# Cosmic implications of a low-scale solution to the axion domain wall problem

## TL;DR

This paper proposes a novel mechanism using the Instanton Interference Effect to solve the axion domain wall problem, avoiding fine-tuning and introducing potential dark matter and gravitational wave signals.

## Contribution

It introduces a new interaction that breaks PQ symmetry at a lower scale, providing a natural solution to the domain wall issue without fine-tuning or inverse phase transitions.

## Key findings

- Mechanism avoids domain walls without fine-tuning.
- Predicts QCD axion mass in 1-15 meV range.
- Suggests new dark matter candidates and gravitational wave signals.

## Abstract

The post-inflationary breaking of Peccei-Quinn (PQ) symmetry can lead to the cosmic domain wall catastrophe. In this Letter we show how to avoid domain walls implementing the Instanton Interference Effect (IIE) with a new interaction which itself breaks PQ symmetry and confines at an energy scale smaller than $\Lambda_{QCD}$. We give a general description of the mechanism and consider its cosmological implications and constraints within a minimal model. Contrary to other mechanisms we do not require an inverse phase transition neither fine-tuned bias terms. Incidentally, the mechanism leads to the introduction of new self-interacting dark matter candidates and the possibility of producing gravitational waves in the frequency range of SKA. Unless a fine-tuned hidden sector is introduced, the mechanism predicts a QCD axion in the mass range $1\text{ meV}-15\text{ meV}$.