Extreme axions unveiled: A novel fluid approach for cosmological modeling

Axion-like particles (ALPs) are a well-motivated dark matter candidate that solves 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 extended Baryonic Oscillation Spectroscopic Survey (eBOSS) DR14 Lyman-$\ensuremath{\alpha}$ (Ly-$\ensuremath{\alpha}$) 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 axionCAMB software as well as implementing a series of computational innovations to efficiently simulate the extreme axions. We find that, for axion masses between ${10}^{\ensuremath{-}23}\text{ }\text{ }\mathrm{eV}\ensuremath{\lesssim}{m}_{\mathrm{ax}}\ensuremath{\lesssim}{10}^{\ensuremath{-}22.5}\text{ }\text{ }\mathrm{eV}$, constraints on the axion fraction imposed by the eBOSS DR14 Ly-$\ensuremath{\alpha}$ forest can be significantly weakened by allowing them to be in the form of extreme axions with a starting angle between $\ensuremath{\pi}\ensuremath{-}{10}^{\ensuremath{-}1}\ensuremath{\lesssim}{\ensuremath{\theta}}_{i}\ensuremath{\lesssim}\ensuremath{\pi}\ensuremath{-}{10}^{\ensuremath{-}2}$. This work motivates and enables a more robust hydrodynamical analysis of extreme axions in order to compare them to high-resolution Ly-$\ensuremath{\alpha}$ forest data in the future.