GW detection

A GW detection (gravitational wave detection) is the reception of a gravitational wave (GW) pattern that is recognized as the signal of an astronomical event (a gravitational wave event or GW event) by a gravitational-wave detector such as LIGO. Detected events so far have generally been GWs from black hole mergers due to the orbital decay of a pair of co-orbiting black holes, but also include a few cases where one or both the co-orbiting objects have been neutron stars. Detecting such merger-signals was the aim of these first ground-based GW detectors: the merger process had been analyzed regarding the signal they would produce and the successful detections confirm current science of black holes and neutron stars as well as the general relativity model of gravity.

To be detectable, a signal must have sufficient amplitude and a sufficiently-distinct pattern to stand out from the gravitational wave background, i.e., all the other random GWs produced across the universe. These events combined the nearness and strength to stand out, and the singles consists of a chirp (waves rising in frequency) followed by a short period of lesser waves called the ringdown. The chirp is because the final orbits produce waves in proportion to the orbital periods, and the orbits speed up as the orbit decays. The final impact produces a few more less-regular waves from a rearrangement of masses, producing the ringdown. The signal reveals information analytically (e.g., the chirp mass), and more through numerical simulation of mergers of pairs of objects of various sizes and rotations. There is an inclination-distance degeneracy because the signal is to some degree directional: if viewed from edge-on to the orbit, the signal is twice as strong as if viewed from perpendicular to the orbit. The ringdown helps reduce this ambiguity. Any determination of the angle between the merging objects' rotational axes gives a clue regarding whether the two objects were born as a binary star versus capture subsequent to their formation.

The first six detections accepted:

Subsequently, some additional candidate detections within the above time period have been accepted as real. As of 2024 there have been over 100 detections and candidate events, including additional neutron star mergers and some neutron-star black-hole mergers. The four detectors (LIGO's two, plus Virgo and KAGRA) began an operation-run in May 2023, after some years of upgrades to the detectors: Virgo and KAGRA ran only briefly, due to issues, and LIGO ran until January 2024. In April 2024, LIGO and Virgo resumed operation, but KAGRA is under repair due to earthquake damage.

The term sub-threshold gravitational wave (sub-threshold GW) is used for apparent detections that don't meet an agreed-upon detection threshold. Such a near-detection might remain an un-confirmable candidate, but subsequent analysis of data may lead to it being dismissed or confirmed. (The term sub-threshold GW might also be taken to include the GWs passing through Earth that are so weak that they don't even raise such a suspicion.)