Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Axial Gravitational Waves in Bianchi I Universe   In this paper, we have studied the propagation of axial gravitational waves
in Bianchi I universe using the Regge-Wheeler gauge. In this gauge, there are
only two non-zero components of $ h_{\mu\nu} $ in the case of axial waves:
$h_0(t,r)$ and $h_1(t,r)$. The field equations in absence of matter have been
derived both for the unperturbed as well as axially perturbed metric. These
field equations are solved simultaneously by assuming the expansion scalar
$\Theta$ to be proportional to the shear scalar $\sigma$ (so that $a= b^n$,
where $a$, $b$ are the metric coefficients and $n$ is an arbitrary constant),
and the wave equation for the perturbation parameter $h_0(t,r)$ have been
derived. We used the method of separation of variables to solve for this
parameter, and have subsequently determined $h_1(t,r)$. We then discuss a few
special cases in order to interpret the results. We find that the anisotropy of
the background spacetime is responsible for the damping of the gravitational
waves as they propagate through this spacetime. The perturbations depend on the
values of the angular momentum $l$. The field equations in the presence of
matter reveal that the axially perturbed spacetime leads to perturbations only
in the azimuthal velocity of the fluid leaving the matter field undisturbed.

37
The categories of this paper are: supr-con
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
31
The categories of this paper are: ao-sci
22
The categories of this paper are: chao-dyn
13
The categories of this paper are: quant-ph
7
The categories of this paper are: hep-th
18
The categories of this paper are: econ
12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
33
The categories of this paper are: plasm-ph
34
The categories of this paper are: chem-ph
6
The categories of this paper are: astro-ph
19
The categories of this paper are: eess
2
The categories of this paper are: math