From: LARRY KLAES (ljk4_at_msn.com)
Date: Tue Aug 23 2005 - 21:15:29 UTC
Paper (*cross-listing*): gr-qc/0508045
Date: Thu, 11 Aug 2005 04:49:42 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: 19 pages, uses iopart.cls
\\
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)
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\\
Paper (*cross-listing*): gr-qc/0508047
Date: Thu, 11 Aug 2005 11:18:48 GMT (7kb)
Title: Secular increase of the Astronomical Unit and perihelion precessions
as
tests of the Dvali-Gabadadze-Porrati multi-dimensional braneworld scenario
Authors: Lorenzo Iorio
Comments: LaTex2e, 7 pages, no figures, no tables, 13 references
\\
An unexpected secular increase of the Astronomical Unit, the length scale of
the Solar System, has recently been reported by three different research
groups
(Krasinsky and Brumberg, Pitjeva, Standish). The latest JPL measurements
amount
to 7+-2 m cy^-1. At present, there are no explanations able to accommodate
such
an observed phenomenon, neither in the realm of classical physics nor in the
usual four-dimensional framework of the Einsteinian General Relativity. The
Dvali-Gabadadze-Porrati braneworld scenario, which is a multi-dimensional
model
of gravity aimed to the explanation of the observed cosmic acceleration
without
dark energy, predicts, among other things, a perihelion secular shift, due
to
Lue and Starkman, of 5 10^-4 arcsec cy^-1 for all the planets of the Solar
System. It yields a variation of about 6 m cy^-1 for the Earth-Sun distance
which is compatible at 1-sigma level with the observed rate of the
Astronomical
Unit. The recently measured corrections to the secular motions of the
perihelia
of the inner planets of the Solar System are in agreement, at 1-sigma level,
with the predicted value of the Lue-Starkman effect for Mercury and Mars and
at
2-sigma level for the Earth.
\\ ( http://arXiv.org/abs/gr-qc/0508047 , 7kb)
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\\
Paper (*cross-listing*): gr-qc/0508053
Date: Fri, 12 Aug 2005 14:53:09 GMT (12kb)
Date (revised v2): Sat, 13 Aug 2005 07:09:18 GMT (12kb)
Title: Dark energy and the future fate of the Universe
Authors: Yungui Gong and Yuan-Zhong Zhang
Comments: 2 figures, 11 pages
\\
We consider the possibility of observing the onset of the late time
acceleration of our patch of the Universe. The Hubble size criterion and the
event horizon criterion are applied to several dark energy models to discuss
the problem of future inflation of the Universe. We find that the
acceleration
has not lasted long enough to be confirmed by present observations for the
dark
energy model with constant equation of state, the holographic dark energy
model
and the generalized Chaplygin gas (GCG) model. For the flat $\Lambda$CDM
model
with $\Omega_{m0}=0.3$, we find that if we use the Hubble size criterion, we
need to wait until the scale factor $a_v$ reaches 3.59 times of the scale
factor $a_T$ when the Universe started acceleration, to confirm the onset of
acceleration, and we need to wait until $a_v=2.3 a_T$ to confirm the onset
of
acceleration if we use the event horizon criterion. For the flat holographic
dark energy model with $d=1$, we find that $a_v=3.46 a_T$ and $a_v=2.34
a_T$,
respectively. For the flat GCG model with the best supernova fitting
parameter
$\alpha=1.2$, we find that $a_v=5.50 a_T$ and $a_v=2.08 a_T$, respectively.
\\ ( http://arXiv.org/abs/gr-qc/0508053 , 12kb)
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Paper (*cross-listing*): hep-th/0508101
Date: Mon, 15 Aug 2005 12:23:51 GMT (27kb)
Title: (A)symmetric Tachyon Rolling in de Sitter Spacetime: An universe
devoid
of Planck density
Authors: Harvendra Singh
Comments: 17 pages, 5 fugures
Report-no: SINP/TNP/05-20
\\
In a recent paper we studied rolling tachyon flat FRW cosmologies, but those
admitting only time-reversal asymmetric boundary conditions. The
time-reversal
symmetric cosmologies have been studied by Sen previously. We show
explicitly
here that through appropriate choice of initial conditions, the time
evolution
of the Hubble parameter in these two types of solutions can be made
completely
identical for $t>0$, except near $t=0$. The rolling tachyon solution also
gives
rise to necessary inflation. We find that universe does start as a string
size
object (with string scale $10^{15} GeV$) with a string mass density $\simeq
10^{78} gm. cm^{-3}$ and not with Planck density.
\\ ( http://arXiv.org/abs/hep-th/0508101 , 27kb)
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Paper (*cross-listing*): hep-th/0508135
Date: Thu, 18 Aug 2005 22:31:51 GMT (9kb)
Title: Cosmic strings: progress and problems
Authors: Alexander Vilenkin
Comments: 7 pages, contribution to "Inflating Horizons of Particle
Astrophysics
and Cosmology", honoring Katsuhiko Sato on his 60th birthday
\\
Recent developments in cosmic strings are reviewed, with emphasis on
unresolved problems.
\\ ( http://arXiv.org/abs/hep-th/0508135 , 9kb)
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