Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gravitational waves in an expanding Universe   We study the tensorial modes of the two-fluid model, where one of this fluids
has an equation of state $p = - \rho/3$ (variable cosmological constant, cosmic
string fluid, texture) or $p = - \rho$ (cosmological constant), while the other
fluid is an ordinary matter (radiation, stiff matter, incoherent matter). In
the first case, it is possible to have a closed Universe whose dynamics can be
that of an open Universe providing alternative solutions for the age and
horizon problems. This study of the gravitational waves is extended for all
values of the effective curvature $k_{eff}=k-\frac{8\pi G}{3}\rho_{0s}$, that
is, positive, negative or zero, $k$ being the curvature of the spacelike
section. In the second case, we restrict ourselves to a flat spatial section.
The behaviour of gravitational waves have, in each case, very particular
features, that can be reflected in the anisotropy spectrum of Cosmic Microwave
Background Radiation. We make also some considerations of these models as
candidate to dark matter models.

21
The categories of this paper are: adap-org
2
The categories of this paper are: math
14
The categories of this paper are: math-ph
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
28
The categories of this paper are: alg-geom
12
The categories of this paper are: nucl-th
31
The categories of this paper are: ao-sci
10
The categories of this paper are: q-bio
36
The categories of this paper are: cmp-lg
7
The categories of this paper are: hep-th
24
The categories of this paper are: dg-ga
3
The categories of this paper are: physics
20
The categories of this paper are: acc-phys
17
The categories of this paper are: q-fin
37
The categories of this paper are: supr-con