Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The ultimate frontier of 21cm cosmology   We present the most detailed computation to date of the 21-cm global signal
and fluctuations at $z\gtrsim 500$. Our calculations include a highly precise
estimate of the Wouthuysen-Field (WF) effect and the first explicit calculation
of the impact of free-free processes, the two dominant components of the signal
at $z\gtrsim 800$. We implement a new high-resolution Ly$\alpha$ radiative
transfer calculation, coupled to a state-of-the-art primordial recombination
code. Using these tools, we find a global signal from 21-cm processes alone of
roughly 0.01mK at $z\sim1000$, slightly larger than it would be without the WF
effect, but much weaker than previous estimates including this effect. We also
find that this signal is swamped by a smooth $1-2$ mK signal due to free-free
absorption at high redshift by the partially ionized gas along the line of
sight. In addition, we estimate the amplitude of 21-cm fluctuations, of order
$\sim 10^{-7}$ mK at $z\sim1000$. Unfortunately, we find that due to the
brightness of the low-frequency sky, these fluctuations will not be observable
beyond $z\sim$ a few hundred by even extremely futuristic observations. The 21
cm fluctuations are exponentially suppressed at higher redshifts by the large
free-free optical depth, making this the ultimate upper redshift limit for
21-cm surveys.

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The categories of this paper are: cs
18
The categories of this paper are: econ
24
The categories of this paper are: dg-ga
29
The categories of this paper are: funct-an
14
The categories of this paper are: math-ph
27
The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
11
The categories of this paper are: hep-lat
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-ex
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The categories of this paper are: stat
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The categories of this paper are: nucl-th
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The categories of this paper are: q-fin
31
The categories of this paper are: ao-sci
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The categories of this paper are: eess