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A gravitational wave (GW) is a phenomenon of general relativity (which models gravity as curves in spacetime, i.e., curvature), specifically of waves of spacetime, propagating from disturbances. Gravitational wave astronomy is, naturally, the study of such waves. Gravitational waves carry energy as they travel, i.e., some of the energy of the disturbance that caused them. Gravitational wave strain is essentially the amplitude of the wave.
A similar-sounding term, gravity wave is entirely different: a wave on a boundary surface of a fluid using gravity (buoyancy) as its restoring force. An ocean wave is an example.
The first indirect evidence of gravitational waves was the observation of the orbital decay of a binary system (the Hulse-Taylor Binary), which matched predictions of the effects of gravitational waves, the orbit's shrinkage due to the extraction of energy carried away by the waves. All orbits decay by this effect, but it is typically insignificant and non-detectable. Heavy objects (e.g., compact objects) in small orbits can produce waves sufficiently intense to detect directly, and a final collision between objects would be the most detectable, i.e., a gravitational wave event.
Clear detection of well-explained gravitational waves (GW detections) requires a signal that stands out from the expected background of extremely slight waves from ongoing phenomena, the gravitational wave background (GWB). For example, the expected waves of the final fall of binary black holes as they merge would have to show above the plethora of lesser waves from binary black holes not yet so close to each other and/or more distant from us. The merging of such massive objects is a runaway process: as they draw nearer, the gravitational ways they produce grow more efficient at radiating away the kinetic energy.
Among the efforts to detect gravitational waves are ground-based Michelson interferometers such as LIGO, Virgo (which have spotted waves of black hole mergers and neutron star mergers) and KAGRA, planned ground-based detectors such as the Einstein Telescope, space-based gravitational-wave detector missions such as LISA that incorporate the equivalent of Michelson interferometers, and ground-based pulsar-timing analysis efforts, including NANOGrav, Parkes Pulsar Timing Array, International Pulsar Timing Array, and European Pulsar Timing Array. Detecting GWs in pulsar-timing histories requires considerable data (possibly a decade's worth, thus considerable data storage) and processing.
The signal from a gravitational wave detection of a compact object merger is increasing GW frequency, and is referred to as a chirp. Detectable gravitational waves produced by impending binary SMBH mergers (resulting from galaxy mergers) time have extremely low frequencies for a long time, e.g., within the range of 10-9 Hz (about a cycle per year) to 10-6 Hz (about a cycle per day) and are a target of pulsar timing arrays (PTAs).
The formation of GWs requires a type of asymmetry in the motion of masses: for example, a perfectly symmetric supernova (matter ejected uniformly in all directions) will not trigger waves. An orbit does, which is why the observation of a decaying orbit (involving objects massive enough to detect such a decay) was taken as evidence of GWs. The quadrupole moment of mass produces the waves, called gravitational quadrupole radiation. Albert Einstein, in his development of general relativity, derived the quadrupole formula which describes such waves produced by a reconfiguration of mass.
The waves are transverse, affecting distances across the path of the wave. Those from orbits propagate roughly out over the plane of the orbit, making it substantially directional, maximal along all directions of the plane of the orbit. Doubling a detector's sensitivity (so as to sense a strain half as large as before) multiplies the volume of potential sources by eight, i.e., the cube of the ratio of improvement.
The first clear gravitational wave detection was in 2015. Detections have been labeled GW followed by a six-digit date. The first six accepted detections were all detected by LIGO, some with Virgo as well after it was upgraded to similar sensitivity. The six are GW150914, GW151226, GW170104, GW170608, GW170814, and GW170817. They are grouped in time because the detectors are only operating for limited periods of time, often down for maintenance and upgrades. Over time, upgrades are increasing the detectors' sensitivity and more should be detected. Having three detectors (LIGO's two detectors plus Virgo) allows the direction of the events to be limited to two possible regions of the celestial sphere totaling about 60 square degrees, greatly improving the chances of identifying other associated signals such as electromagnetic radiation; the term multi-messenger astronomy is used for studying a single phenomenon with such differing kinds of signals. As additional such detectors achieve similar sensitivity, better localization will result and more EMR counterparts will be identified.
The first six detections provided data regarding the frequency of the occurrence of such events within the volume to which the detectors are sensitive. Additionally, they adjust the previous notion that neutron star mergers would be the most common source, since five out of six were black hole mergers, and the black holes merging were larger than what was expected to be detected. Subsequent operation of the detectors has produced more than 60 candidate detections as of 2/2020, and subsequent operation that began in May 2023 has recorded candidates at a rate of nearly one a day.