LIGO, or the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. There are two LIGO observatories in the United States—one in Hanford, Washington, and the other in Livingston, Louisiana. These observatories use laser interferometry to measure the minute ripples in spacetime caused by passing gravitational waves from cosmic events, such as the collisions of black holes or neutron stars.