Extreme Axions Unveiled: a Novel Fluid Approach for Cosmological Modeling

Abstract

Axion-like particles (ALPs) are a well-motivated dark matter candidate that solve some of the problems in the clustering of large scale structure in cosmology. ALPs are often described by a simplified quadratic potential to specify the dynamics of the axion field, and are included in cosmological analysis codes using a modified fluid prescription. In this paper we consider the extreme axion: a version of the axion with a high initial field angle that produces an enhancement (rather than a suppression) of structure on small scales around the Jeans length, which can be probed by measurements of clustering such as the eBOSS DR14 Ly-$\alpha$ forest. We present a novel method of modeling the extreme axion as a cosmological fluid, combining the Generalized Dark Matter model with the effective fluid approach presented in the \texttt{axionCAMB} software, as well as implementing a series of computational innovations to efficiently simulate the extreme axions. We find that for axion masses between $10^{-23}\mathrm{eV}\lesssim m_{\mathrm{ax}}\lesssim 10^{-22.5}\mathrm{eV}$, constraints on the axion fraction imposed by the eBOSS DR14 Ly-$\alpha$ forest can be significantly weakened by allowing them to be in the form of extreme axions with a starting angle between $\pi -10^{-1}\lesssim {\theta }_i\lesssim \pi -10^{-2}$. This work motivates and enables a more robust hydrodynamical analysis of extreme axions in order to compare them to high-resolution Ly-$\alpha$ forest data in the future.

Comments: Minor edits and additions in response to journal referee comments, and a new Fig 11 illustratring the non-trivial range of axion starting angles that can alleviate observational constraints