Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: THERMODYNAMIC PROPERTIES OF $\gamma$--FLUIDS AND THE QUANTUM VACCUM   The thermodynamic behaviour of a relativistic perfect simple fluid obeying
the equation of state $p=(\gamma-1)\rho $, where $0 \le \gamma \le 2$ is a
constant, has been investigated. Particular cases include: vacuum($p=-\rho $,
$\gamma=0$), a randomly oriented distribution of cosmic strings ($p=-{1 \over
3} \rho $, $\gamma =2/3$), blackbody radiation ($p={1\over 3} \rho$, $\gamma
=4/3$) and stiff matter ($p=\rho$, $\gamma=2$). Fluids with $\gamma <1$ become
hotter when they expand adiabatically ($T\propto V^{1- \gamma}$). By assuming
that such fluids may be regarded as a kind of generalized radiation, the
general Planck's type form of the spectrum is deduced. As a limiting case, a
new Lorentz invariant spectrum of the vacuum which is compatible with the
equation of state and other thermodynamic constraints is proposed. Some
possible consequences to the early universe physics are also discussed.

31
The categories of this paper are: ao-sci
25
The categories of this paper are: patt-sol
8
The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
2
The categories of this paper are: math
26
The categories of this paper are: bayes-an
7
The categories of this paper are: hep-th
20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
6
The categories of this paper are: astro-ph
18
The categories of this paper are: econ
1
The categories of this paper are: cs
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat