Gravitational Waves from Quasi-stable Strings

TL;DR

This paper models the gravitational wave spectrum from quasi-stable cosmic strings, showing how inflation and monopole production suppress low-frequency signals, aligning with observational bounds and accessible to future experiments.

Contribution

It introduces a new model of cosmic strings that are effectively stable due to monopole production and inflation, modifying the gravitational wave spectrum predictions.

Findings

Suppressed low-frequency gravitational wave spectrum compared to stable strings.

Compatible with current PPTA bounds on string tension.

Predicts detectable signals for upcoming experiments within specified tension range.

Abstract

We estimate the stochastic gravitational wave spectrum emitted from a network of cosmic strings in which the latter are effectively stable against breaking by monopole pair creation. The monopoles are produced at a higher scale from an earlier symmetry breaking and experience significant inflation before reentering the horizon. This gives rise to monopole-antimonopole pairs connected by string segments and the string loop formation essentially ceases. As a consequence, the lower frequency portion of the gravitational wave spectrum is suppressed relative to the no-inflation case with stable strings, which evades the stringent PPTA bound on the dimensionless string tension $G\mu$. We display the modified spectrum, accessible in the ongoing and future experiments, for $G\mu$ values in the range $10^{-10} - 10^{-15}$. We show how this `quasi-stable' string network is realized in realistic grand unified theories.