LIGO, or the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment and observatory designed to detect gravitational waves. It consists of two massive detectors located in Hanford, Washington and Livingston, Louisiana, USA. These detectors use laser interferometry to measure tiny changes in distance between mirrors placed at the ends of 4 km long vacuum tubes. The LIGO detector network is the most sensitive instrument ever built for observing gravitational waves.
Gravitational waves are ripples in space-time that are produced by some of the most energetic events in the Universe, such as colliding black holes or neutron stars. These waves travel at the speed of light and are incredibly hard to detect due to their extremely weak nature. LIGO was designed specifically for detecting these types of signals from astrophysical sources.
Since beginning operations in 2002, LIGO has not yet detected any gravitational waves directly. However, indirect evidence for their existence has been found through observations of binary star systems. In 2015, LIGO made its first direct detection of gravitational waves from two merging black holes using both of its detectors simultaneously. This groundbreaking discovery opened up a new window onto the Universe and ushered in the era of gravitational wave astronomy.
LIGO is currently undergoing an upgrade called Advanced LIGO (aLIGO), which will increase its sensitivity by an order of magnitude compared to the original design. With this upgrade, LIGO should be able to detect signals from much more distant sources, such as colliding supermassive black holes located at cosmological distances. aLIGO is expected to begin taking data sometime in 2019–2020.