Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Back-Reaction of Gravitational Waves Revisited   We study the back-reaction of gravitational waves in early universe
cosmology, focusing both on super-Hubble and sub-Hubble modes. Sub-Hubble modes
lead to an effective energy density which scales as radiation. Hence, the
relative contribution of such gravitational waves to the total energy density
is constrained by big bang nucleosynthesis. This leads to an upper bound on the
tensor spectral slope n_T which also depends on the tensor to scalar ratio r.
Super-Hubble modes, on the other hand, lead to a negative contribution to the
effective energy density, and to an equation of state of curvature. Demanding
that the early universe is not dominated by the back-reaction leads to
constraints on the gravitational wave spectral parameters which are derived.

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The categories of this paper are: chem-ph
15
The categories of this paper are: nucl-ex
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The categories of this paper are: supr-con
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The categories of this paper are: plasm-ph
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The categories of this paper are: nlin
8
The categories of this paper are: hep-ex
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The categories of this paper are: acc-phys
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The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
23
The categories of this paper are: solv-int
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The categories of this paper are: dg-ga
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The categories of this paper are: patt-sol
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The categories of this paper are: mtrl-th
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The categories of this paper are: chao-dyn
16
The categories of this paper are: stat
36
The categories of this paper are: cmp-lg