Gravitational Waves from Domain Wall Collapse, and Application to Nanohertz Signals with QCD-coupled Axions

TL;DR

This paper models gravitational waves from collapsing domain walls, incorporating bias effects via lattice simulations, and links these signals to observed nanohertz gravitational waves potentially caused by QCD-coupled axion domain walls.

Contribution

It provides the first lattice simulation of domain wall collapse with bias effects and connects these gravitational waves to observed nanohertz signals, suggesting an axion QCD-coupled origin.

Findings

Gravitational waves from domain wall collapse match nanohertz observations.

QCD effects induce bias leading to rapid domain wall collapse.

Potential for future axion searches and primordial black hole detection.

Abstract

We study for the first time the gravitational waves generated during the collapse of domain walls, incorporating the potential bias in the lattice simulations. The final stages of domain wall collapse are crucial for the production of gravitational waves, but have remained unexplored due to computational difficulties. As a significant application of this new result, we show that the observed NANOGrav, EPTA, PPTA, and CPTA data, which indicate stochastic gravitational waves in the nanohertz regime, can be attributed to axion domain walls coupled to QCD. In our model, non-perturbative effects of QCD induce a temperature-dependent bias around the QCD crossover, inducing the rapid collapse of the domain walls. We use sophisticated lattice simulations that account for the temperature-dependent bias to measure the gravitational waves resulting from the domain wall annihilation. We also discuss the future prospects for accelerator-based searches for the axion and the potential for the formation and detection of primordial black holes.