High-harmonic cosmic strings and gravitational waves

TL;DR

This thesis investigates high-harmonic cosmic string loops and their gravitational wave signals, developing models to predict observable waveforms and rates, with implications for gravitational wave detection.

Contribution

It introduces a detailed analysis of high-harmonic cosmic string loops, including their dynamics, cusp formation, and gravitational wave emission, extending previous models to include high-harmonic effects.

Findings

High-harmonic content affects the number of cusps per loop cycle.

Derived gravitational waveforms for cusp emissions in a cosmological context.

Estimated gravitational wave signal amplitudes and event rates for Earth-based detectors.

Abstract

In this thesis we describe high-harmonic cosmic string loops in a general relativistic context, and study the implications of high-harmonic content for the predicted gravitational wave signal from cosmic string networks. Initially, we introduce the variational principle, spacetime concepts and other mathematical tools that we will need for the calculations in the following chapters. We introduce the FLRW universe and the $\Lambda CDM$ universe. We then describe the Nambu-Goto cosmic string in a curved spacetime, its equations of motion and its energy-momentum tensor. Fixing the spacetime to be flat, and fixing the gauge, we find the motion of the cosmic string and we present and discuss special solutions. Using the odd-harmonic family of cosmic string loops, we calculate the number of cusps per period and the values of the second derivatives of the left- and right-moving harmonic modes at the cusp, and study their dependence on the harmonic order. We then develop a toy model that calculates the stable daughter loops produced from a parent loop using a statistical approach based on a binary tree description of the loop chopping. We also use the toy model to calculate the average number of cusps produced from a system of loops that self intersect over their course of existence. We derive the gravitational waveform emitted from a cusp as observed away from the cusp, in any direction of observation. We then propagate this result in an FLRW spacetime to reach an expression of its amplitude on Earth. Assuming two different cosmic string network models, we implement our above mentioned high-harmonic results to find the amplitude of the signal and the rate at which these signals reach an observer on Earth.