Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Constraints on the sound speed of dark energy   We have studied constraints on the equation of state, $w$, and speed of
sound, c_s, of the dark energy from a joint analysis of data from the cosmic
microwave background, large scale structure and type-Ia supernovae. We find
that current observations have no significant sensitivity to c_s. However,
there is a slight difference between models in which there are no dark energy
perturbations and models in which dark energy behaves as a fluid. Assuming that
there are no dark energy perturbations shifts the allowed region for $w$ to
slightly higher values. At present models with and without dark energy
perturbations provide roughly equally good fits to observations, but the
difference is potentially important for future parameter estimations. Finally,
we have also performed error forecasts for future measurements of c_s.

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The categories of this paper are: cond-mat
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The categories of this paper are: math
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The categories of this paper are: quant-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-fin
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The categories of this paper are: eess
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The categories of this paper are: cmp-lg
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The categories of this paper are: physics
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The categories of this paper are: supr-con
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The categories of this paper are: chao-dyn