Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Investigation of the electromagnetically induced transparency in era of
  cosmological hydrogen recombination   Investigation of the cosmic microwave background formation processes is one
of the most actual problem at present time. In this paper we analyze the
response of the hydrogen atom to the external photon fields. Field
characteristics are defined via conditions corresponding to the recombination
era of universe. Approximation of three-level atom is used to describe the
"atom - fields" interaction. It is found that the phenomena of the
electromagnetically induced transparancy takes place in this case.
Consideration of EIT phenomena makes it necessary to update astrophysical
description of the processes of the cosmic microwave background formation and,
in particular, Sobolev escape probability. Additional terms to the optical
depth in definition of Sobolev escape probability on the level about 1% are
found.

23
The categories of this paper are: solv-int
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
27
The categories of this paper are: comp-gas
12
The categories of this paper are: nucl-th
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
21
The categories of this paper are: adap-org
37
The categories of this paper are: supr-con
19
The categories of this paper are: eess
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
7
The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph
18
The categories of this paper are: econ