On Dec. 26, 2015, the entire planet experienced a spacetime disturbance in the form of gravitational waves. These waves are far too slight to affect our everyday lives, but their detection marks a revolution in physics. Scientists of the Laser Interferometer Gravitational-wave Observatory, or LIGO, had already discovered gravitational waves a few months earlier on Sept. 14, but detecting these waves for the second time confirmed it wasn't a fluke: we really do live in a universe awash with gravitational waves and we can use them to "see" a previously dark dimension of the cosmos, such as previously invisible black hole mergers.
Image: A graphical representation of two black holes rapidly spinning before merging as one. The spiral is gravitational waves being generated by this violent event. Credit: Caltech/MIT/LIGO