Radiation from Global Topological Strings using Adaptive Mesh Refinement: Methodology and Massless Modes

TL;DR

This paper uses adaptive mesh refinement simulations to study the dynamics and radiation of global topological strings, validating analytic models and exploring implications for axions and gravitational waves.

Contribution

It introduces a detailed AMR simulation approach for global strings, comparing numerical results with analytic radiation models across a range of string parameters.

Findings

Excellent agreement between simulation and analytic radiation predictions.

Backreaction decay scales with inverse tension and inverse square of amplitude.

Thin-string (Nambu-Goto) limit accurately models cosmological global string radiation.

Abstract

We implement adaptive mesh refinement (AMR) simulations of global topological strings using the public numerical relativity code, GRChombo. We perform a quantitative investigation of the dynamics of single sinusoidally displaced string configurations, studying a wide range of string energy densities $\mu \propto \ln{\lambda}$, defined by the string width parameter $\lambda$ over two orders of magnitude. We investigate the resulting massless (Goldstone boson or axion) radiation signals, using quantitative diagnostic tools to determine the eigenmode decomposition. Given analytic radiation predictions, we compare the oscillating string trajectory with a backreaction model accounting for radiation energy losses, finding excellent agreement. We establish that backreaction decay is accurately characterised by the inverse square of the amplitude being proportional to the inverse tension $\mu$ for $3\lesssim \lambda \lesssim 100$. We conclude that analytic radiation modelling in the thin-string (Nambu-Goto) limit provides the appropriate cosmological limit for global strings. We contextualise these results with respect to axions and gravitational waves produced by cosmic string networks.