Stochastic gravitational-wave background from metastable cosmic strings

TL;DR

This paper calculates the complete stochastic gravitational-wave background from metastable cosmic strings, showing it could explain recent pulsar timing signals and be detectable by current and future gravitational wave observatories.

Contribution

It provides the first comprehensive computation of the gravitational-wave background from metastable cosmic strings, including loop and segment contributions with technical improvements.

Findings

Spectrum consistent with NANOGrav and Parkes signals for Gμ ~ 10^-11 to 10^-7.

Main contribution from string loops in the relevant parameter space.

Detectable signals in ground- and space-based detectors for certain string tensions.

Abstract

A metastable cosmic-string network is a generic consequence of many grand unified theories (GUTs) when combined with cosmic inflation. Metastable cosmic strings are not topologically stable, but decay on cosmic time scales due to pair production of GUT monopoles. This leads to a network consisting of metastable long strings on superhorizon scales as well as of string loops and segments on subhorizon scales. We compute for the first time the complete stochastic gravitational-wave background (SGWB) arising from all these network constituents, including several technical improvements to both the derivation of the loop and segment contributions. We find that the gravitational waves emitted by string loops provide the main contribution to the gravitational-wave spectrum in the relevant parameter space. The resulting spectrum is consistent with the tentative signal observed by the NANOGrav and Parkes pulsar timing collaborations for a string tension of G\mu ~ 10^-11...-7 and has ample discovery space for ground- and space-based detectors. For GUT-scale string tensions, G\mu ~ 10^-8...-7, metastable strings predict a SGWB in the LIGO-Virgo-KAGRA band that could be discovered in the near future.