Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Conformal time dependent Painleve-Gullstrand spacetime   The properties of an anisotropic fluid outside a star or a black hole
embedded in an expanding universe are investigated. One finds that, in
Painleve-Gullstrand coordinates, the heat flux of the cosmological fluid
vanishes, in spite of the nonzero value of a non-diagonal component of the
stress tensor. The pressures and energy density of the fluid are regular at $r
= 2m$, divergent at $r = 0$ and change sign at certain values of their
arguments.

32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
30
The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
15
The categories of this paper are: nucl-ex
3
The categories of this paper are: physics
21
The categories of this paper are: adap-org
0
The categories of this paper are: hep-ph
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas