Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Stephani Universe, k-essence and strings in the 5-th dimension   The Stephani universe is an inhomogeneous alternative to $\Lambda{\rm CDM}$
.We show that the exotic fluid driving the Stephani exact solution of
Einstein's equations is an unusual form of k-essence that is linear in
"velocity". Much as the Stephani universe can be embedded into (a section of)
flat 5-d Minkowski space-time, we show that the k-essence obtains through
dimensional reduction of a 5-d strongly coupled non-linear "electrodynamics"
that, in the empty Stephani universe, is similar to the Nielsen-Olesen field
theory of the dual string.

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The categories of this paper are: nlin
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The categories of this paper are: q-alg
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The categories of this paper are: gr-qc
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The categories of this paper are: stat
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The categories of this paper are: q-fin
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The categories of this paper are: funct-an
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The categories of this paper are: math
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The categories of this paper are: mtrl-th
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The categories of this paper are: chao-dyn
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The categories of this paper are: econ
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The categories of this paper are: comp-gas
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The categories of this paper are: bayes-an
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The categories of this paper are: adap-org
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The categories of this paper are: nucl-th
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The categories of this paper are: astro-ph
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The categories of this paper are: nucl-ex