Gravitational wave spectrum from expanding string loops on domain walls: Implication for nanohertz pulsar timing array signal

TL;DR

This paper analytically derives the gravitational wave spectrum emitted by expanding string loops on domain walls, revealing a unique linear frequency dependence and implications for nanohertz pulsar timing array observations.

Contribution

It introduces a model-independent analytical calculation of the GW spectrum from domain wall string loops, highlighting a distinctive linear frequency dependence at high frequencies.

Findings

The GW spectrum is linearly proportional to frequency at high frequencies.

String loops radiate GWs even when perfectly circular, unlike bubble collision sources.

The results are relevant for interpreting nanohertz pulsar timing array signals.

Abstract

We analytically calculate the spectrum of stochastic gravitational waves (GWs) emitted by expanding string loops on domain walls in the scenario where domain walls decay by nucleation of string loops. By introducing macroscopic parameters characterizing the nucleation of the loops, the stochastic GW spectrum is derived in a way that is independent of the details of particle physics models. In contrast to GWs emitted from bubble collisions of the false vacuum decay, the string loops do radiate GWs even when they are perfectly circular before their collisions, resulting in that more and more contribution to the spectrum comes from the smaller and smaller loops compared to the typical size of the collided loops. Consequently, the spectrum is linearly proportional to the frequency at the high-frequency region, which is peculiar to this GW source. Furthermore, the results are compared with the recent nano-Hertz pulsar timing array signal, as well as the projected sensitivity curves of future gravitational wave observatories.