Stochastic Gravitational Wave Background from Global Cosmic Strings

TL;DR

This paper shows that gravitational waves from global cosmic strings, predicted in theories beyond the Standard Model, could be detectable and serve as probes of the early universe's expansion rate before Big Bang nucleosynthesis.

Contribution

It demonstrates the potential observability of gravitational waves from global strings and their use in probing pre-BBN cosmology, which was previously unexplored.

Findings

GWs from global strings are potentially detectable with current or future detectors.

The GW spectrum can reveal information about the Hubble rate before BBN.

Global strings can produce observable signals despite being subdominant to Goldstone emission.

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 or 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.