Axion domain walls and thermal friction

TL;DR

The work addresses axion dark matter production in the post-inflationary PQ scenario, where a network of domain walls significantly influences the final axion abundance. It introduces an analytical framework based on nonequilibrium quantum field theory, leveraging the Schwinger-Keldysh formalism and the 2PI effective action to model domain-wall dynamics within a Sine-Gordon potential. By coarse-graining and assuming isotropy, it derives effective transport equations for macroscopic network quantities, notably the average energy density $\rho$ and rms velocity $v$, incorporating thermal, self-interaction, plasma-induced and quantum effects through generalized collision terms. Preliminary results show thermal friction can slow wall motion and modify scaling behavior, offering a controlled pathway to refine axion abundance estimates for QCD axions and high-mass axion-like particles.

Abstract

The post-inflationary Peccei-Quinn symmetry-breaking scenario provides a rich theoretical framework to study axion dark matter production through the dynamics oftopological defects. Accurate predictions for the axion abundance require a detailed understanding of the formation and evolution of cosmic strings and domain walls, which are inevitably produced in this scenario. Most existing studies rely on large-scale numerical simulations of the classical equations of motion, which are subject to significant systematic uncertainties. In this contribution, we briefly review the main features of the post-inflationary scenario and the current limitations in the literature, and present preliminary results from a new analytical framework for axion domain wall networks. Our approach is based on nonequilibrium quantum field theory. It employs the two-particle-irreducible effective action to derive effective Fokker-Planck equations for macroscopic quantities such as the average energy density and root-mean-square velocity of the network.We discuss the relevance of thermal, self-interactions, plasma-induced and quantum effects for cosmological axion abundance estimates, with applications to QCD axions and high-mass axion-like particles.

Figures (1)

• Figure 1: Evolution of the rescaled rms velocity (right) of an axion domain wall network as a function of conformal time $\eta$. We observe that the point of parameter space we consider here is far from the usual friction-dominated region of the literature (usually $m_a \gtrsim 10^{8}\,\rm{eV}$ and larger $g_{a \gamma \gamma}$). Figure from Ref. Favitta:2025neq, where we also show the dynamical evolution of the rescaled energy density $\rho \eta$.