# Cosmological signatures of ultralight dark matter with an axionlike potential

## TL;DR

This paper investigates how nonlinearities in axion-like dark matter potentials affect cosmological evolution, revealing delays in oscillation onset and small-scale instabilities that could influence structure formation.

## Contribution

It provides the first detailed analysis of nonlinear potential effects on axion-like dark matter using the CLASS code, highlighting new dynamical features.

## Key findings

- Delay in axion field oscillation onset due to nonlinearities
- Identification of tachyonic instability in linear modes
- Potential constraints on axion parameters from Ly-α observations

## Abstract

Nonlinearities in a realistic axion field potential may play an important role in the cosmological dynamics. In this paper we use the Boltzmann code CLASS to solve the background and linear perturbations evolution of an axion field and contrast our results with those of CDM and the free axion case. We conclude that there is a slight delay in the onset of the axion field oscillations when nonlinearities in the axion potential are taken into account. Besides, we identify a tachyonic instability of linear modes resulting in the presence of a bump in the power spectrum at small scales. Some comments are in turn about the true source of the tachyonic instability, how the parameters of the axionlike potential can be constrained by Ly-$\alpha$ observations, and the consequences in the stability of self-gravitating objects made of axions.