Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Perturbations in a regular bouncing Universe   We consider a simple toy model of a regular bouncing universe. The bounce is
caused by an extra time-like dimension, which leads to a sign flip of the
$\rho^2$ term in the effective four dimensional Randall Sundrum-like
description. We find a wide class of possible bounces: big bang avoiding ones
for regular matter content, and big rip avoiding ones for phantom matter.
  Focusing on radiation as the matter content, we discuss the evolution of
scalar, vector and tensor perturbations. We compute a spectral index of
$n_s=-1$ for scalar perturbations and a deep blue index for tensor
perturbations after invoking vacuum initial conditions, ruling out such a model
as a realistic one. We also find that the spectrum (evaluated at Hubble
crossing) is sensitive to the bounce. We conclude that it is challenging, but
not impossible, for cyclic/ekpyrotic models to succeed, if one can find a
regularized version.

21
The categories of this paper are: adap-org
32
The categories of this paper are: atom-ph
28
The categories of this paper are: alg-geom
4
The categories of this paper are: cond-mat
9
The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th
25
The categories of this paper are: patt-sol
15
The categories of this paper are: nucl-ex
3
The categories of this paper are: physics
24
The categories of this paper are: dg-ga
11
The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
10
The categories of this paper are: q-bio
36
The categories of this paper are: cmp-lg
23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas