1,375 research outputs found
Cosmic Acceleration and Modified Gravitational Models
There is now overwhelming observational evidence that our Universe is
accelerating in its expansion. I discuss how modified gravitational models can
provide an explanation for this observed late-time cosmic acceleration. We
consider specific low-curvature corrections to the Einstein-Hilbert action.
Many of these models generically contain unstable de Sitter solutions and,
depending on the parameters of the theory, late-time accelerating attractor
solutions.Comment: 11 pages, 3 figures, references added; Based on talks given at
MRST-04, PASCOS-04 and COSMO-0
Moduli Stabilization and Inflation Using Wrapped Branes
We demonstrate that a gas of wrapped branes in the early Universe can help
resolve the cosmological Dine-Seiberg/Brustein-Steinhardt overshoot problem in
the context of moduli stabilization with steep potentials in string theory.
Starting from this mechanism, we propose a cosmological model with a natural
setting in the context of an early phase dominated by brane and string gases.
The Universe inflates at early times due to the presence of a wrapped two brane
(domain wall) gas and all moduli are stabilized. A natural graceful exit from
the inflationary regime is achieved. However, the basic model suffers from a
generalized domain wall/reheating problem and cannot generate a scale invariant
spectrum of fluctuations without additional physics. Several suggestions are
presented to address these issues.Comment: 7 pages, 5 figures, REVTeX; References added, note adde
Loitering Phase in Brane Gas Cosmology
Brane Gas Cosmology (BGC) is an approach to M-theory cosmology in which the
initial state of the Universe is taken to be small, dense and hot, with all
fundamental degrees of freedom near thermal equilibrium. Such a starting point
is in close analogy with the Standard Big Bang (SBB) model. The topology of the
Universe is assumed to be toroidal in all nine spatial dimensions and is filled
with a gas of p-branes. The dynamics of winding modes allow, at most, three
spatial dimensions to become large, thus explaining the origin of our
macroscopic 3+1-dimensional Universe. Here we conduct a detailed analysis of
the loitering phase of BGC. We do so by including into the equations of motion
that describe the dilaton gravity background some new equations which determine
the annihilation of string winding modes into string loops. Specific solutions
are found within the model that exhibit loitering, i.e. the Universe
experiences a short phase of slow contraction during which the Hubble radius
grows larger than the physical extent of the Universe. As a result the brane
problem (generalized domain wall problem) in BGC is solved. The initial
singularity and horizon problems of the SBB scenario are solved without relying
on an inflationary phase.Comment: 19 pages, 6 figures; version to appear in Nucl. Phys.
Universe Generation from Black Hole Interiors
We point out that scenarios in which the universe is born from the interior
of a black hole may not posses many of the problems of the Standard Big-Bang
(SBB) model. In particular we demonstrate that the horizon problem, flatness,
and the structure formation problem might be solved naturally, not necessarily
requiring a long period of cosmological inflation. The black hole information
loss problem is also discussed. Our conclusions are completely independent of
the details of general models.Comment: 6 pages, 2 figure
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