New astrophysical bounds on ultralight axionlike particles

Understand

Motivated by tension between the predictions of ordinary cold dark matter (CDM) and observations at galactic scales, ultralight axionlike particles (ULALPs) with mass of the order $10^{-22}~{\rm eV}$ have been proposed as an alternative CDM candidate.

• We consider cold and collisionless ULALPs produced in the early Universe by the vacuum realignment mechanism and constituting most of CDM.
• The ULALP fluid is commonly described by classical field equations.
• However, we show that, like QCD axions, the ULALPs thermalize by gravitational self-interactions and form a Bose-Einstein condensate, a quantum phenomenon.