Far from equilibrium dynamics of Bose-Einstein condensation for Axion Dark Matter

Understand

Axions and similar very weakly interacting particles are increasingly compelling candidates for the cold dark matter of the universe.

• Having very low mass and being produced non-thermally in the early Universe, axions feature extremely high occupation numbers.
• It has been suggested that this leads to the formation of a Bose-Einstein condensate with potentially significant impact on observation and direct detection experiments.
• In this note we aim to clarify that if Bose-Einstein condensation occurs for light and very weakly interacting dark matter particles, it does not happen in thermal equilibrium but is described by a far-from-equilibrium state.