Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Why do cosmological perturbations look classical to us?   According to the inflationary scenario of cosmology, all structure in the
Universe can be traced back to primordial fluctuations during an accelerated
(inflationary) phase of the very early Universe. A conceptual problem arises
due to the fact that the primordial fluctuations are quantum, while the
standard scenario of structure formation deals with classical fluctuations. In
this essay we present a concise summary of the physics describing the
quantum-to-classical transition. We first discuss the observational
indistinguishability between classical and quantum correlation functions in the
closed system approach (pragmatic view). We then present the open system
approach with environment-induced decoherence. We finally discuss the question
of the fluctuations' entropy for which, in principle, the concrete mechanism
leading to decoherence possesses observational relevance.

18
The categories of this paper are: econ
24
The categories of this paper are: dg-ga
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
7
The categories of this paper are: hep-th
25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics