  Gravitational waves are ripples in spacetime generated by the acceleration of
astrophysical objects. A direct consequence of general relativity, they were
first directly observed in 2015 by the twin Laser Interferometer
Gravitational-Wave Observatory (LIGO) observatories. I review the first five
years of gravitational wave detections. More than fifty gravitational waves
events have been found, emitted by pairs of merging compact objects such as
neutron stars and black holes. These signals yield insights into the formation
of compact objects and their progenitor stars, enable stringent tests of
general relativity and constrain the behavior of matter at densities higher
than an atomic nucleus. Mergers that emit both gravitational and
electromagnetic waves probe the formation of short gamma ray bursts, the
nucleosynthesis of heavy elements, and measure the local expansion rate of the
Universe.
