Bose-Einstein Condensation of Dark Matter Axions (0901.1106v4)

Abstract: We show that cold dark matter axions thermalize and form a Bose-Einstein condensate. We obtain the axion state in a homogeneous and isotropic universe, and derive the equations governing small axion perturbations. Because they form a BEC, axions differ from ordinary cold dark matter in the non-linear regime of structure formation and upon entering the horizon. Axion BEC provides a mechanism for the production of net overall rotation in dark matter halos, and for the alignment of cosmic microwave anisotropy multipoles.

• The paper shows that gravitational interactions enable axions to form a Bose-Einstein condensate, fundamentally impacting nonlinear structure formation.
• The paper uses gravitational thermalization to overcome weak axion self-interactions, ensuring continuous rethermalization of the condensate.
• The paper calculates a small Jeans length and predicts vortex formation, offering insights into anomalous rotational dynamics in galactic halos.

Bose-Einstein Condensation of Dark Matter Axions: A Summary

The paper "Bose-Einstein Condensation of Dark Matter Axions" by P. Sikivie and Q. Yang presents a compelling theoretical analysis of the potential for axions to form a Bose-Einstein Condensate (BEC) in the context of cosmology. Axions, originally proposed to address the strong CP problem in quantum chromodynamics (QCD), are well-established candidates for cold dark matter (CDM) due to their expected low mass and weak interactions.

Key Findings

The authors demonstrate that axions satisfy the conditions necessary for thermalization and subsequent formation of a BEC through gravitational interactions, a scenario largely unexplored prior to this work. The ramifications of axions achieving this condensed state are significant:

1. Difference in Structure Formation: The presence of a BEC modifies the dynamics of structure formation in the nonlinear regime. Axion BEC could influence the large-scale rotational characteristics of dark matter halos and affect CMB anisotropies. The existing evidence suggests this can result in phenomena such as vortices within the axion fluid, which are consistent with rotational features that cannot be easily explained by other CDM models.
2. Gravitational Thermalization: Although axion self-interactions appear insufficient for maintaining thermal equilibrium, gravitational interactions serve as a robust mechanism for continuous rethermalization of the axion BEC. This interaction continues to facilitate thermal equilibrium at large cosmic scales, reflecting the long-range nature of gravity.
3. Non-linear Cosmic Implications: The paper introduces the idea that an axion BEC encompasses features that could reconcile existing puzzles in dark matter research, particularly regarding rotational dynamics and velocity dispersion within galactic structures. Notably, the paper describes the occurrence of vortices that may offer a solution to discrepancies observed in the angular momentum distributions of galaxies.
4. Jeans Length Calculation: The authors derive the Jeans length for the axion condensate, finding it to be remarkably small compared to observable scales, implying that axion BEC phenomena would not affect the linear evolution of density perturbations on scales observable with current astrophysical methods.

Implications and Speculation

The implications of these findings are profound both theoretically and observationally. If axion BECs exist, they offer a unique prediction for the angular momentum distribution in galaxy halos and might contribute to the resolution of the cosmic microwave background multipole alignment problem. Furthermore, the conditioned behavior of axion BEC perturbations on entering the cosmic horizon highlights potential observational signatures distinct from other dark matter candidates.

Future advancements in observational cosmology or experimental axion searches could provide pivotal tests for the BEC hypothesis. Detection of rotational signatures or patterns corresponding to axion BEC properties would significantly bolster this model.

In sum, this paper provides a thorough and mathematically substantiated exploration of the transformative role axions could play as part of the cosmic dark matter inventory, specifically through the formation of a Bose-Einstein Condensate. The work motivates further theoretical investigations and empirical validation of axion-driven phenomena in addressing longstanding questions in cosmology.