Gravitational waves—ripples in spacetime produced by merging black holes, colliding neutron stars, detonating supernovae and other cosmic cataclysms—have sparked a revolution in astrophysics. First observed in 2015, a century after Albert Einstein predicted their existence, these elusive whispers in the fabric of reality are already revealing otherwise hidden details of the exotic objects that produce them. Studies of gravitational waves have provided researchers with the first direct evidence that black holes exist, produced new estimates of the cosmic expansion rate, and shown that neutron stars are the main sources of the universe's supply of gold, platinum and other heavy elements. Eventually they could allow researchers to glimpse the universe as it was in the first fractions of a second after the big bang.