Primordial Gravitational Waves

Abstract

Cosmological inflation represents a successful paradigm for early universe cosmology, which makes well-definite predictions in excellent agreement with current observations. One of these predictions, the existence of a stochastic gravitational wave background from the early universe, has not been tested yet and represents one of the major challenges and opportunities for future research. In this chapter, we provide a brief theoretical overview of the mechanism underlying gravitational wave production during inflation. We explain how the detection of inflationary gravitational waves can provide invaluable information on the physics driving inflation, as well as on aspects of high-energy physics and gravity at the highest energy scales. In particular, we discuss how different realizations of inflation – from single-field inflation to more sophisticated scenarios motivated from high-energy physics – lead to distinctive properties for the gravitational wave background they produce. Given the future prospects on the observational and experimental side of gravitational wave physics, a detailed theoretical analysis of these properties will be essential for a correct interpretation of data. The theoretical ideas we discuss here aim to provide a general orientation on well-motivated physical effects from the early universe, to be searched for with experiments aimed to detect a stochastic background of gravitational waves.