[Paper Review] The dark matter is mostly an axion BEC

P. Sikivie|arXiv (Cornell University)|Dec 7, 2010

Scientific Research and Discoveries1 references3 citations

TL;DR

This paper proposes that dark matter is primarily axions forming a Bose-Einstein condensate (BEC), which leads to net overall rotation in galactic halos and produces distinct caustic ring structures in phase space. Observational support for caustic rings in isolated disk galaxies like the Milky Way aligns precisely with axion BEC predictions but contradicts the irrotational, chaotic phase space structure expected from WIMPs or sterile neutrinos.

ABSTRACT

Axions differ from ordinary cold dark matter, such as WIMPs or sterile neutrinos, because they form a Bose-Einstein condensate (BEC). As a result, axions accreting onto a galactic halo fall in with net overall rotation. In contrast, ordinary CDM accretes onto galactic halos with an irrotational velocity field. The inner caustics are different in the two cases. It is shown that if the dark matter is axions, the phase space structure of the halos of isolated disk galaxies, such as the Milky Way, is precisely that of the caustic ring model for which observational support exists. The other dark matter candidates predict a far more chaotic phase space structure for galactic halos.

Motivation & Objective

• To resolve the long-standing problem of dark matter identity by testing axion BEC against alternative cold dark matter candidates.
• To explain the observed phase space structure of galactic halos, particularly the presence of caustic rings in isolated disk galaxies.
• To demonstrate that axion BEC uniquely produces net overall rotation and ordered caustic structures, unlike other CDM models.
• To validate the caustic ring model using tidal torque theory and self-similarity arguments in cosmological evolution.
• To establish that axion BEC is the only CDM candidate consistent with both observed halo kinematics and phase space morphology.

Proposed method

• Model axion dark matter as a Bose-Einstein condensate (BEC) using the Klein-Gordon action for the axion field with mass m and decay constant f.
• Calculate the number density and momentum dispersion of cold axions from the QCD phase transition, showing they are in a highly occupied quantum state.
• Apply Bose-Einstein condensation criteria: high occupation number (10^61) and low effective temperature, confirming BEC formation despite weak coupling.
• Use tidal torque theory to derive the time evolution of angular momentum on the turnaround sphere, showing time-independent rotation axis and ℓ(t) ∝ t^(5/3).
• Compare the predicted phase space structure—caustic rings in the galactic plane—with the observed structure in isolated disk galaxies.
• Contrast axion BEC behavior with WIMPs and sterile neutrinos, which produce curl-free, irrotational velocity fields and tent-like inner caustics.

Experimental results

Research questions

• RQ1 Can the observed caustic ring structure in galactic halos be explained by a Bose-Einstein condensate of axions?
• RQ2 How does axion BEC produce net overall rotation in galactic halos, unlike other cold dark matter candidates?
• RQ3 Is the time dependence of angular momentum on the turnaround sphere consistent with tidal torque theory and the caustic ring model?
• RQ4 Why do WIMPs and sterile neutrinos fail to reproduce the ordered, planar caustic structure observed in disk galaxies?
• RQ5 What distinguishes the phase space structure of axion BEC halos from that of conventional cold dark matter models?

Key findings

• Axion BEC forms due to extremely high occupation number (~10^61) and low effective temperature, satisfying the conditions for Bose-Einstein condensation despite weak interactions.
• Axion BEC produces a net overall rotation in galactic halos, resulting in caustic rings confined to the galactic plane, which matches observational data.
• In contrast, WIMPs and sterile neutrinos produce irrotational, curl-free velocity fields and tent-like inner caustics with random orientations.
• The time evolution of specific angular momentum on the turnaround sphere follows ℓ(t) ∝ t^(5/3), consistent with tidal torque theory and the caustic ring model for ε = 0.33.
• The self-similarity of the angular momentum distribution and the constancy of the rotation axis over time are predicted by axion BEC and confirmed by observations.
• Axion BEC is the only dark matter candidate that simultaneously explains the observed phase space structure, rotation, and time evolution of galactic halos.