Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cosmological perturbation theory and conserved quantities in the
  large-scale limit   The linear cosmological perturbation theory of an almost homogeneous and
isotropic perfect fluid universe is reconsidered and formally simplified by
introducing new covariant and gauge-invariant variables with physical
interpretations on hypersurfaces of constant expansion, constant curvature or
constant energy density. The existence of conserved perturbation quantities on
scales larger than the Hubble scale is discussed. The quantity which is
conserved on large scales in a flat background universe may be expressed in
terms of the fractional, spatial gradient of the energy density on constant
expansion hypersurfaces or, alternatively, with the help of expansion or
curvature perturbation variables on hypersurfaces of constant energy density.
For nonvanishing background curvature the perturbation dynamics is most
suitably described in terms of energy density perturbations on hypersurfaces of
constant curvature.

25
The categories of this paper are: patt-sol
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
8
The categories of this paper are: hep-ex
11
The categories of this paper are: hep-lat
7
The categories of this paper are: hep-th
4
The categories of this paper are: cond-mat
18
The categories of this paper are: econ
37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th
17
The categories of this paper are: q-fin
32
The categories of this paper are: atom-ph