Bose-Einstein condensate cosmology within the framework of QCD axion

A bstract Bose-Einstein Condensation (BEC) cosmology is analyzed in the framework of a string-inspired axion model. The dispersion relation of the axionic mode includes both gravitational and self-interaction terms, the latter being small in magnitude but crucial for inducing instability of the condensate. The generation rate of BEC around redshift z ≈ 30 is primarily governed by gravity, consistent with a phenomenological value Γ ≈ 10 − 31 eV adopted in previous work and realizable for the QCD axion. The relation between this early BEC epoch and the later formation of supermassive black holes at z ≈ 5 is also discussed.