As Einstein laid out in his general theory of relativity, gravity is a result of space and time being warped by the presence of mass, much like an electric field is warped by the presence of a charge. When a mass accelerates, it changes the curvature of space-time, producing a gravitational wave. This wave then ripples out through the cosmos at the speed of light, slightly distorting the fabric of space-time as it goes.
LIGO, a sophisticated pair of laser interferometers separated by a distance of nearly 2,000 miles, is designed to detect the largest of these faint cosmic ripples. And on September 14, 2015, it did. After traveling over 1.3 billion light-years, gravitational waves generated by the merger of two closely orbiting black holes finally reached Earth, producing an iconic "chirp" from LIGO, proving Einstein was right.