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-alpha (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 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 eV max 10-22.5 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 theta i 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.