Bose Einstein condensation of the classical axion field in cosmology?

Understand

The axion is a motivated cold dark matter candidate, which it would be interesting to distinguish from weakly interacting massive particles.

• Sikivie has suggested that axions could behave differently during non-linear galaxy evolution, if they form a Bose-Einstein condensate, and argues that ``gravitational thermalisation'' drives them to a Bose-Einstein condensate during the radiation dominated era.
• Using classical equations of motion during linear structure formation, we explore whether the gravitational interactions of axions can generate entropy.
• At linear order in Newton's constant, we interpret that the principle activities of gravity are to expand the Universe and grow density fluctuations.