Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Multi-fluid cosmology in Einstein gravity: analytical solutions   We review analytical solutions of the Einstein equations which are expressed
in terms of elementary functions and describe
Friedmann-Lema\^itre-Robertson-Walker universes sourced by multiple (real or
effective) perfect fluids with constant equations of state. Effective fluids
include spatial curvature, the cosmological constant, and scalar fields. We
provide a description with unified notation, explicit and parametric forms of
the solutions, and relations between different expressions present in the
literature. Interesting solutions from a modern point of view include
interacting fluids and scalar fields. Old solutions, integrability conditions,
and solution methods keep being rediscovered, which motivates a review with
modern eyes.

32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
24
The categories of this paper are: dg-ga
18
The categories of this paper are: econ
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
17
The categories of this paper are: q-fin
34
The categories of this paper are: chem-ph
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
11
The categories of this paper are: hep-lat
27
The categories of this paper are: comp-gas
35
The categories of this paper are: mtrl-th
16
The categories of this paper are: stat