LIGO (or Laser Interferometer Gravitational-Wave Observatory) is a pair of Gravitational Wave observatories that work in unison, in Livingston, Louisiana (LIGO L or LLO for LIGO Livingston Observatory) Hanford, Washington (LIGO H or LHO for LIGO Hanford Observatory), 3002 kilometers apart. They are Interferometers, looking for unexplained changes in distance between two lengths at right angles, i.e., Michelson Interferometers, with arms 4km in length. The delta distance they aim to detect is on the order of 10-21, which less than the width of an atomic nucleus. Other sources of distance change such as ground movements like Earthquakes, are filtered out by detecting them independently, and by the use of two separated detectors which should both register something affecting the whole Earth at once, i.e., an actual gravitational wave. The use of two detectors also allows some determination of the direction from which the wave came. The two operated from 2002 to 2010, shutting down for a planned four year upgrade as Advanced LIGO (AdLIGO) which aims at a sensitivity that covers a thousand times the volume for a given wave-strength, thus can detect a thousand times the number of events, or see something a thousand times more often. Further plans: a 2020 upgrade called A+ to double sensitivity, and a 2027 upgrade named Voyager to double the sensitivity again and also extend the Frequency range.
In Fall 2015, when Advanced LIGO first went live for testing, it detected a gravitational wave event (GW150914) suggesting a Black Hole merger. It had been predicted that the first detected waves would be from a Neutron Star merger. The second detection was in the Spring of 2016 and also appears to be a Compact Object merger.
Gravitational Wave (GW)
International Pulsar Timing Array (IPTA)
Pulsar Timing Array (PTA)