Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The spin temperature of neutral hydrogen during cosmic pre-reionization   We re-examine the role of collisions in decoupling the HI 21-cm spin
temperature from the cosmic microwave background (CMB). The cross section for
de-exciting the 21-cm trasition in collisions with free electrons is more than
10 times larger than it is in collisions with other atoms. If the fraction of
free electrons in the diffuse cosmic gas is between 10 and 30 per cent then
collisions alone can decouple the spin temperature from the cosmic microwave
background (CMB), even in moderately under-dense regions at z >15. This
decoupling is especially important during the very early stages of
re-ionization when a Ly-alpha continuum background had yet to be established.
As a detailed example, we develop a semi-analytic model to quantify 21-cm
emission signatures from a diffuse gas which is partially ionized at z>10 by an
X-ray background. We find 21-cm differential brightness temperature
fluctuations with a mean of ~8 mK and a rms value as large as 5 mK, for a
frequency resolution bandwidth of 100 KHz and a beamsize of 3 arcmin. Another
example where free electron-atom collisions are important is during the
recombination of bubbles ionized by short-lived UV sources. When the ionized
fraction in these bubbles drops to 10-20 per cent their differential
temperature can be as high as 10 mK. We also consider the correlation of the
21-cm temperature as a function of frequency. By isotropy, a comparison with
the angular projected correlation in the plane of the sky should yield an
estimate for the matter density parameter, Omega_m, and the dark energy
component. This variant of the Alcock-Paczyski test is applicable for any
reionization scenario.

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The categories of this paper are: q-bio
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The categories of this paper are: econ
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The categories of this paper are: adap-org
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The categories of this paper are: funct-an
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The categories of this paper are: acc-phys
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The categories of this paper are: bayes-an
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The categories of this paper are: solv-int
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The categories of this paper are: q-fin
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The categories of this paper are: cs
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The categories of this paper are: physics
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The categories of this paper are: quant-ph
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The categories of this paper are: math
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The categories of this paper are: plasm-ph
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The categories of this paper are: q-alg
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The categories of this paper are: eess
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The categories of this paper are: chao-dyn