Nanohertz Gravitational Waves from Axion Domain Walls Coupled to QCD

Abstract

We show that the recently reported NANOGrav, EPTA, PPTA, and CPTA data suggesting the existence of stochastic gravitational waves in the nanohertz region can be explained by axion domain walls coupled to QCD. In this scenario, the non-perturbative effects of QCD generate a temperature-dependent bias for the domain wall around the QCD phase transition, leading to an immediate collapse of the domain walls. We perform dedicated lattice simulations of the axion domain walls, taking into account the temperature dependence of the bias, to estimate the gravitational waves emitted during the domain wall annihilation process. 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.Comment: 17 pages, 4figure