Simulations of axion-like particles in the post-inflationary scenario

TL;DR

This paper presents numerical simulations of axion-like particles in the post-inflationary scenario, exploring how different temperature dependencies affect dark matter production and the evolution of topological defects.

Contribution

It extends previous QCD axion simulations to a broader class of ALPs with various temperature dependencies, analyzing their nonlinear dynamics and impact on dark matter abundance.

Findings

A temperature-independent ALP model yields 25% more dark matter than standard calculations.

QCD axion models produce nearly six times less dark matter than generic ALPs.

The study quantifies the contribution of string-wall networks to dark matter across different models.

Abstract

Axions and axion-like particles (ALPs) are some of the most popular candidates for dark matter, with several viable production scenarios that make different predictions. In the scenario in which the axion is born after inflation, its field develops significant inhomogeneity and evolves in a highly nonlinear fashion. Understanding the eventual abundance and distribution of axionic dark matter in this scenario therefore requires dedicated numerical simulations. So far the community has focused its efforts on simulations of the QCD axion, a model that predicts a specific temperature dependence for the axion mass. Here, we go beyond the QCD axion, and perform a suite of simulations over a range of possible temperature dependencies labelled by a power-law index. We study the complex dynamics of the axion field, including the scaling of cosmic strings and domain walls, the spectrum of non-relativistic axions, the lifetime and internal structure of axitons, and the seeds of miniclusters. In particular, we quantify how much the string-wall network contributes to the dark matter abundance as a function of how quickly the axion mass grows. We find that a temperature-independent model produces 25\% more dark matter than the standard misalignment calculation. In contrast to this generic ALP, QCD axion models are almost six times less efficient at producing dark matter. Given the flourishing experimental campaign to search for ALPs, these results have potentially wide implications for direct and indirect searches.