  The LIGO and Virgo gravitational-wave detectors carried out the first half of
their third observing run from April through October 2019. During this period,
they detected 39 new signals from the coalescence of black holes or neutron
stars, more than quadrupling the total number of detected events. These
detections included some unprecedented sources, like a pair of black holes with
unequal masses (GW190412), a massive pair of neutron stars (GW190425), a black
hole potentially in the supernova pair-instability mass gap (GW190521), and
either the lightest black hole or the heaviest neutron star known to date
(GW190814). Collectively, the full set of signals provided astrophysically
valuable information about the distributions of compact objects and their
evolution throughout cosmic history. It also enabled more constraining and
diverse tests of general relativity, including new probes of the fundamental
nature of black holes. This review summarizes the highlights of these results
and their implications.
