Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 21-cm cosmology   Imaging the Universe during the first hundreds of millions of years remains
one of the exciting challenges facing modern cosmology. Observations of the
redshifted 21 cm line of atomic hydrogen offer the potential of opening a new
window into this epoch. This would transform our understanding of the formation
of the first stars and galaxies and of the thermal history of the Universe. A
new generation of radio telescopes is being constructed for this purpose with
the first results starting to trickle in. In this review, we detail the physics
that governs the 21 cm signal and describe what might be learnt from upcoming
observations. We also generalize our discussion to intensity mapping of other
atomic and molecular lines.

3
The categories of this paper are: physics
13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
7
The categories of this paper are: hep-th
17
The categories of this paper are: q-fin
5
The categories of this paper are: gr-qc
30
The categories of this paper are: q-alg
14
The categories of this paper are: math-ph
16
The categories of this paper are: stat
18
The categories of this paper are: econ
19
The categories of this paper are: eess
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
26
The categories of this paper are: bayes-an