Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical Depth of the Cosmic Microwave Background due to Scattering and
  Absorption   It is shown that, in addition to the Thomson scattering, the absorption due
to the electron-electron, electron-ion and the electron -atom collisions in a
partially ionized cosmic plasma would also contribute to the optical depth of
the cosmic microwave background (CMB). The absorption depth depends on the
plasma temperature and frequency of the CMB radiation. The absorption effects
are prominent at the low frequency part of the CMB spectrum. These effects when
included in the interpretation of the CMB spectrum may necessitate a revised
view of the ioniziation of the universe.

3
The categories of this paper are: physics
35
The categories of this paper are: mtrl-th
0
The categories of this paper are: hep-ph
21
The categories of this paper are: adap-org
32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
4
The categories of this paper are: cond-mat
33
The categories of this paper are: plasm-ph
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an