Axion-like particle dark matter in the linear regime of structure formation

Understand

If axion-like particles (ALPs) constitute a major part of dark matter (DM), due to the bosonic nature and a relative small mass, they could behave differently from the point like dark matter particles on the formation of the cosmic structures.

• When studying the structure formation, it is often useful to consider DM as a special fluid with a given density and a given velocity.
• ALP fluid obeys a same continuity equation comparing to the point-like collisionless DM but has a different first-order velocity equation.
• In the linear regime of structure formation, the resulted observational differences are negligible for the QCD axions but can be interesting for very light ALPs.