# Early-Universe Simulations of the Cosmological Axion

## TL;DR

This paper presents high-resolution simulations of early-universe axion models, revealing the formation of ultracompact dark matter minihalos and providing insights into their primordial spectrum and implications for axion detection.

## Contribution

It introduces detailed simulations of axion-induced minihalo formation from the PQ transition to matter-radiation equality, highlighting the generated perturbation spectrum and refining axion mass estimates.

## Key findings

- Primordial perturbation spectrum characterized.
- DM density consistent with axion mass around 25.2 μeV.
- Implications for axion dark matter detection strategies.

## Abstract

Ultracompact dark matter (DM) minihalos at masses at and below $10^{-12}$ $M_\odot$ arise in axion DM models where the Peccei-Quinn (PQ) symmetry is broken after inflation. The minihalos arise from density perturbations that are generated from the non-trivial axion self interactions during and shortly after the collapse of the axion-string and domain-wall network. We perform high-resolution simulations of this scenario starting at the epoch before the PQ phase transition and ending at matter-radiation equality. We characterize the spectrum of primordial perturbations that are generated and comment on implications for efforts to detect axion DM. We also measure the DM density at different simulated masses and argue that the correct DM density is obtained for $m_a = 25.2 \pm 11.0$ $\mu\mathrm{eV}$.