron stars can be detected with a matched-filter signal-to-noise ratio of 8.

Binary system – Two stellar objects (e.g., stars, neutron stars, black holes) in orbit around each other. (Wikipedia)

Black hole – A region of space-time with gravity so intense that it prevents anything, including light, from escaping. Black holes come in different sizes: the stellar-mass black holes originate from stellar collapses and their masses range from a few solar masses to about 65 solar masses. The intermediate-mass black holes range in mass from around 100 solar masses to 105 solar masses. Finally, the supermassive black holes range from more than 105 solar masses to more than 109 solar masses. (Wikipedia)

Black hole mimicker – A black-hole-like region of space-time that is enough like a black hole to be detected by its gravitational waves in a merger but, upon careful inspection, fails to have all the properties required by general relativity.

Bosons – Class of elementary particles. Bosons do not obey the Pauli exclusion principle obeyed by fermions (another class of elementary particles, such as electrons). This property allows many bosons to occupy the same quantum state at the same time and form macroscopic objects such as the boson clouds we consider. (Wikipedia)

Buchdahl limit – This limit describes the maximum amount of mass that can exist in a sphere before the sphere must undergo gravitational collapse to a black hole. (Wikipedia)

Burst search – A search for coincident excess energy in a network of GW detectors that operates without assuming a specific waveform model.

Calibration – Determination of the correlation between an input and an output quantity. In the case of the gravitational-wave detectors, this procedure allows us to obtain the differences in arm lengths from the detector output. (Wikipedia)

Cepheid – Type of pulsating variable star that undergoes periodic changes in radius and temperature, leading to regular, periodic changes in their luminosity. By measuring their pulsation period, astronomers can reliably estimate the distance of Cepheid variable stars. (Wikipedia)

Characteristic age – The “age” of a pulsar as determined using its current frequency and spin-down rate, and an assumption about the mechanism(s) that is slowing it down, i.e., through gravitational-wave emission.

Chi-square test – Chi-square is a statistical test commonly used to compare observed data with predicted data calculated using a specific hypothesis. (Wikipedia)

Chirp – A chirp is the name of the gravitational wave signal or waveform shape we typically observe as a pair of super dense objects, such as black holes or neutron stars, inspiral towards each other before merging. In a chirp signal, the frequency and amplitude increases with time. (Wikipedia)

Chirp mass – A mathematical combination of masses for each compact object in a binary (see Wikipedia for the formula). The chirp mass determines the leading-order orbital evolution of the system as a result of energy loss from emitting gravitational waves. (Wikipedia)

Coherent – When two or more detectors record closely related signals, a coherent analysis is one that uses that expected relationship to distinguish real signals from detector noise. Noise tends to produce unrelated false signals in each detector. (Wikipedia)

Coherent analysis – A method for simultaneously analyzing data recorded by multiple detectors which requires them all to be consistent with a single physical signal. This approach can be significantly more effective than simply adding together the outputs from the detectors, especially when the pattern of signals in the different detectors depends on some factor like location in the sky.

Coherent WaveBurst (cWB) – The cWB algorithm is a method for detecting gravitational wave signals without relying on templates of predicted gravitational-wave signals. The algorithm works by comparing signals measured across multiple detectors to see if an event stands out above the noise background in a consistent manner.

Compact binary – A system made of two compact stellar remnants, e.g. neutron stars or black holes, orbiting around each other very closely.

Compact binary coalescence – commonly abbreviated as CBC, it consists of two black holes, two neutron stars, or one black hole and one neutron star that inspiral and eventually merge. The whole process produces gravitational waves that increase in frequency and amplitude as the two objects get closer to each other and accelerate. The resulting object of the merger can be either a neutron star or a black hole, depending on the initial system. The objects forming the binary are called its components, the primary component being defined as the one having the largest mass. (SoundsOfSpacetime, MPG-AEI)

Compactness – Quantity that measures how compact a star is. It depends on the star’s radius and mass.

Compact object – An extremely dense astrophysical object such as a black hole, neutron star, or white dwarf. (Wikipedia)

Confidence level – The percentage of the time the true value is expected to be contained within the quoted range. (Wikipedia)

Constraining CCSN models and model exclusion – Since we do not know how to simulate accurately CCSN signals, several different CCSN models and approaches are under consideration. By analysing GW data that was collected at the same time as an observed supernova we can place limits on certain parameters, i.e. the numbers that characterise the details of the CCSN models, e.g. the amount of emitted GW energy. We can also make statements that might allow us to exclude some models (e.g. extreme emission models) when GWs are not detected by reasoning that if these models were a correct description of a CCSN event then a GW signal should have been detected.

Continuous gravitational waves – Long-lasting form of gravitational radiation. See here for more details.

Core-collapse supernova – In a star, the pressure of its gas is constantly compensating the gravitational pull from its core. When nearing the end of its life, the pressure drops and the star cannot any longer withstand the gravitational pull. It suffers an extremely rapid gravitational collapse towards its core which has several possible outcomes. The sudden collapse can create an extremely high pressure in the star causing it to explode in a supernova — hence the name “core-collapse supernova”. The supernova can then leave behind a neutron star or a black hole in the case of fallback. If the star was too massive to begin with, it will collapse directly into a black hole, skipping the supernova step. (Wikipedia)

Cosmic distance ladder – The combination of methods by which astronomers determine the distance of objects in the universe. Distances to remote objects, which are usually based on empirical relationships between their properties, are built upon more direct, geometrical, measurements of distances to nearby objects – usually within the Milky Way galaxy. (Wikipedia)

Cosmic microwave background (CMB) – Soon after the Big Bang, when the first atoms formed, the Universe became transparent. The electromagnetic radiation that escaped at that time appears as a faint background in the microwave region of the radio spectrum; this is also known as relic radiation. Today, the CMB is uniform to roughly one part in 100,000, and yet, the small, measurable temperature fluctuations of this background contain the imprint of the Universe content as it was at this time. (Wikipedia)

Cosmic string – Hypothetical one-dimensional objects that may have formed in the early Universe, while it was cooling down and expanding. (Wikipedia)

Cosmological distances – This refers to distances much larger than the size of our galaxy and the local group. Typically cosmological distances are expressed in terms of Gigaparsecs. One Gigaparsec (often abbreviated as Gpc) corresponds to about three billion light years or 3×10²² km. That is, a million times the size of our galaxy. (Wikipedia)

Cosmology – The study of the origin, evolution, structure and ultimate fate of the Universe. (Wikipedia)

Counterparts – EM counterparts are events that can be associated with a corresponding gravitational wave event.

Coupling – When one particle interacts with others in a specific way. (Wikipedia)

Credible level – Interval within which an uncertain parameter value falls with a particular probability. (Wikipedia)

Cross-correlation – Measure of the similarity of two (or more) sets of data. If the data from two separate gravitational wave detectors is found to be correlated, this may indicate the presence of the gravitational wave background (provided other possible sources of correlation are ruled out). (Wikipedia)

Crust – Like the Earth, neutron stars have solid crusts on top of a liquid lower layer. Like the Earth’s crust, these could be rough, with mountains and so on, which is good for gravitational wave emission. In addition to elastic forces which support mountains similar to the situation on Earth, neutron stars’ magnetic fields are strong enough to support “mountains” in the liquid layers too.

Cubic gigaparsec – The volume of a cube each of whose edges is a gigaparsec long.

Cusp – A fixed point on a curve at which a point tracing the curve would exactly reverse its direction of motion.

Dark energy – Mysterious, unknown, component of the matter and energy content of the cosmos that dominates the behavior of the Universe on its largest scales and is believed to be causing the expansion of the universe to accelerate. The simplest model for dark energy is that of a so-called cosmological constant that exerts a negative pressure, resulting in an accelerated expansion. (Wikipedia)

Dark matter – This mysterious form of matter makes up about 85% of the mass in the Universe. It is dark because it doesn’t emit light or interact electromagnetically. Many theories of dark matter predict that it is some type of fundamental particle, but it is also interesting to consider the possibility that the darkest objects we know of (primordial black holes) could be a component of dark matter. (Wikipedia)

Dark matter halo – A dark matter halo is the inferred halo of dark matter that permeates and surrounds individual galaxies. (Wikipedia)

Degeneracy pressure – Degeneracy pressure is a quantum effect that arises when identical particles (such as electrons or neutrons) cannot share the same quantum state. This results in a quantum pressure that can be sufficient to prevent the gravitational collapse of a star, even after all its fuel has been exhausted. (Wikipedia)

Detector noise – Fluctuation in the gravitational-wave measurement signal due to various instrumental and environmental effects. The sensitivity of a gravitational-wave detector is limited by different sources of noise.

Directed search – A search for a continuous gravitational wave signal when something is known about a potential source and included in the search method, but not all parameters of the potential source are known with high precision. This type of analysis lies between searches for unknown sources (all-sky searches) and targeted searches (all parameters are well-constrained).

Dispersion – When the speed with which a wave travels depends on its frequency. (Wikipedia)

Distant universe – Due to the finite speed of light, the further we look into the distant universe, the further we also look back in time. Hence, binary mergers detected from large distances actually happened when the Universe was much younger than today, and so we constrain a different epoch of its history than with more local observations. Since the Universe expands, larger distances also correspond to higher redshifts of observed signal wavelengths.

Doppler shift – Change in the frequency of a wave due to the relative motion of the source and the observer. (Wikipedia)

Dynamical formation – The assembly of a compact object binary via a chance gravitational encounter between two compact objects in a dense environment. (Wikipedia)

Eccentricity – This parameter is used to quantify the ellipticity of the orbit of a binary black hole system. Higher values of eccentricity denote a more pronounced elongation of the orbit. For binary black holes with wide orbits, the eccentricity can be approximated by Keplerian eccentricity. As the orbital separation decays, relativistic effects become more pronounced, and the Keplerian eccentricity diverges from the eccentricity measurable by gravitational waves. (Wikipedia)

Effective inspiral spin – A well-measured parameter encoding spin information in a gravitational-wave signal. It describes how much of each individual black hole’s spin is rotating in the same way as the orbital rotation (e.g., if the spin and the orbit are both clockwise or anticlockwise).

Efficiency – The fraction of detected simulated signals, assuming a random arrival direction and time. Expressed as a function of waveform type and strength.

Einstein – In 1915 Albert Einstein first described the general theory of gravitation. In this theory, gravity is the result of curvature in spacetime caused by concentrations of mass or energy. It predicted both gravitational waves and gravitational lensing. (Wikipedia)

Einstein@Home – A system that uses the idle time on volunteer computers to solve scientific problems that require large amounts of computer power, such as processing data from GW detectors and performing all-sky searches for continuous wave signals. The reader interested to become one of these volunteers, and help us to find GWs, can join the Einstein@Home project following instructions at this link.

Electromagnetic – Light that can span the entire spectrum including Gamma-rays, X-rays, ultraviolet, infra-red and radio. (Wikipedia)

Electromagnetic coupling – Strength of interaction between charged particles. (Wikipedia)

Electromagnetic radiation – Visible (optical) light stretches from red to violet, but beyond this range the spectrum continues. Past red light there are infrared light, microwaves, and radio waves, while past violet there are ultraviolet light, X-rays, and gamma rays. Astronomers use different parts of the spectrum to view different aspects of the Universe. However, some things are dark and therefore hard to see with any part of this spectrum. (Wikipedia)

Electromagnetic spectrum – Visible light stretches from red to violet, but outside the range our eyes can see, this spectrum continues. Beyond red light there is infra-red, microwaves and radio waves, and beyond violet there is ultraviolet, X rays and gamma rays. This is the spectrum of electromagnetic radiation, and astronomers use each part of the spectrum to learn more about the Universe. All electromagnetic radiation takes the form of ripples in electric and magnetic fields, and differ in their frequency or wavelength (the length of a ripple). (Wikipedia)

Electromagnetic (EM) waves – Waves composed of coupled electric and magnetic fields. These waves are commonly known as light, although only a small portion of EM wavelengths can be seen by the human eye. In order of increasing wavelength, the kinds of EM light mentioned in this recent paper are: Gamma-rays, X-rays, UV (ultraviolet), visible (optical), radio. (Wikipedia)

Electron degeneracy pressure – Electron degeneracy pressure arises from the Pauli exclusion principle, which states that no two electrons can share identical quantum states. This means that even under extreme gravitational forces, such as at the heart of stars, there is a limit to how compressed electrons can become. (Wikipedia)

Electrons – The lightest stable subatomic particle known. These are found surrounding the nucleus of an atom. (Wikipedia)

Ellipticity – Roughly, the ellipticity can be thought of as the ratio between the size Δr of the deformation, or “mountain” on the surface of a neutron star, compared to the star’s radius, r: Δr/r. Equatorial ellipticity measures how far from spherical a body is, defined as the relative deformation across the equatorial plane with respect to the deformation along the perpendicular direction in the same plane. Similarly, polar ellipticity measures the difference between a plane passing through the stellar poles and the equatorial plane. (Wikipedia)

Energy flux – A measure of how much energy is reaching a detector per unit area per unit time. For example, energy flux can have units of ergs per square centimeter per second. This definition is useful when the detector is very far from the source because then the amount of energy gathered by a detector is proportional to its size (area) and to how long it waits. However, the energy flux may be present for only a fraction of a second in a transient astrophysical event such as a short GRB. (Wikipedia)

Energy density – The amount of energy stored in a given system or region of space per unit volume. (Wikipedia)

Equation of state – The internal structure of a NS can be described by the Equation of State, which tells us about the relationship between pressure and density inside the NS. Since we still do not know what is the precise internal structure of these objects, we use different models of the equation of state to cover several possibilities. (Wikipedia)

Equatorial coordinates – Astronomers define the position of objects on the sky using the equatorial coordinate system. In this system an object’s position is defined by its right ascension and declination, which are equivalent to a longitude and latitude on the sky based on a plane formed by projecting the Earth’s equator onto the celestial sphere. (Wikipedia)

Equatorial ellipticity – Measure of how far from spherical a body is, defined as the relative deformation across the equatorial plane with respect to the deformation along the perpendicular direction in the same plane.

eV: The abbreviation for electronvolt, a unit of energy commonly used in atomic and particle physics. Because of the relation between energy and mass established by Einstein, E = mc², masses of particles can be given in units of energy divided by the square of the speed of light, i.e., eV/c². For example, the mass of the electron is 5.11 ⨉ 10⁵ eV/c² while the mass of the neutrino, the lightest massive particle currently known, is less than 0.120 eV/c². When the natural unit system is used (which sets c = 1), masses are expressed in eV. (Wikipedia)

Exclusion distance – The exclusion distance is the distance within which 90% of a population of simulated signals would be recovered at least as confidently as the loudest candidate event caused by detector noise near the time of the GRB.

Exotic compact objects – These are objects which mimic properties of black holes, but are not black holes. Distinguishing them from BHs is a challenge.

Fallback – In the scenario of a core-collapse supernova that forms a neutron star, residual matter can “fall back” towards the neutron star. This accretion of matter can drive the mass of the neutron star above its maximum mass and lead to the formation of a black hole.

False-alarm probability – The probability that a detector noise fluctuation could produce an event similar to a candidate being considered. (Wikipedia)

False alarm rate – The false alarm rate is used to quantify how likely an event is to have been caused by noise. It is computed by simulating events coming from noise and looking at their signal strength, to derive a distribution of the expected rate of such events as a function of the signal strength. In more concrete terms, if an event has a false alarm rate of 1 per day, this means that we expect the noise of our detector to produce such an event about once every day. We would therefore have little confidence in this event. (Wikipedia)

Fast radio burst – A short-duration and high-energy burst of electromagnetic waves at radio frequencies. (Wikipedia)

Feedback control loop – A feedback control loop is a system that manages and controls the behavior of a device. A common illustration of a servo control loop is the cruise control device in many automobiles. Once set, the loop maintains the speed of the car without needing input from the driver.

Field theories – Frameworks used to describe subatomic particles in particle physics. (Wikipedia)

Folded data – Due to the Earth’s daily rotation about its axis, nearly every 24 hours the observed sky looks the same. By taking advantage of this temporal symmetry, we can compress the entire data set of several hundreds of days into one (sidereal) day. This compressed data is called the folded data.

Fourier transform – A mathematical transformation employed to transform signals between time domain and frequency domain. (Wikipedia)

Frequency bins – Intervals between samples when analyzing mathematical functions or physical signals with respect to frequency, rather than time.

Frequency derivative – The rate of change of the frequency.

Fused silica – A common material for glass. An extremely pure fused silica, or silicon dioxide, is the material used for test mass mirrors in LIGO and Virgo, and for auxiliary mirrors in KAGRA. (Wikipedia)

Galaxies – These are gravitationally bound systems composed mostly of stars, gas and dark matter. (Wikipedia)

Galaxy clusters – Galaxies can also form larger gravitational systems known as clusters. (Wikipedia)

Gamma ray(s) – Light with a very large amount of energy per photon. Despite the word “ray” in the name, gamma rays often behave like individual particles. Typically, a photon with an energy of 100 keV (kilo electron volts) or greater is considered a gamma-ray photon, although this is not a firm definition. Gamma rays can be produced in nuclear reactions or the collisions or radioactive decays of especially high-energy particles. (Wikipedia)

Gamma-ray burst (GRB) – A flash of gamma rays coming from a distant astrophysical source and lasting for up to hundreds of seconds, in many cases even less than a few seconds. (Wikipedia)

General relativity – The theory of gravity proposed by Albert Einstein in 1915. In this theory, space and time are like a malleable fabric that warps in the presence of matter and energy, and objects follow trajectories through this curved space. (Wikipedia)

GEO600 – A gravitational-wave detector situated in Hannover, Germany. It is a laser interferometer with 600-m long arms. The laser beam travels through each arm twice, providing an optical arm length of 1.2 km. (Wikipedia)

Gigaparsec – Unit of distance equal to 3262 million light years, or 3,0857×10²⁵ meters. (Wikipedia)

GLADE+ – New, extended, compilation of galaxy catalogs, containing data for about 22 million galaxies, used to provide redshift information for potential host galaxies of our GW events. A free-to-access scientific paper describing the original GLADE catalog is available here.

Glitch – Burst of noise in gravitational-wave data, analogous to a pop of static heard from a speaker, that can sometimes be confused for or mask out a real gravitational-wave signal. (Wikipedia)

Globular cluster – A very dense group of stars bound together by gravity. (Wikipedia)

Graphics Processing Unit (GPU) – Specialized hardware suited for the processing of data using massive parallelization. (Wikipedia)

Gravitational radiation – Ripples in space-time created by accelerating massive objects. Like electromagnetic radiation, they travel at the speed of light. They are predicted by Einstein’s theory of general relativity and are commonly known as gravitational waves. If you would like to know more, you have come to the right place! Try looking at our other pages on gravitational-wave science.

Gravitational-wave astronomy – Astronomy which uses gravitational waves—ripples in spacetime created by accelerating objects—to observe the Universe. Using gravitational waves, we can detect systems, like colliding black holes, which are difficult (if not impossible) to discover otherwise. If you’d like to know more about gravitational-wave astronomy, you’ve come to the right place! (Wikipedia)

Gravitational-wave data analysis – Gravitational waves are extremely weak by the time they reach us, so that specialized and highly sophisticated software is required to “dig out” their signal from noisy data. It is very hard to detect a GW signal using only one detector, since there are many local effects that could mimic – or indeed completely swamp – the signal in our observed data. Consequently we require to analyse GW data from at least two different detectors. There are two types of analysis that we can carry out. We can analyse real data from the GW detectors and assess how likely it is that the strongest candidate GW signals in these data could have been caused simply by background noise. Alternatively we can inject simulated GW signals into the detector data and thus determine how efficiently and successfully our analysis algorithms can find these simulated signals.

Gravitational-wave energy density – The fraction of the total energy in the Universe contained in gravitational waves. (Wikipedia)

Gravitational waveform – A representation of a gravitational-wave signal’s evolution with time. (Wikipedia)

Gravitational waveform template – A predicted model of how the disturbance caused by a gravitational wave varies with time.

Gravitational-wave polarizations – The geometric shape of the stretching and squeezing of space-time caused by a gravitational wave as it moves. General relativity only predicts one specific type, so-called tensor polarization, while some alternative theories of gravity also predict additional polarizations. (Wikipedia)

Gravitational waves – These are ripples in space-time that are generated by some of the most violent processes in the universe, such as merging neutron stars or black holes. See here for more information.

Gravitational-wave spectrum – A representation of how strong is a gravitational wave signal at different frequencies. (Wikipedia)

Gravitational-wave strain – A measure of deformation of space-time by a passing gravitational wave. (Wikipedia)

GRB – Gamma-ray burst (GRB) is an astronomical phenomenon in which a distant object suddenly emits a very powerful flash of gamma rays and X-rays, often followed by light at lower energies (ultraviolet, visible, infrared, and radio waves). Most GRBs are believed to be produced by collapsing massive stars or by mergers of neutron stars in binary systems. (Wikipedia)

GRB-targeted searches – Those are targeted searches for GW signals associated with GRBs reported during the run. By targeting the times and sky positions of GRBs we can potentially detect weaker associated GW signals than would be identified with all-sky searches. For the O3GK data, LVK collaboration has performed a matched-filter search for the BNS and NSBH coalescences and generic unmodeled bursts.

GW150914 – The first confident direct observation of gravitational waves, which was made on 14 September 2015 by the LIGO Scientific and Virgo Collaborations. See here for more information.

GW200105_162426 – A possible neutron star-black hole coalescence reported previously. This candidate has a 36% probability of being a real astrophysical signal. It is a particularly challenging source to analyse for two reasons. The first is that it is a single detector observation by LIGO Livingston. At the time of the event, LIGO Hanford was not taking data. Virgo was taking data, but no such signal could be identified. An event being visible in one detector but invisible in another is not unexpected: it depends on the relative sensitivity of the instruments and on the position of the source of the gravitational-wave signal in the sky (there was a similar situation for GW170817). It is always challenging to estimate the significance of a signal if there is data from only one detector. The second reason is that at this stage little is known about the population of neutron star-black hole binary systems as we do not have many observations of them. This makes reliably classifying neutron star-black hole coalescences difficult. As we observe more neutron star-black hole systems, we will learn more about these binaries and can come back to check candidates like GW200105_162426.

GWTC-3 – The third Gravitational-wave Transient Catalog (GWTC-3) describes signals, most likely generated by gravitational waves, which were detected with Advanced LIGO and Advanced Virgo up to the end of their third observing run.

Hertz – A unit of frequency equal to one cycle per second. (Wikipedia)

Hidden Markov model – Technique of looking for signals whose frequency changes randomly. (Wikipedia)

Higher-order modes – Sub-dominant terms in the spherical harmonics expansion that describes GWs beyond the dominant, quadrupolar term. The quadrupole term is analogous to the fundamental tone in sound waves.

Hough transform – Algorithm to identify well-described shapes in images such as the ones described by a spectrogram. (Wikipedia)

Hubble constant – Parameter used to measure the expansion rate of the universe. Its present-day value is denoted by the symbol H₀ and it is measured to be about 70 km s^-1 Mpc^-1. (Wikipedia)

Hyper-massive – An object with too much mass to be stable. Neutron stars with masses bigger than a couple times the mass of our sun can’t exist for long because of the huge gravitational pull and will therefore collapse into a black hole before long.

Inclination angle – Angle between the spin axis of a neutron star and a reference direction such as the line of sight.

Inhomogeneities from the Big Bang – Fluctuations in the density of the primordial plasma caused by the quantum fields dominating the Early Universe.

Initial LIGO – The first epoch of LIGO, spanning 2002 – 2010. The search described in this publication used data from late 2010. (Wikipedia)

Inspiral – The slow shrinking of the orbit of a binary due to the effect of gravitational wave emission. It’s the first and longest phase in the coalescence of a binary.

Interferometer – Instrument, used in many fields of science and engineering, that operates by merging two or more sources of light to create an interference pattern, which can be measured and analyzed and contains information about the object or phenomenon being studied. See also here. (Wikipedia)

Instability – A state that is not stable to slight changes, like a pin balanced on its head. (Wikipedia)

Isolated binary evolution – The evolution of a binary star system from the birth to the death of each of the two stars terminating in a compact object binary.

J-statistic – Mathematical tool that intakes information from the binary to convert the raw data from the interferometer into the likelihood of a signal being present at a given frequency. (Wikipedia)

KAGRA – An underground gravitational-wave detector situated at Kamioka in Gifu prefecture, Japan. It is a laser interferometer with 3 km long arms and cryogenically cooled sapphire test mass mirrors. (Wikipedia)

Kerr black holes – Black holes in GR that are completely described by its mass and the spin (rotation). (Wikipedia)

Kiloparsec (kpc) – A thousand parsecs. A parsec is an astronomical unit of length corresponding to roughly 3 light-years or 30 trillion kilometers. (Wikipedia)

Kink – A discontinuity on a curve. (Wikipedia)

Kozai-Lidov mechanism – A dynamical phenomenon affecting the orbit of a binary system perturbed by a distant third body. (Wikipedia)

Large Magellanic Cloud – A dwarf galaxy companion to the Milky Way at a distance of 50,000 parsecs. Both the Large and Small Magellanic Clouds are visible to the eye in the southern hemisphere. (Wikipedia)

Light scattering – When laser light hits an optical component, a small proportion of it can be scattered (reflected at a random angle). The scattered light can be reflected off of other surfaces and make its way back into the detector laser beam and cause scattered light glitches. For more information about these glitches and other, have a look at Gravity Spy.

Light-year – Unit of distance. 1 light-year is the distance travelled by light in one year, computed based on the velocity of light in vacuum. (Wikipedia)

LIGO – The Laser Interferometric Gravitational-Wave Observatory (LIGO) is a US-based pair of gravitational-wave detectors. One is situated near Livingston, Louisiana, and the other near Hanford, Washington. Both detectors are laser interferometers with two perpendicular 4-km long arms. (Wikipedia)

LIGO, Virgo and KAGRA – Respectively located in the USA, Italy and Japan, these are the instruments that allow us to detect gravitational waves. The basic concept of the LIGO, Virgo and KAGRA detectors consists of two arms of kilometer scale forming an “L” shape, with mirrors at their end and in which a laser beam circulates. We use the laser to measure relative variations in the length of the arms caused when gravitational waves cross the Earth. The distance between the arms of each detector is constantly monitored and constitutes the data in which we search for gravitational waves signatures.

Long GRB – The duration is longer than 2 seconds. This is thought to occur when the core of massive stars collapse. (Wikipedia)

Lower mass gap – A mass range where we expect no or few compact objects to exist. This range extends roughly from 3 M_☉ for the maximum mass of a neutron star, to 5 M_☉ for the minimum mass of a black hole. (Wikipedia)

Low-mass X-ray binary (LMXB) – A binary system consisting of a compact object such as a white dwarf, a neutron star or a black hole, and a lower-mass companion star, in which the compact object is accreting matter from the companion, generating X-rays. (Wikipedia)

Luminosity distance – This is the distance between the observer and the astrophysical object, as inferred by the apparent brightness of the object and its actual luminosity (assuming it is known). (Wikipedia)

Machine learning – The study of computer algorithms that can improve automatically through experience and by the use of data. (Wikipedia)

Magnetar – A highly magnetized neutron star. (Wikipedia)

Magnetar giant flare – A much larger version of the short burst, emitting the amount of energy the Sun would in 100,000 years in less than a second.

Marginalize – A calculation method used in statistics to focus on a quantity of interest by averaging over the uncertain values of one or more other quantities that influence it. The averaging is typically done using likelihood or probability weighting so that the marginalized value represents a best estimate.

Mass ratio – Denoted by q. Defined as: q = m₂/m₁, where m₂ and m₁ are the lighter and heavier masses, respectively.

Massive star – Massive stars have masses more than 8 times the mass of the Sun. Only stars that massive can form a neutron star after they explode as a supernova. If they have lower masses, the remnant becomes a white dwarf star.

Matched-filtering – A method we use to analyse the detector data and detect compact binary coalescence events. It involves comparing at all times the data from our detectors with theoretically predicted signals which depend on the properties of the binary system, looking for correlations. When a real gravitational wave crosses the Earth we should find a good match between the detector data and the predicted signal. (Wikipedia)

Median – The value exactly in the middle of a distribution, so that half of the other values lie above and half of them below the median. (Wikipedia)

Megaparsec (Mpc) – A unit of distance. A million times the distance of a parsec, equal to about 3.26 million light-years. (Wikipedia)

Merger rate density – The number of compact-object binaries that are expected to merge per unit volume of space, per year.

Metric theory – Metric theories are a large class of theories of gravity characterized by the fact that they describe the effect of gravity on matter and energy via a simple mathematical object called a metric tensor. General relativity is a metric theory, and so are essentially all its viable alternatives (e.g. Brans-Dicke gravity). For more on alternative theories, see this review (technical).

MHD simulation – Magneto-hydrodynamical simulation. A model of a star (or other astronomical phenomenon) that treats matter as a fluid. Yes, similar equations govern fluids and stars! The pressures inside stars are so high that their interiors behave as a fluid. In this case, MHD simulations also include the influence of magnetic fields in the model. (Wikipedia)

Microlensing experiments – Gravitational lensing is an optical effect that occurs when a distribution of matter (such as a cluster of galaxies) sits between a distant source of light and the observer on Earth; the gravitational effect of this matter bends the light’s path, like a lens. If the lens is smaller than a galaxy (for example a star), the effect is known as ‘microlensing’ and is seen as a change in brightness of the source. (Wikipedia)

Millisecond – This is equal to 1 millionth of a second.

Millisecond pulsar – A rapidly rotating pulsar with a rotational period less than about 30 milliseconds and a very low spin-down rate. (Wikipedia)

Modulus – The modulus of a number x, indicated as |x|, is its non-negative value, disregarding its sign.

Multi-messenger astronomy – When the same object or system is studied with different wavelengths of photons, this is called multi-wavelength astronomy. For example, the neutron star Scorpius X-1 is observed in radio waves and X-rays, while its companion star was detected by analyzing the visible light from the system. Expanding this concept to include astrophysical messengers that aren’t part of the electromagnetic spectrum, like gravitational waves, is called multi-messenger astronomy. (Wikipedia)

Multimessenger counterpart – We refer to light or neutrinos originating from a gravitational-wave source as counterpart signals.

Multivariate analysis – Analysis technique where multiple parameters are used simultaneously to statistically enhance an effect of interest. This multi-dimensional approach differs from standard analyses where parameters are used one at a time. (Wikipedia)

Narrowband gravitational waves – Gravitational waves emitted at approximately the same frequency.

Network sensitivity – A description of a network of detectors’ ability to detect a signal. A network composed of detectors with lower noise is able to detect weaker signals and therefore is said to have higher sensitivity.

Neutrino – A fundamental particle produced in some radioactive decay processes and high-energy particle reactions. A distinctive property of neutrinos is that they can normally pass through other matter with very little chance of interacting with it, so they are sometimes called ghost particles. Neutrinos are produced in huge numbers by the Sun, in supernovas, and possibly also in binary mergers. (Wikipedia)

Neutron star – A relic of a massive star. When a massive star has exhausted its nuclear fuel, it dies in a catastrophic way—a supernova—that may result in the formation of a neutron star: an object so massive and dense (though not as much as a black hole) that atoms cannot sustain their structure as we normally perceive them on Earth. These stars are about as massive as our sun, but with a radius of about ten kilometers. (Wikipedia)

Neutron star-black hole binary – A system consisting of one black hole and one neutron star in close orbit around each other.

Noise – Fluctuations in the gravitational-wave measurement signal due to various instrumental and environmental effects. The sensitivity of a gravitational-wave detector is limited by noise.

Non-Gaussian – A process that deviates from Normal (Gaussian) distributions; in the case of gravitational-wave detectors, the data generally follows Gaussian distributions with sharp deviations due to environmental and instrumental effects. (Wikipedia)

Non-linear crystal– A crystal in which you can inject light of one color and get out light of another color. (Wikipedia)

Nuclear fusion – Nuclear fusion occurs when two nuclei merge together into a single nucleus. For elements lighter than iron, this results in a release of energy. This energy powers stars. (Wikipedia)

Nuclear saturation density – The density of matter when nucleons (the protons and neutrons that compose the nucleus) begin to touch. (Wikipedia)

O3 observing run – The third data taking run in the Advanced LIGO and Virgo era began April 1, 2019 and ended March 27, 2020. The data used in this search came from LIGO and roughly the first half of the run, namely from April 1 to October 1, 2019.

Observing run – A period of observation in which gravitational-wave detectors are taking data.

Offline search – Offline analyses are searches that are performed on data from a previous observation period, typically during breaks when the detectors are not operating, for maintenance and upgrades. They complement real-time analyses (also called online analyses) which are carried out during the observation periods.

Optical cavity – An optical cavity is formed by two mirrors, forcing the laser to bounce back and forth multiple times. If the separation between the mirrors is an exact multiple of the laser wavelength, the laser power will build up in the cavity, reaching a value that is higher than the input power. For this to work, the cavity length and alignment have to be precisely controlled. In Advanced LIGO this is achieved with a number of servo control loops. (Wikipedia)

Pair-instability supernova (PISN) – Type of supernova explosion predicted to occur in a star with a mass greater than about 130 solar masses. The production of electron-positron pairs in the core causes a dramatic drop in the pressure supporting the star, leading to a runaway thermonuclear explosion which leaves behind no stellar remnant. (Wikipedia)

Parameter – A quantity that influences the form of a GW signal, such as signal frequency or binary orbital period. (Wikipedia)

Parameter estimation – A statistical technique used to infer the astrophysical parameters corresponding to a gravitational-wave signal. (Wikipedia)

Parameter space – Every GW signal is characterized by its properties (parameters) like duration, amplitude, peak frequency, etc. The whole set of these properties is called a parameter space. (Wikipedia)

Parsec (pc) – a unit of distance that is equal to approximately 3.26 light-years (31 trillion kilometres or 19 trillion miles). (Wikipedia)

Phase transition – Thermodynamical transformation of a system from one state to another. An example of a phase transition is when water cools and becomes ice. A phase transition could have happened in the early universe. (Wikipedia)

Phenomenological model – A model which has parameters are that not necessarily directly derived from first physical principles. (Wikipedia)

Photon – A particle of light which carries a certain amount of energy. In the early 1900s it became clear that light, which was previously thought of as a wave phenomenon, can act like a particle in certain ways. That was a key piece of evidence that led to the development of quantum mechanics (see, for example, History of quantum mechanics in Wikipedia). All light is composed of photons, with the amount of energy in a photon determining how it interacts with matter. (Wikipedia)

Pipeline – A process that is used as part of the workflow. The GUANO pipeline involves securing satellite data automatically in response to sudden reports of interesting astrophysical events.

Point source – A source of gravitational waves where the size of a source is negligible, when compared to the distance to the source. (Wikipedia)

Polar ellipticity – Similar to the equatorial ellipticity, but measures the difference between a plane passing through the stellar poles and the equatorial plane.

Polarization – Refers to the independent ways in which a wave can oscillate. For gravitational waves in GR, there are two independent modes of polarization, called plus and cross. This means that a passing gravitational wave would stretch and squeeze a ring of particles to the form of a + and x. (Wikipedia)

Popcorn background – The particular kind of stochastic background produced by binary black hole collisions. The gravitational-wave signals from binary black holes are loud, short, and separated in time. They arrive at Earth at random times, creating a stochastic background that sounds like the popping of individual kernels of corn.

Population model – A theoretical model which gives the abundance of compact objects of a given type as a function of any combination of binary parameters. (Wikipedia)

Posterior probability – How likely certain values of a given physical property are after analyzing our data, estimated through a process known as Bayesian inference. (Wikipedia)

Power law mass distribution – The probability of a black hole existing at a given mass is equivalent to the mass to some power: p(m) ∝ m^α where α is the slope of the power law (i.e., how steeply the probability changes with increasing mass). (Wikipedia)

Precessing binaries – Binary systems where the components’ spin direction is misaligned with the direction of their orbital angular momentum. 𝝌_eff, called the “effective aligned spin” is a parameter that characterizes a particular combination of the compact objects’ spins before they merge. A negative value of 𝝌_eff indicates anti-aligned spins with respect to the angular momentum of the system. 𝝌_p, called the “effective precession spin”, is another combination of individual spins that measures how much precession is present in the system, with a higher value indicating more precession.

Primordial black hole – A hypothetical BH that may have formed in the early universe, soon after the Big Bang. Predicted masses range from roughly 10^-18 M_☉ to 10³⁶ M_☉. A theoretical type of black hole formed in the early Universe. Fluctuations in the energy density of the Universe could have led to regions of space that were so dense that they spontaneously collapsed to form black holes. Since they are not formed via the collapse of massive stars, primordial black holes could conceivably exist below one solar mass or merge at redshifts higher than the first stars form. (Wikipedia)

Prior – An initial guess or hypothesis as to the shape of a probability distribution. (Wikipedia)

Probability of being astrophysical – This quantifies the probability that a signal is a real gravitational wave. It depends both on how loud the signal is compared to background noise and on our understanding of the population that the source belongs to. Having many observations of binary black holes aids our understanding of the binary black hole population and helps us to compute the probability of candidates signals of that type to be astrophysical. As we have fewer observations of neutron star-black hole mergers, this quantity is harder to compute and we may need to revisit the current estimates once we have more observations and know more about the population.

Pulsar – From “pulsating radio source”: a highly magnetized rotating compact star that emits beams of electromagnetic radiation out of its magnetic poles. (Wikipedia)

Pulsar glitch – A pulsar is a neutron star that has been observed through its pulses of electromagnetic radiation (usually in the radio band). Not all neutron stars can be observed as a pulsar, because they do not emit electromagnetic radiation in the direction of the Earth, or because they do not emit at all. A fraction of the neutron star population is known to show transient glitches, measured by electromagnetic observations of pulsars. The two most-explored mechanisms in the literature for these pulsar glitches are star quakes and superfluid-crust interactions. (Wikipedia)

p-value – A measure of the statistical significance of a given measurement. Loosely, the p-value answers the question “how likely is it that this result is simply background noise?” Low p-value events are more likely to be caused by actual gravitational wave signals. (Wikipedia)

Radio emission – Long-wavelength electromagnetic radiation generated by accelerated electric charges.

Radiometry – A method of searching for persistent signals, based on time delay between detectors that potential signals would induce. (Wikipedia)

Radio telescopes – Telescopes that measure radio waves coming from across the Universe. Radio telescopes come in many forms. In some cases they look like single large satellite dishes (for example, the Lovell Telescope at the Jodrell Bank Observatory) that can point to where they hope to detect radio waves. In other cases there are many “smaller” satellite dishes (~10m across, like the MeerKAT telescope) whose signals are combined to improve their sensitivities to weak radio waves. Finally, there are some radio telescopes that are long, rigid metal structures that do not necessarily “point” but instead are sensitive to the sky directly above them (one example is CHIME). We use data from all of these different types of telescopes in this analysis. (Wikipedia)

Recycled pulsar – A pulsar that may not necessarily rotate fast enough to be classified as a millisecond pulsar, but is expected to have acquired its high rotational velocity by accreting matter from a companion star.

Redshift – The stretching of the wavelength of light, or gravitational waves, traveling through the expanding Universe. (Wikipedia)

Ringdown – The phase of a black hole merger where the distorted black hole that forms in the merger emits gravitational waves that cause the distortions to disappear.

r-modes – Waves in a rotating fluid, also known as Rossby waves and driven by the Coriolis force. They have a frequency that is comparable to the spin frequency of the star, so for young neutron stars, they could be in the LIGO and Virgo frequency band. (Wikipedia)

S5 and S6 data – Data from the fifth and sixth LIGO science runs in 2005-2007 and 2009-2010 respectively. Although S6 was slightly shorter than S5, it achieved better (lower) noise levels.

Sapphire – KAGRA uses artificial sapphire for its test mass mirrors. Natural sapphires are blue due to impurities, but the artificial ones are pure aluminium oxide and are colorless. Sapphire mirrors have excellent optical and thermal properties at cryogenic temperatures. (Wikipedia)

Schumann resonances – Extremely low-frequency radio waves generated by lightning that remain captured between the Earth’s surface and the ionosphere, an atmospheric layer starting from about 60 kilometers altitude. (Wikipedia)

Science run – A period of observation in which data is taken. Initial LIGO had six science runs between 2000 and 2010.

Scintillator detector – A type of particle detector that produces a flash of light when an energetic particle enters it and is stopped or scattered. The light is collected, converted to an electrical signal by an optical sensor such as a photomultiplier tube, and measured to determine the amount of energy that was deposited in the detector. (Wikipedia)

Scorpius X-1 – Scorpius X-1 is an X-ray source located roughly 9000 light years away in the constellation Scorpius. Scorpius X-1 was the first extrasolar X-ray source discovered, and outside of the Sun, is the brightest persistent X-ray (1-10 keV) source in the sky. It was the first of the LMXBs to be discovered. (Wikipedia)

Search pipeline – Computing programs that consist of a chain of several processes. They condition the data for analysis, filter them and then compute various quantities in order to reject as many noise events as possible and estimate the significance of candidate astrophysical events. Some pipelines run in real-time, some run offline and some do both. More information can be found on the IGWN public alerts user guide.

Selection effects – Gravitational wave detectors are more likely to pick up events with certain intrinsic properties (for example in the right range of masses) and from certain regions in the sky. This leads to finding more pairs of similar events than one could at first expect. (Wikipedia)

Semi-major axis – The semi-major axis is the long axis of an elliptical orbit. (Wikipedia)

Sensitive band – The range of frequencies in which a gravitational wave detector is sensitive to gravitational wave signals. For the LIGO-Virgo interferometers, this usually spans from ~20 Hz to ~1 kHz.

Sensitive volume-time – The space-time volume within which our search algorithm can detect gravitational wave events with significance higher than a predetermined threshold.

Sensitivity – A description of a detector’s ability to detect a signal. Detectors with lower noise are able to detect weaker signals and therefore are said to have higher (or greater) sensitivity. (Wikipedia)

Sensitivity curve – The sensitivity of a GW detector is determined by a large number of noise sources corresponding to many different physical phenomena (e.g., seismic or electronic noise). The sum of all these noise sources determines the sensitivity of the detector at each frequency, giving its sensitivity curve.

Servo control loop – A servo control loop is a system that manages and controls the behavior of a device. A common illustration of a servo control loop is the cruise control device in many automobiles. Once set, the loop maintains the speed of the car without needing input from the driver. (Wikipedia)

Short GRB – The duration is less than 2 seconds. This is thought to come from the coalescence of compact objects (BNS or NSBH). (Wikipedia)

Signal-to-noise ratio (SNR) – The ratio of the signal power to the noise power. It measures the strength of the signal compared with the sources of noise that contaminate it. (Wikipedia)

Sine Gaussian – A type of artificial data that we add to the real data for determining our sensitivity. A Sine Gaussian has the same approximate shape and properties as the signal that we would expect from an f-mode in a magnetar.

Solar mass ( M_☉) – Mass of the Sun, used as a standard mass unit in astronomy. It equals roughly 2×10³⁰ kg. (Wikipedia)

Solar system barycenter – The center of mass of our local solar system. The solar system barycenter is the point around which all bodies in the Solar System orbit, so signals received at that point would experience no Doppler shift in the frame of our Solar System. (Wikipedia)

Spectral index – A model parameter that describes how the saturation energy of the modes evolves with frequency. (Wikipedia)

Spectrogram – Visual representation of the frequency composition of a time series. (Wikipedia)

Spin – Angular momentum of a rotating black hole; one of the defining properties of black holes, along with mass and charge. (Wikipedia)

Spin-down – Pulsars are rotating neutron stars which spin so regularly on a long timescale that they may be treated as reference position beacons. Their rotational speed is generally seen to decrease with time (equivalent to an increase in rotational period), as they lose rotational energy over time. This slowing of the rotation speed of the neutron star is referred to as spin-down. Conversely, an increase of rotation rate is called spin-up — this may happen if a pulsar is in a binary system and is spinning up by accretion of matter from its stellar companion.

Spin-down limit – The limit placed on the amplitude of gravitational waves from a pulsar based on the assumption that all the rotational kinetic energy lost by the star as it spins-down is through gravitational radiation. This assumes a precisely known distance to the pulsar, whereas in reality pulsar distances can be uncertain by up to a factor of about two. However, we do know that there are other ways that pulsars lose energy, with the main assumed mechanism being magnetic dipole radiation.

Spin frequency – The rotation of a pulsar around its own axis (its “spin”) can be measured with radio, X-ray and other telescopes by counting the arrival of pulses (when the “lighthouse beam” sweeps over the Earth) and the average time between them. The standard unit of frequency is the Hertz (Hz).

Squeezed vacuum state – The “squeezed vacuum” state, like the “vacuum state”, is a state in which there are no sources of light, and it obeys the Heisenberg Uncertainty Principle. However, the uncertainty in “amplitude” and “phase” are not the same. In particular, some uncertainty can be “squeezed” from amplitude to make the phase uncertainty bigger, and vice versa. (Wikipedia)

Standard Model – Theory describing three of the four known fundamental forces (electromagnetic, weak, and strong interactions) excluding gravity in the universe and classifying all known elementary particles. (Wikipedia)

Star formation rate – The rate at which gas is turned into stars over cosmic time. (Wikipedia)

Starquake – As our planet has earthquakes, shifts of the crust layer of a neutron star can create starquakes. While the study of earthquakes on our planet is called seismology, the study of similar phenomena in stars is called asteroseismology. (Wikipedia)

Stellar evolution models – These are mathematical models which are used to study how the physical parameters of a star evolve with time, from the moment when it is born until it becomes a stellar remnant. (Wikipedia)

Stochastic – Non-deterministic, random, can only be analyzed using probability and cannot be predicted exactly. (Wikipedia)

Stochastic gravitational-wave background – A random gravitational-wave background formed by overlapping signals that are too weak to detect individually. (Wikipedia)

Strain – The fractional change in the distance between two reference points due to the deformation of spacetime by a passing gravitational wave. The typical strain of even the strongest gravitational waves reaching Earth is very small — typically less than 10^-21. (Wikipedia)

Stratification – The formation of distinct layers in parts of a neutron star, e.g. an outer crust.

Strength of the GWB – The energy density in gravitational waves. This is expressed as the fraction of the total energy in the Universe in the form of gravitational waves.

String theory – A theoretical framework in which elementary particles are replaced by one-dimensional strings. (Wikipedia)

Subsolar-mass object – An astrophysical object is considered to be a subsolar-mass object if its mass is lower than that of the Sun, which is denoted by the symbol M_☉.

Supercomputer – Collection of interconnected computers, which allows to run very expensive computational routines in parallel, considerably reducing the run time. (Wikipedia)

Superfluid – A peculiar state of matter where the fluid flows without viscosity. (Wikipedia)

Supernova – A violent explosion, often spotted a rapidly appearing bright object in the sky, which then fades away. A supernova may outshine the rest of its galaxy. There are a variety of different supernovae. Some come from the collapse of massive stars, others may come from the collision of two white dwarfs. (Wikipedia)

Supernova lensing – Gravitational lensing caused by compact objects along the line of sight between Earth and a Type I supernova can magnify the brightness of the supernova. This effect depends on primordial BH models and can be used to set constraints on their population.

Superradiance – A process in which particles extract rotational energy from a spinning massive object. In the case of boson clouds, a bosonic field in the vicinity of a rotating black hole can be amplified through superradiant scattering. (Wikipedia)

Template – In this context, a time or frequency domain representation of a gravitational-wave signal. Usually, they are generated with the help of computers based on a particular theoretical model and set of parameters.

Tensor polarization – Gravitational-wave polarization allowed by general relativity. (Wikipedia)

Threshold – Signal strength chosen to define the dividing line between “most likely noise” and “could be a real event”. Any signal stronger than the threshold is considered a candidate event and is investigated more thoroughly.

Tidal deformation – Deformation of an object induced by the gravitational field of another object. As an example, on the Earth tides are caused by the Sun and the Moon and produce a deformation of the surface of the oceans with consequent daily fluctuation in ocean level.

Time domain – The analysis of mathematical functions or physical signals with respect to time. (Wikipedia)

Time-shift analysis – Standard method in gravitational-wave science to assess the confidence of a candidate. It is performed by applying different time-shifts between the data streams from two or more detectors. Eventual triggers coming from this time-shifts analysis are generated by accidental coincidences and they resemble the statistical distribution of the candidates in the original data (not shifted). Repeating these time-shifts a huge number of times allows us to increase the confidence on the significance assessment.

Topological – Properties of space that are preserved under continuous deformations. (Wikipedia)

Total mass – The sum of the masses of the two compact objects in the binary.

Transient – Astronomical phenomenon of short timescales; in contrast to astrophysical events lasting from thousands to billions of years.

Transient gravitational waves – gravitational waves resulting from a short, often cataclysmic event, e.g. binary compact mergers. Most transient gravitational waves appear in the detector only for seconds or less.

Type Ia supernova – Particular explosion mechanism of a white dwarf, accreting material from a red giant companion star, whose mass becomes greater than the Chandrasekhar limit of 1.4 times the solar mass. The distances of Type Ia supernovae can be reliably estimated since they are all found to explode with a quite similar peak intrinsic brightness, or luminosity – making them useful standard candles. (Wikipedia)

Upper limit – A statement about the maximum value some quantity can have while still being consistent with the non-detection. Here, we use the concept to place constraints on the strength of the forces from baryon minus lepton vector dark matter at different masses or frequencies. We use a 95% degree-of-belief limit, i.e., given the data there is a 95% probability that the quantity is below this limit.

Upper mass gap – The absence of black holes with masses in the range of about ~50-120 solar masses due to pair-instability supernovae. (Wikipedia)

Vacuum state and “zero-point” fluctuations – The “vacuum state” is a state in which there are no sources of light. However, it is still described by an electro-magnetic field which obeys the Heisenberg Uncertainty Principle. In particular, the vacuum state has uncertainty in “amplitude” and “phase” which are equally distributed, typically referred to as “zero-point” fluctuations. (Wikipedia)

Vector boson – A class of particles that has a spin of 1. For example, a photon is a vector boson, and it carries electromagnetic force. Here we consider a vector boson and its force which are not included in the standard model of particle physics. (Wikipedia)

Vector/scalar polarizations – Additional polarizations predicted by some alternative theories of gravity, but forbidden by general relativity.

Vela pulsar – A pulsar located in the constellation of Vela, remnant from a Supernova explosion. (Wikipedia)

Virgo – A gravitational-wave detector situated near Pisa, Italy. It is a laser interferometer with 3-km long arms. (Wikipedia)

Waveform – The behavior of the signal’s amplitude as a function of time. (Wikipedia)

Wave optics regime – Wave optics refers to the regime in which the wave properties of a gravitational wave or electromagnetic radiation become important and interference and diffraction effects must be taken into account. (Wikipedia)

X-ray(s) – Light with a large amount of energy per photon, but not as much as a gamma ray. Photons with energies between about 100 eV (electron volts) and 100 keV are generally considered X-ray photons, although there is a seamless transition between “hard” (high-energy) X-rays and gamma rays. X-rays can be produced in the collisions of high-energy particles, as well as by the decay of “excited” (temporarily elevated energy) states of electrons in atoms. Electromagnetic radiation (light) that falls in a frequency range that humans cannot see. Frequency range of 10¹⁶ – 10²⁰ Hz. (Wikipedia)

X-ray telescopes – Telescopes that measure X-rays from across the Universe. Typically, these telescopes count individual photons that come in as opposed to measuring a steady stream of light the way a radio telescope works. (Wikipedia)