Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: New family of inhomogeneous $\gamma$-law cosmologies: example of
  gravitational waves in a homogeneous $p=\varrho/3$ background   We present an explicit three-parameter class of $p=\gamma \varrho$,
($-1/3\leq\gamma<1$), cosmological models admitting a two-dimensional group
$G_{2}$ of isometries acting on spacelike surfaces. The family is self-similar
in the sense that it has a further homothetic vector field and it contains
subfamilies of both (previously unknown) tilted and non-tilted Bianchi models
with that equation of state. This is the first algebraically general class of
solutions of this kind including dust inhomogeneous solutions. The whole class
presents a universal spacelike big-bang singularity in the finite past. More
interestingly, the case $p=\varrho /3$ constitutes a new two-parameter
inhomogeneous subfamily which can be viewed as a Bianchi V background with a
gravitational wave travelling orthogonally to the surfaces of transitivity of
the $G_{2}$ group. This wave generates the {\it inhomogeneity} of the spacetime
and is related to the sound waves {\it tilting} the perfect fluid. It seems to
be the first explicit exact example of a gravitational wave travelling along a
homogeneous background that has a realistic equation of state $p=\varrho /3$.

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The categories of this paper are: solv-int
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The categories of this paper are: nucl-ex
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The categories of this paper are: ao-sci
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The categories of this paper are: bayes-an
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The categories of this paper are: nlin
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The categories of this paper are: physics
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The categories of this paper are: hep-ex
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The categories of this paper are: cs
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The categories of this paper are: eess
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The categories of this paper are: quant-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: econ
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The categories of this paper are: stat
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The categories of this paper are: dg-ga
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The categories of this paper are: chem-ph
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The categories of this paper are: hep-ph