[Paper Review] Axion BEC Dark Matter

Ozgur Erken, P. Sikivie+2 more|arXiv (Cornell University)|Jan 1, 2011

Dark Matter and Cosmic Phenomena2 references6 citations

TL;DR

This paper proposes that axion cold dark matter undergoes Bose-Einstein condensation (BEC) due to gravitational self-interactions when the photon temperature reaches ~500 eV, leading to observable differences from standard cold dark matter (CDM). The BEC rethermalizes during galactic infall, acquiring net rotation via tidal torquing, resulting in distinct, ring-like inner caustics—unlike the irrotational, 'tent-like' caustics of WIMPs—offering a testable observational signature for axion BEC dark matter.

ABSTRACT

Cold dark matter axions thermalize through gravitational self-interactions and form a Bose-Einstein condensate when the photon temperature reaches approximately 500 eV. Axion Bose-Einstein condensation provides an opportunity to distinguish axions from the other dark matter candidates on the basis of observation. The rethermalization of axions that are about to fall in a galactic potential well causes them to acquire net overall rotation, whereas ordinary cold dark matter falls in with an irrotational velocity field. The inner caustics of galactic halos are different in the two cases.

Motivation & Objective

• . The paper investigates whether axion BEC dark matter can be observationally distinguished from standard cold dark matter (CDM).
• It addresses the fundamental question of when axions behave as CDM versus when they deviate due to quantum condensation.
• The objective is to determine if axion BEC leads to observable dynamical differences, particularly in halo structure and caustic formation.
• It focuses on the role of gravitational self-interactions in enabling thermalization and BEC formation in axion dark matter.

Proposed method

• . The study models axion dark matter as a quantum Bose gas undergoing gravitational self-interaction.
• It uses the self-similar infall model to estimate the size L and rate of momentum change ˙p of axion regions during galactic infall.
• The condition for rethermalization is derived as 4πGnm²L ≳ ˙p, ensuring axions remain in the lowest energy state.
• The analysis applies the Friedmann equation post-equality to estimate the matter density parameter and gravitational forces.
• It compares the velocity field of axion BEC (with non-zero vorticity) to that of WIMPs (irrotational), predicting distinct caustic morphologies.
• The paper evaluates the relaxation rate Γ ∼ 4πGnm²ℓ² from gravitational scattering, showing it exceeds Hubble rate at ~500 eV, enabling BEC formation.

Experimental results

Research questions

• RQ1 . Can axion dark matter form a Bose-Einstein condensate through gravitational self-interactions, and at what temperature does this occur?
• RQ2 . What are the observable consequences of axion BEC formation compared to standard cold dark matter?
• RQ3 . How does tidal torquing from neighboring galaxies affect the angular momentum and velocity field of infalling axion BEC?
• RQ4 . Does rethermalization of axion BEC during galactic infall lead to a rigidly rotating state with non-zero vorticity?
• RQ5 . Can the resulting caustic structure in galactic halos—specifically ring-like caustics—distinguish axion BEC from WIMP dark matter?

Key findings

• . Axion BEC forms when the photon temperature reaches approximately 500 eV, corresponding to f ≈ 10¹² GeV.
• The relaxation rate due to gravitational self-interactions exceeds the Hubble rate at this temperature, enabling thermalization and BEC formation.
• Axion BEC rethermalizes during galactic infall, maintaining the lowest energy state consistent with angular momentum from tidal torquing.
• The resulting velocity field of axion BEC has non-zero vorticity (∇×v ≠ 0), unlike the irrotational flow of WIMPs.
• This leads to the formation of ring-like inner caustics in galactic halos, in contrast to the 'tent-like' caustics predicted for WIMPs.
• The observed presence of caustic rings in galaxies is consistent with the phase-space structure predicted by axion BEC dark matter.