Gravitational Waves from Global Cosmic Strings and Cosmic Archaeology

TL;DR

This paper provides an extensive analysis of gravitational wave signals from global cosmic strings, incorporating recent developments, model uncertainties, and non-standard cosmologies, highlighting their potential to probe the early universe before BBN.

Contribution

It offers a comprehensive, updated analytical framework for predicting GW signals from global cosmic strings, including effects of model variations and alternative cosmic histories.

Findings

GW signals from global strings are potentially detectable with current and future detectors.

Variations in model parameters can significantly influence the GW spectrum.

The GW background can probe cosmic history up to temperatures of 10^8 GeV.

Abstract

Global cosmic strings are predicted in many motivated extensions to the Standard Model of particle physics, with close connections to axion dark matter physics. Recent studies suggest that, although subdominant relative to Goldstone emission, gravitational wave (GW) signals from global strings can be detectable with current and planned GW detectors such as LIGO, LISA, DECIGO/BBO, ET/CE and AEDGE/AION, as well as pulsar timing arrays such as PPTA, NANOGrav and SKA. This work is an extensive, updated study on GWs from a global cosmic string network, taking into account of the most recent developments related to the subject. The main analysis is based on the analytical Velocity-dependent One-Scale (VOS) model calibrated with recent simulation results, which provides a generic protocol for such calculations with details given. We also demonstrate how the GW signal can be influenced with variations to the baseline model: this includes considering the uncertainties of model parameters and the potential deviation from the conventional VOS model prediction (i.e. the scaling behavior) as suggested by some of the recent simulation results. Furthermore, we investigated in detail the effect of a non-standard cosmology (e.g. early matter domination or kination) or new particle species on the GW signals from global strings. We demonstrate that the frequency spectrum of GW background from global cosmic strings can be used to probe the cosmic history prior to the Big Bang nucleosynthesis (BBN) (i.e. the primordial dark age) up to a temperature of $T\sim 10^8$ GeV.