Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Two-point functions in (loop) quantum cosmology   The path-integral formulation of quantum cosmology with a massless scalar
field as a sum-over-histories of volume transitions is discussed, with
particular but non-exclusive reference to loop quantum cosmology. Exploiting
the analogy with the relativistic particle, we give a complete overview of the
possible two-point functions, pointing out the choices involved in their
definitions, deriving their vertex expansions and the composition laws they
satisfy. We clarify the origin and relations of different quantities previously
defined in the literature, in particular the tie between definitions using a
group averaging procedure and those in a deparametrized framework. Finally, we
draw some conclusions about the physics of a single quantum universe (where
there exist superselection rules on positive- and negative-frequency sectors
and different choices of inner product are physically equivalent) and
multiverse field theories where the role of these sectors and the inner product
are reinterpreted.

16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
12
The categories of this paper are: nucl-th
4
The categories of this paper are: cond-mat
23
The categories of this paper are: solv-int
37
The categories of this paper are: supr-con
27
The categories of this paper are: comp-gas
22
The categories of this paper are: chao-dyn
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
35
The categories of this paper are: mtrl-th
21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph