WKB approximation to boson dark matter

TL;DR

This paper applies the WKB approximation to solve the nonlinear Schrödinger-Poisson equations describing ultralight axion dark matter halos, showing the approximation's near-exactness and its implications for halo structure modeling.

Contribution

It introduces a WKB-based method to analyze eigenstates of ultralight axion dark matter halos, providing a new analytical approach consistent with simulation results.

Findings

WKB approximation is nearly exact for galactic halos.

Large number of bound states contribute to the halo profile.

Results align with numerical simulations.

Abstract

Galactic dark matter halos may be composed of ultralight axions (ULAs) ($m_a \lesssim 1$ eV) with wave functions that satisfy nonlinear Schr\"{o}dinger-Poisson equations (SPA). We find eigenstates of SPA in WKB approximation. The expansion parameter of the WKB approximation is $\delta=1/\sqrt{S}$, where $S=2 M R G m_a^{2}$, with $M$ being the total mass, $R$ the radius of the halo, and $G$ the gravitational constant. $S\gg 1$ for almost all galaxies, even if the ULA mass is as small as $m_a=10^{-22} $ eV, making the leading order WKB approximation almost exact. As the level spacing of bound states is roughly proportional to $\delta$, the number of states in the gravitational well is huge. We do not see a reason why not all or most of them contribute to the halo. Using an appropriate distribution function allows the summation of states to construct the profile of the halo as a function of the gravitational potential, which can be found solving the Poisson equation. Using various energy distribution functions, we obtain results similar to those in simulations. Future plans include investigations of collapse through time dependent generalizations, and inclusion of self-interactions, which also induce decay processes of the halo.