The infant cry of the universe——stochastic gravitational wave background from inflation

This article recounts the origin of gravitational waves from the very birth of the universe. Cosmic inflation is the short period of accelerated expansion prior to the onset of the hot Big Bang. Initially generated from quantum fluctuations, both curvature and tensor perturbations are stretched to cosmological scales; the former serve as the seeds of large-scale structures and the anisotropies of the cosmic microwave background, while the latter form primordial gravitational waves. At the next order, the quadrupole moment of the curvature perturbations can also act as a source of gravitational waves. In particular, enhanced curvature perturbations can simultaneously form abundant primordial black holes and observable secondary gravitational waves. The latter constitute one of the key scientific goals for space-based gravitational wave detectors.