Gravitational waves from black hole merger
This image, extracted from a numerical simulation, depicts the gravitational waves that are released when two supermassive black holes merge.
Supermassive black holes are found at the centre of most massive galaxies. When two or more galaxies merge, the central supermassive black holes sink to the centre of the new galactic system and coalesce. These are among the most energetic events in the Universe, and are expected to be one of the strongest sources of gravitational waves.
Gravitational waves are disturbances to the structure of spacetime, produced by some accelerated massive bodies. They have not yet been directly detected, but indirect proof of their existence was found in the late 1970s.
LISA Pathfinder is paving the way for the detection of gravitational waves from space by testing the technology needed for future missions.
(Update 12 February 2016: high-frequency gravitational waves, emitted by a pair of merging black holes, were directly detected for the first time with the Advanced Laser Interferometer Gravitational-Wave Observatory.)