Gravitational Waves from Broken Cosmic Strings: The Bursts and the Beads

TL;DR

This paper studies gravitational wave signals from metastable cosmic strings with breakage events caused by monopoles, predicting detectable bursts and backgrounds for current and future gravitational wave observatories.

Contribution

It introduces a model of cosmic string breakage by monopoles and analyzes the resulting gravitational wave signatures, including bursts and a scale-invariant stochastic background.

Findings

Detectable gravitational wave bursts from cosmic string breakage at LIGO and LISA.

A scale-invariant stochastic background from ultra-relativistic beads.

Constraints on cosmic string tensions as low as Gμ ~ 10^{-12}.

Abstract

We analyze the gravitational wave signatures of a network of metastable cosmic strings. We consider the case of cosmic string instability to breakage, with no primordial population of monopoles. This scenario is well motivated from GUT and string theoretic models with an inflationary phase below the GUT/string scale. The network initially evolves according to a scaling solution, but with breakage events resulting from confined monopoles (beads) being pair produced and accelerated apart. We find these ultra-relativistic beads to be a potent source of gravitational waves bursts, detectable by Initial LIGO, Advanced LIGO, and LISA. Indeed, Advanced LIGO could observe bursts from strings with tensions as low as $G\mu \sim 10^{-12}$. In addition, we find that ultra-relativistic beads produce a scale-invariant stochastic background detectable by LIGO, LISA, and pulsar timing experiments. The stochastic background is scale invariant up to Planckian frequencies. This phenomenology provides new constraints and signatures of cosmic strings that disappear long before the present day.