Non-linear hydrodynamics of axion dark matter: relative velocity effects and "quantum forces"

TL;DR

This paper derives non-linear hydrodynamic equations for axion dark matter, incorporating effects of expansion and Hubble drag, and explores their implications for cosmic structure formation and potential observational constraints.

Contribution

It introduces a simplified derivation of hydrodynamic equations for axion DM, including a moving-background perturbation theory and a quantum force analogy for non-linear modeling.

Findings

Axion mass does not affect the coherence length of relative velocity.

Modified MBPT equations include axion effective sound speed.

Potential to improve axion mass constraints via 21cm power spectrum.

Abstract

The non-linear hydrodynamic equations for axion/scalar field dark matter (DM) in the non-relativistic Madelung-Shcr\"{o}dinger form are derived in a simple manner, including the effects of universal expansion and Hubble drag. The hydrodynamic equations are used to investigate the relative velocity between axion DM and baryons, and the moving-background perturbation theory (MBPT) derived. Axions massive enough to be all of the DM do not affect the coherence length of the relative velocity, but the MBPT equations are modified by the inclusion of the axion effective sound speed. These MBPT equations are necessary for accurately modelling the effects of axion DM on the formation of the first cosmic structures, and suggest that the 21cm power spectrum could improve constraints on axion mass by up to four orders of magnitude with respect to the current best constraints. A further application of these results uses the "quantum force" analogy to model scalar field gradient energy in a smoothed-particle hydrodynamics model of axion DM. Such a model can treat axion DM in the non-linear regime and could be incorporated into existing N-body codes.