Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

5
The categories of this paper are: gr-qc
4
The categories of this paper are: cond-mat
3
The categories of this paper are: physics
31
The categories of this paper are: ao-sci
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
18
The categories of this paper are: econ
37
The categories of this paper are: supr-con
32
The categories of this paper are: atom-ph
33
The categories of this paper are: plasm-ph
10
The categories of this paper are: q-bio
21
The categories of this paper are: adap-org
11
The categories of this paper are: hep-lat
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
2
The categories of this paper are: math