Cosmic Simulations of Axion String-Wall Networks: Probing Dark Matter and Gravitational Waves for Discovery

TL;DR

This paper uses advanced 3D lattice simulations to study gravitational waves and axion emissions from string-wall networks in the early universe, refining dark matter relic abundance estimates and exploring detectability of signals across different models.

Contribution

It provides new insights into the gravitational wave spectrum and axion relic abundance from string-wall networks, especially for scenarios with multiple domain walls.

Findings

GW spectrum is nearly independent of bias term and N_DW

QCD axion model predicts undetectable GW emissions

Axion-like particles can produce detectable GW signals in nano-Hertz to milli-Hertz range

Abstract

We simultaneously study gravitational waves (GWs) and free axions emitted from axionic string-wall networks in the early universe using advanced 3D lattice simulations. Our simulations start before the Peccei-Quinn phase transition and end with the destruction of string-wall networks below the QCD scale. The axion dark matter (DM) relic abundance radiated from string-wall networks are updated and refined for the scenarios of $N_{\rm DW}>1$. In this scenario, we observe that the GW spectrum is almost independent of the bias term and $N_{\rm DW}$, and $\Omega_{\rm GW}h^2\propto f^{1.29}(f^{-0.43})$ in the IR and middle-frequency regions. After considering the constraints from DM relic abundance, we found that the QCD axion model predicts undetectable GW emissions, and the axion-like particles model allows for a detectable GW signal in the nano-Hertz to the milli-Hertz frequency range corresponding to axion masses range from KeV to TeV. For $N_{\rm DW}=1$, the GW energy density appears undetectable for QCD axions and axion-like particles.