# Foamy Dark Matter from Monodromies

## TL;DR

This paper explores axion-like particle dark matter with monodromy potentials, revealing how non-linear dynamics lead to large fluctuations that impact structure formation and dark matter density.

## Contribution

It demonstrates the non-linear evolution of ALP dark matter with monodromy, showing how fluctuations dominate and influence cosmological structures.

## Key findings

- Complete conversion of homogeneous field into fluctuations for strong wiggles
- Dark matter density exhibits large fluctuations at sub-astronomical scales
- Fluctuations affect structure formation and current dark matter density constraints

## Abstract

We investigate the dynamics of axion-like particle (ALP) dark matter where the field range is enlarged by a monodromy. The monodromy potential allows sufficient production of dark matter also at larger couplings to the Standard Model particles. The potential typically features a number of "wiggles" that lead to a rapid growth of fluctuations. Using classical-statistical field theory simulations we go beyond the linear regime and treat the system in the non-linear and even non-perturbative regime. For sufficiently strong wiggles the initially homogeneous field is completely converted into fluctuations. The fluctuations correspond to dark matter particles with a non-vanishing velocity and we consider the corresponding restrictions from structure formation as well as the effects on today's dark matter density. Since all the dark matter is made up from these strong fluctuations, the dark matter density features large, $\mathcal{O}(1)$ fluctuations at scales $\lesssim 10^{6}\,{\rm km}\sqrt{{\rm eV}/m_a}$.