Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Density perturbation and cosmological evolution in the presence of
  magnetic field in $f(R)$ gravity models   In this paper, we have investigated the density perturbations and
cosmological evolution in the FLRW universe in presence of a cosmic magnetic
field, which may be assumed to mimic primordial magnetic fields. Such magnetic
fields have sufficient strength to influence galaxy formation and cluster
dynamics, thereby leaving an imprint on the CMB anisotropies. We have
considered the FLRW universe as a representative of the isotropic cosmological
model in the 1+3 covariant formalism for $f(R)$ gravity. The propagation
equations have been determined and analyzed, where we have assumed that the
magnetic field is aligned uniformly along the $x$-direction, resulting in a
diagonal shear tensor. Subsequently,the density perturbation evolution
equations have been studied and the results have been interpreted. We have also
indicated how these results change in the general relativistic case and briefly
mentioned the expected change in higher-order gravity theories.

13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
3
The categories of this paper are: physics
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
29
The categories of this paper are: funct-an
4
The categories of this paper are: cond-mat
23
The categories of this paper are: solv-int
35
The categories of this paper are: mtrl-th
11
The categories of this paper are: hep-lat
37
The categories of this paper are: supr-con