Stochastic Gravitational Wave Background from Global Cosmic Strings

Abstract Global cosmic strings are generically predicted in particle physics beyond the Standard Model, e.g., a post-inflationary global U ( 1 ) symmetry breaking which may associate with axion-like dark matter. We demonstrate that although subdominant to Goldstone emission, gravitational waves (GWs) radiated from global strings can be observable with current/future GW detectors. The frequency spectrum of such GWs is also shown to be a powerful tool to probe the Hubble expansion rate of the Universe at times prior to the Big Bang nucleosynthesis where the standard cosmology has yet to be tested.