Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ekpyrotic and Cyclic Cosmology   Ekpyrotic and cyclic cosmologies provide theories of the very early and of
the very late universe. In these models, the big bang is described as a
collision of branes - and thus the big bang is not the beginning of time.
Before the big bang, there is an ekpyrotic phase with equation of state w=P/rho
>> 1 (where P is the average pressure and rho the average energy density)
during which the universe slowly contracts. This phase resolves the standard
cosmological puzzles and generates a nearly scale-invariant spectrum of
cosmological perturbations containing a significant non-gaussian component. At
the same time it produces small-amplitude gravitational waves with a blue
spectrum. The dark energy dominating the present-day cosmological evolution is
reinterpreted as a small attractive force between our brane and a parallel one.
This force eventually induces a new ekpyrotic phase and a new brane collision,
leading to the idea of a cyclic universe. This review discusses the detailed
properties of these models, their embedding in M-theory and their viability,
with an emphasis on open issues and observational signatures.

1
The categories of this paper are: cs
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math
23
The categories of this paper are: solv-int
19
The categories of this paper are: eess
11
The categories of this paper are: hep-lat
3
The categories of this paper are: physics
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
14
The categories of this paper are: math-ph
26
The categories of this paper are: bayes-an
33
The categories of this paper are: plasm-ph