Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: ``Natural'' Vacua in Hyperbolic Friedmann-Robertson-Walker Spacetimes   Recent evidence indicates that the Universe is open, i.e., spatially
hyperbolic, longstanding theoretical preferences to the contrary
notwithstanding. This makes it possible to select a vacuum state, Fock space,
and particle definition for a quantized field, by requiring concordance with
ordinary flat-spacetime theory at late times. The particle-number basis states
thus identified span the physical state space of the field at all times. This
construction is demonstrated here explicitly for a massive, minimally coupled,
linear scalar field in an open, radiation-dominated Friedmann-Robertson-Walker
spacetime.

4
The categories of this paper are: cond-mat
7
The categories of this paper are: hep-th
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
31
The categories of this paper are: ao-sci
14
The categories of this paper are: math-ph
15
The categories of this paper are: nucl-ex
27
The categories of this paper are: comp-gas
11
The categories of this paper are: hep-lat
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
9
The categories of this paper are: nlin