Gravitational waves originated from the early universe: A review and perspective
Abstract
With the observation of gravitational wave signals from binary black hole mergers by LIGO, a new window for exploring the universe has been opened. The stochastic gravitational wave background originating from the early universe could exist on various frequency bands in the current universe, which is an important scientific goal for space-based gravitational wave detectors and pulsar timing arrays. In this paper, we review significant progress in this field, particularly focusing on primordial gravitational waves from inflation, scalar-induced secondary gravitational waves, gravitational waves originating from topological defects such as cosmic strings and domain walls, and those generated by the bubble walls and sound waves from first-order phase transition. The characteristic spectral shapes of these gravitational wave signals are critical scientific objectives for future gravitational wave detectors, offering valuable insights into the laws of nature governing the very early universe and high-energy physics.
