Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Bose-Einstein condensation of photons in the matter-dominated universe   In 1914, Planck introduced the concept of a white body. In nature, no true
white bodies are known. We assume that the universe after last-scattering is an
ideal white body that contains a tremendously large number of thermal photons
and is at an extremely high temperature. Bose-Einstein condensation of photons
in an ideal white body is investigated within the framework of quantum
statistical mechanism. The computation shows that the transition temperature
$T_c$ is a monotonically increasing function of the number density $n$ of
photons. At finite temperature, we find that the condensate fraction $N_0(T)/N$
decreases continuously from unity to zero as the temperature increases from
zero to the transition temperature $T_c$. Further, we study the radiation
properties of an ideal white body. It is found that in the condensation region
of $T<T_c$, the spectral intensity $I(\omega,T)$ of white body radiation is
identical with Planck's law for blackbody radiation.

15
The categories of this paper are: nucl-ex
4
The categories of this paper are: cond-mat
28
The categories of this paper are: alg-geom
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
9
The categories of this paper are: nlin
12
The categories of this paper are: nucl-th
25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat
29
The categories of this paper are: funct-an
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas