A form of energy that is emitted by an accelerating mass in the form of waves that travel through spacetime at the speed of light. Gravitational radiation is different in nature from normal electromagnetic radiation and has different causes. Pulses of gravitational radiation are expected to be emitted by supernovae, objects falling into black holes, and the mergers of neutron stars or black holes in binary systems. Short-period cataclysmic binaries (orbital periods less than about two hours) are also believed to evolve to shorter orbital periods as a result of the emission of gravitational radiation. The first direct detection of gravitational radiation was made in 2015 September by the Laser Interferometer Gravitational-wave Observatory (LIGO), which consists of a pair of detectors in Hanford, Washington state, and Livingston, Louisiana. The radiation is thought to have been emitted by the merger of a pair of black holes in a galaxy over 1 billion l.y. from Earth. Each star had a mass about 30 times that of the Sun; in the merger about three solar masses were converted into energy and emitted as gravitational waves. LIGO made a second detection of gravitational waves from another source in 2015 December, although in this case the black holes involved were less massive and the signal was weaker. In addition, the decay in the orbital period of the double pulsar J0737–3039AB is attributed to loss of energy through gravitational radiation, although the waves have not been detected directly.

https://www.ligo.caltech.edu/page/what-is-ligo Official web page of the Laser Interferometer Gravitational-Wave Observatory (LIGO).