Gravitational waves from low-scale cosmic strings

TL;DR

This paper explores unique gravitational wave signatures from low-scale cosmic strings, revealing features like a sharp cutoff frequency and oscillating patterns in the spectrum, which can be detected by future GW observatories.

Contribution

It identifies novel spectral features of GWs from low-scale cosmic strings and discusses their potential for direct detection, advancing understanding of early Universe physics.

Findings

GW spectrum exhibits a sharp cutoff frequency $f_{cut}$

Oscillating pattern in the GW spectrum with minima at multiples of $f_{cut}$

Features detectable by future experiments like BBO and DECIGO

Abstract

Cosmic strings are a common prediction in many grand unified theories and a promising source of stochastic gravitational waves (GWs) from the early Universe. In this paper, we point out that the GW signal from cosmic strings produced at a comparatively low energy scale, $v \lesssim 10^9 \textrm{GeV}$, exhibits several novel features that are not present in the case of high-scale cosmic strings. Our findings notably include (i) a sharp cutoff frequency $f_{\rm cut}$ in the GW spectrum from the fundamental oscillation mode on closed string loops and (ii) an oscillating pattern in the total GW spectrum from all oscillation modes whose local minima are located at integer multiples of $f_{\rm cut}$. These features reflect the fact that string loops produced in the early Universe fail to shrink to zero size because of GW emission within the age of the Universe, if their tension is low enough. In addition, they offer an exciting opportunity to directly probe the discrete spectrum of oscillation modes on closed string loops in GW observations. For strings produced at a scale $v \sim 10^9\textrm{GeV}$, the novel features in the GW spectrum are within the sensitivity reach of future experiments such as BBO and DECIGO.