Axion cosmology with long-lived domain walls

TL;DR

This paper studies the cosmological effects of long-lived domain walls in axion models with multiple domain wall numbers, showing they overproduce dark matter axions and produce undetectable gravitational waves, thus constraining these models.

Contribution

It provides the first detailed lattice simulation analysis of long-lived domain walls in axion cosmology, constraining model parameters based on relic densities.

Findings

Long-lived domain walls overproduce axion dark matter.

Gravitational wave signals from these walls are too weak to detect.

Most parameter regions of such models are cosmologically excluded.

Abstract

We investigate the cosmological constraints on axion models where the domain wall number is greater than one. In these models, multiple domain walls attached to strings are formed, and they survive for a long time. Their annihilation occurs due to the effects of explicit symmetry breaking term which might be raised by Planck-scale physics. We perform three-dimensional lattice simulations and compute the spectra of axions and gravitational waves produced by long-lived domain walls. Using the numerical results, we estimated relic density of axions and gravitational waves. We find that the existence of long-lived domain walls leads to the overproduction of cold dark matter axions, while the density of gravitational waves is too small to observe at the present time. Combining the results with other observational constraints, we find that the whole parameter region of models are excluded unless an unacceptable fine-tuning exists.