LIGO

LIGO (or Laser Interferometer Gravitational-wave Observatory) is a pair of gravitational-wave detectors that work in unison, located in Livingston, Louisiana (LIGO L or LLO for LIGO Livingston Observatory) and Hanford, Washington (LIGO H or LHO for LIGO Hanford Observatory), 3002 kilometers separated. The detectors are interferometers, watching for otherwise-unexplained changes in distance between two lengths at right angles to each other, i.e., Michelson interferometers with arms 4 km in length. Gravitational waves (GWs) are "ripples in space itself" such that distances between two points in space change by tiny amounts far too small for us to perceive. The detectors look for effects on the light travel time over the 4 km arms, looking to sense tiny distance-changes that are less than the width of a proton. By nature, the instrument is most sensitive for a wave whose wave-period is twice the interferometer's light path's time of travel, and LIGO uses a Fabry-Pérot cavity, reflecting the light multiple times over the 4 km to put it in the most sensitive regime. It also catches the leftover light that would be lost, reflecting it back to increase the effective power of the laser (a power recycling mirror).

Other sources of distance-change such as ground movements (e.g., from earthquakes) are filtered out by watching for such ground movements with other equipment, and by the use of two well-separated GW detectors which should simultaneously register anything affecting the whole Earth at once, i.e., an actual gravitational wave. The use of pair of detectors also allows some determination of the direction from which the wave came. The pair operated from 2002 to 2010, shutting down for a planned four-year upgrade to become Advanced LIGO (AdLIGO or aLIGO), increasing the sensitivity to detect events of a given wave-strength throughout a larger portion of the universe, a thousand times larger by volume, multiplying the number of events detectable by a thousand. A project to incorporate additional improvements, aiming to double the sensitivity in the early 2020s was called A+ (Advanced plus), and some of the improvements are in service in 2024. Yet another future such project for further upgrades in the latter 2020s has been called Voyager or A# (Advanced sharp), with the intention of doubling the sensitivity again and also extend the frequency range.

In 2015-2017, LIGO detected several gravitational wave events, which analysis showed are generally from black hole mergers, taking place with a redshift in the 0.1-to-0.2 general range. It had been previously thought that neutron star merger detections would be the more common detections. The first GW detection (GW150914) was during Advanced LIGO's September 2015 initial testing, and the second was in December 2015.

LIGO's time alternates between considerable periods of maintenance/upgrades versus periods of observation, the latter periods generally lasting several months (or even years) each. The earliest six observation periods (which were before 2015), termed Science Run 1 through 6, had no detections. With the upgrade to Advanced LIGO, the periods of observation have been labeled Observing Runs:

During the O1/O2 time frame, six candidate-detections were accepted as real, but since then, five more from that time period have been accepted as well. With Virgo's upgrades and the subsequent completion of KAGRA, the observation runs of all three are being coordinated so as to have all the three or four detectors active simultaneously because that provides very useful additional data for the detected events (including additional chance of confirming a particular detection through at least two of the detectors recording analogous signals).

LIGO began its O4 science run (with some of the intended A+ upgrades, with a sensitivity increase on the order of 30%) on May 24, 2023. After a scheduled maintenance outage in early 2024, it resumed official operation on April 10, 2024 with plans to end O4 in February 2025.