Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Separating expansion from contraction in spherically symmetric models
  with a perfect-fluid: Generalization of the Tolman-Oppenheimer-Volkoff
  condition and application to models with a cosmological constant   We investigate spherically symmetric perfect-fluid spacetimes and discuss the
existence and stability of a dividing shell separating expanding and collapsing
regions. We perform a 3+1 splitting and obtain gauge invariant conditions
relating the intrinsic spatial curvature of the shells to the Misner-Sharp mass
and to a function of the pressure that we introduce and that generalizes the
Tolman-Oppenheimer-Volkoff equilibrium condition. We find that surfaces
fulfilling those two conditions fit, locally, the requirements of a dividing
shell and we argue that cosmological initial conditions should allow its global
validity. We analyze the particular cases of the Lema\^itre-Tolman-Bondi dust
models with a cosmological constant as an example of a cold dark matter model
with a cosmological constant (\Lambda-CDM) and its generalization to contain a
central perfect-fluid core. These models provide simple, but physically
interesting illustrations of our results.

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The categories of this paper are: q-bio
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The categories of this paper are: chem-ph
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The categories of this paper are: q-alg
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The categories of this paper are: adap-org
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The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
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The categories of this paper are: quant-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: hep-ex
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The categories of this paper are: math-ph
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The categories of this paper are: q-fin
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The categories of this paper are: econ
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The categories of this paper are: cs
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The categories of this paper are: stat
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The categories of this paper are: alg-geom
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The categories of this paper are: supr-con