Gravitational waves are ripples in space and time, emitted by asymmetrically accelerating sources such as black hole binaries. They were first predicted by Einstein, 100 years ago and is the only theory of his yet to be proven.
Yesterday was a big day for Gravitational Wave astronomy, the laser interferometer gravitational wave observatory (LIGO) announced the first ever direct detection of a gravitational wave emitted by the first ever observation of a binary of Kerr (spinning) black holes. You can read their papers here https://www.ligo.caltech.edu/page/detection-companion-papers
The University of Birmingham astrophysics group are involved both in the detectors and the analysis within LIGO so I made this video to commemorate the day,