Using gravitational waves to see the first second of the Universe

Gravitational waves open a unique window on the earliest times in cosmology because the dense primordial plasma, impenetrable to light or neutrinos, is transparent to gravitational waves up to the instant of the birth of the Universe. This review discusses the possible signals from the phase transitions, topological defects, and other cosmological sources, as well as a range of strategies for detection of the stochastic gravitational waves produced by the earliest events in the history of the Universe.