Paper (*cross-listing*): gr-qc/0508116
Date: Sun, 28 Aug 2005 14:22:51 GMT (38kb)

Title: A late-accelerating universe with no dark energy -- and no big bang

Authors: Richard A. Brown, Roy Maartens, Eleftherios Papantonopoulos, Vassilis
Zamarias

Comments: 7 pages, 6 figs
\\
Brane-world models offer the possibility of explaining the late acceleration
of the universe via infra-red modifications to General Relativity, rather than
a dark energy field. However, one also expects ultra-violet modifications to
General Relativity, when high-energy stringy effects in the early universe
begin to grow. We generalize the DGP brane-world model via an ultra-violet
modification, in the form of a Gauss-Bonnet term in the bulk action. The
combination of infra-red and ultra-violet modifications produces an intriguing
cosmology. The DGP feature of late-time acceleration without dark energy is
preserved, but there is an entirely new feature -- there is no hot big bang in
the early universe. The universe starts with finite density and pressure, from
a "quiet" and "sudden" curvature singularity.

\\ ( http://arXiv.org/abs/gr-qc/0508116 , 38kb)

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Paper (*cross-listing*): gr-qc/0508045
replaced with revised version Tue, 30 Aug 2005 04:11:11 GMT (16kb)

Title: Necessary and sufficient conditions for big bangs, bounces, crunches,
rips, sudden singularities, and extremality events

Authors: Celine Cattoen (Victoria University of Wellington), Matt Visser
(Victoria University of Wellington)

Comments: 20 pages, uses iopart.cls; several references added

The physically relevant singularities occurring in FRW cosmologies had traditionally been thought to be limited to the "big bang", and possibly a "big crunch". However, over the last few years, the zoo of cosmological singularities considered in the literature has become considerably more extensive, with "big rips" and "sudden singularities" added to the mix, as well as renewed interest in non-singular cosmological events such as "bounces" and "turnarounds".

In this article we present a complete catalogue of such cosmological milestones, both at the kinematical and dynamical level. First, using generalized power series, purely kinematical definitions of these cosmological events are provided in terms of the behaviour of the scale factor a(t). The notion of a "scale-factor singularity'" is defined, and its relation to curvature singularities (polynomial and differential) is explored. Second, dynamical information is extracted by using the Friedmann equations (without assuming even the existence of any equation of state) to place constraints on whether or not the classical energy conditions are satisfied at the cosmological milestones. We use these considerations to derive necessary and sufficient conditions for the existence of cosmological milestones such as bangs, bounces, crunches, rips, sudden singularities, and extremality events. Since the classification is extremely general, the corresponding results are to a high degree model-independent: In particular, we provide a complete characterization of the class of bangs, crunches, and sudden singularities for which the dominant energy condition is satisfied.

\\ ( http://arXiv.org/abs/gr-qc/0508045 , 16kb)