Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Minimally coupled scalar fields as imperfect fluids   We revisit the issue of the fluid description of minimally coupled scalar
fields. While in a cosmological setup the interpretation of a time-evolving
scalar field as a perfect fluid is well-understood, the situation is more
intricate when the scalar field is static, but has a spatial gradient, a
situation motivated by black hole perturbations in scalar-tensor theories. Then
the scalar field is interpreted as either a particular imperfect fluid of type
I or a superposition of a pair of leftgoing (incoming) and rightgoing
(outgoing) null dusts with a perfect fluid. Finally, when the scalar gradient
is null, it is equivalent to an imperfect fluid of type II, degenerating into
null dust when the energy conditions are imposed. We also propose the suitable
action in terms of the fluid pressure components for each case and discuss the
variational principle for a generic class of minimally coupled scalar fields.

37
The categories of this paper are: supr-con
17
The categories of this paper are: q-fin
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
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The categories of this paper are: stat
14
The categories of this paper are: math-ph
2
The categories of this paper are: math
13
The categories of this paper are: quant-ph
0
The categories of this paper are: hep-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: chem-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: dg-ga
18
The categories of this paper are: econ
30
The categories of this paper are: q-alg