Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Has the Universe always expanded ?   We consider a cosmological setting for which the currently expanding era is
preceded by a contracting phase, that is, we assume the Universe experienced at
least one bounce. We show that scalar hydrodynamic perturbations lead to a
singular behavior of the Bardeen potential and/or its derivatives (i.e. the
curvature) for whatever Universe model for which the last bounce epoch can be
smoothly and causally joined to the radiation dominated era. Such a Universe
would be filled with non-linear perturbations long before nucleosynthesis, and
would thus be incompatible with observations. We therefore conclude that no
observable bounce could possibly have taken place in the early universe if
Einstein gravity together with hydrodynamical fluids is to describe its
evolution, and hence, under these conditions, that the Universe has always
expanded.

22
The categories of this paper are: chao-dyn
23
The categories of this paper are: solv-int
28
The categories of this paper are: alg-geom
37
The categories of this paper are: supr-con
35
The categories of this paper are: mtrl-th
14
The categories of this paper are: math-ph
15
The categories of this paper are: nucl-ex
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
25
The categories of this paper are: patt-sol
0
The categories of this paper are: hep-ph
29
The categories of this paper are: funct-an
31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
36
The categories of this paper are: cmp-lg
30
The categories of this paper are: q-alg