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