Domain Wall Formation from Level Crossing in the Axiverse

TL;DR

This paper explores how level crossing in the Axiverse can lead to chaotic axion dynamics and domain wall formation, with implications for cosmology, baryogenesis, and gravitational wave detection.

Contribution

It reveals that domain wall formation is more common in the Axiverse due to level crossing and introduces the concept of the axion roulette, a chaotic dynamical behavior during axion evolution.

Findings

Chaotic axion behavior during level crossing

Domain wall formation linked to axion roulette

Potential cosmological signatures in gravitational waves

Abstract

We point out that domain wall formation is a more common phenomenon in the Axiverse than previously thought. Level crossing could take place if there is a mixing between axions, and if some of the axions acquire a non-zero mass through non-perturbative effects as the corresponding gauge interactions become strong. The axion potential changes significantly during the level crossing, which affects the axion dynamics in various ways. We find that, if there is a mild hierarchy in the decay constants, the axion starts to run along the valley of the potential, passing through many crests and troughs, until it gets trapped in one of the minima; the {\it axion roulette}. The axion dynamics exhibits a chaotic behavior during the oscillations, and which minimum the axion is finally stabilized is highly sensitive to the initial misalignment angle. Therefore, the axion roulette is considered to be accompanied by domain wall formation. The cosmological domain wall problem can be avoided by introducing a small bias between the vacua. We discuss cosmological implications of the domain wall annihilation for baryogenesis and future gravitational wave experiments.