Do Cosmic String Segments Emit Gravitational Waves?

TL;DR

This paper investigates whether metastable cosmic string segments significantly contribute to the gravitational wave background, concluding that thermal fluctuations suppress their oscillations and their GW contribution is negligible.

Contribution

It demonstrates that thermal fluctuations inhibit oscillations of cosmic string segments, reducing their expected gravitational wave emissions compared to previous estimates.

Findings

Thermal fluctuations prevent string segment oscillations.

String segments' GW contribution is negligible.

Monopole collisions are frequent due to thermal effects.

Abstract

Cosmic strings are predicted in various extensions of the Standard Model, including grand unified theories. Depending on the symmetry-breaking pattern, they can be either topologically stable or metastable. Intriguingly, metastable strings have been proposed as a possible origin of the gravitational wave (GW) background observed by recent pulsar timing array experiments. When metastable strings decay, they fragment into segments with monopoles and antimonopoles attached at their endpoints. The monopole and antimonopole are strongly pulled by the string tension. Violent oscillations of these segments have been considered as a potential GW source, in addition to contributions from string loops. We show that, in realistic situations, the monopoles frequently collide with thermal fluctuations on the string segments, which act as a resistance and prevent the oscillation. As a result, we find that the contribution from string segments to the GW background is negligible.