Gravitational Wave Emission from a Cosmic String Loop, I: Global Case

TL;DR

This study uses field theory simulations to analyze the decay of global string loops into scalar particles and gravitational waves, finding that GW emission is significantly suppressed compared to particle emission, impacting predictions of GW backgrounds in axion models.

Contribution

It provides the first detailed simulation-based quantification of gravitational wave emission from global string loops, showing suppression relative to scalar particle emission across various initial conditions.

Findings

GW emission is suppressed by a factor of about 10 compared to scalar particles.

The decay timescale depends on initial loop length, energy, and angular momentum.

Results are robust across different loop ratios and initial conditions.

Abstract

We study the simultaneous decay of global string loops into scalar particles (massless and massive modes) and gravitational waves (GWs). Using field theory simulations in flat space-time of isolated loops with initial length $\sim 80-1700$ times their core width, we determine the power emitted into scalar particles, $P_{\varphi}$, and GWs, $P_{\rm GW}$, and characterize the loop decay timescale as a function of its initial length, energy and angular momentum. We quantify infrared and ultraviolet lattice dependencies of our results. For all type of loops and initial conditions considered, GW emission is always suppressed compared to particles as $P_{\rm GW}/P_{\varphi} \approx \mathcal{O}(10)(v/m_\text{p})^2\ll 1$, where $v$ is the vacuum expectation value associated with string formation. These conclusions are robust for the length-to-width ratios considered, with no indication they should change if the ratio is increased. The results suggest that the GW background from a global string network, such as in dark matter axion scenarios, will be suppressed compared to previous expectations.