231 research outputs found
Reinventing spacetime on a dynamical hypersurface
In braneworld models, Space-Time-Matter and other Kaluza-Klein theories, our
spacetime is devised as a four-dimensional hypersurface {\it orthogonal} to the
extra dimension in a five-dimensional bulk. We show that the FRW line element
can be "reinvented" on a dynamical four-dimensional hypersurface, which is {\it
not} orthogonal to the extra dimension, without any internal contradiction.
This hypersurface is selected by the requirement of continuity of the metric
and depends explicitly on the evolution of the extra dimension. The main
difference between the "conventional" FRW, on an orthogonal hypersurface, and
the new one is that the later contains higher-dimensional modifications to the
regular matter density and pressure in 4D. We compare the evolution of the
spacetime in these two interpretations. We find that a wealth of "new" physics
can be derived from a five-dimensional metric if it is interpreted on a
dynamical (non-orthogonal) 4D hypersurface. In particular, in the context of a
well-known cosmological metric in , we construct a FRW model which is
consistent with the late accelerated expansion of the universe, while fitting
simultaneously the observational data for the deceleration parameter. The model
predicts an effective equation of state for the universe, which is consistent
with observations.Comment: References added to the Introduction, and Abstract modified. Accepted
for publication in Mod. Phys. Lett.
A Closer Look at the LkCa 15 Protoplanetary Disk
We present 870 micron observations of dust continuum emission from the LkCa
15 protoplanetary disk at high angular resolution (with a characteristic scale
of 0.25" = 35 AU), obtained with the IRAM Plateau de Bure interferometer and
supplemented by slightly lower resolution observations from the Submillimeter
Array. We fit these data with simple morphological models to characterize the
spectacular ring-like emission structure of this disk. Our analysis indicates
that a small amount of 870 micron dust emission (~5 mJy) originates inside a
large (40-50 AU radius) low optical depth cavity. This result can be
interpreted either in the context of an abrupt decrease by a factor of ~5 in
the radial distribution of millimeter-sized dust grains or as indirect evidence
for a gap in the disk, in agreement with previous inferences from the
unresolved infrared spectrum and scattered light images. A preliminary model
focused on the latter possibility suggests the presence of a low-mass
(planetary) companion, having properties commensurate with those inferred from
the recent discovery of LkCa 15b.Comment: in press at ApJ Letter
Brane Cosmology With Generalized Chaplygin Gas in The Bulk
We find exact solution of the Einstein equations in the context of the brane
world scenario. We have supposed a {generalized chaplygin gas} equation of
state for bulk. This study display a constant energy density and pressure for
bulk in late time. It is shown that our assumptions impose a specific equation
of state on brane. {In this work, we have obtained a decelerate universe in
early time and late time.} In the end, it is shown that under some assumption
we have equation of state of cosmological constant for bulk.Comment: 11 page
Asymmetric radiating brane-world
At high energies on a cosmological brane of Randall-Sundrum type, particle
interactions can produce gravitons that are emitted into the bulk and that can
feed a bulk black hole. We generalize previous investigations of such radiating
brane-worlds by allowing for a breaking of Z_2-symmetry, via different bulk
cosmological constants and different initial black hole masses on either side
of the brane. One of the notable features of asymmetry is a suppression of the
asymptotic level of dark radiation, which means that nucleosynthesis
constraints are easier to satisfy. There are also models where the radiation
escapes to infinity on one or both sides, rather than falling into a black
hole, but these models can have negative energy density on the brane.Comment: sign error in eq. (34) corrected; version to appear Phys. Rev.
Limits on the integration constant of the dark radiation term in Brane Cosmology
We consider the constraints from primordial Helium abundances on the constant
of integration of the dark radiation term of the brane-world generalized
Friedmann equation derived from the Randall-Sundrum Single brane model. We
found that -- using simple, approximate and semianalytical Method -- that the
constant of integration is limited to be between -8.9 and 2.2 which limits the
possible contribution from dark radiation term to be approximately between -27%
to 7% of the background photon energy density.Comment: 8 page
FLRW Universes from "Wave-Like" Cosmologies in 5D
We consider the evolution of a 4D-universe embedded in a five-dimensional
(bulk) world with a large extra dimension and a cosmological constant. The
cosmology in 5D possesses "wave-like" character in the sense that the metric
coefficients in the bulk are functions of the extra coordinate and time in a
way similar to a pulse or traveling wave propagating along the fifth dimension.
This assumption is motivated by some recent work presenting the big-bang as a
higher dimensional shock wave. We show that this assumption, together with an
equation of state for the effective matter quantities in 4D, allows Einstein's
equations to be fully integrated. We then recover the familiar FLRW universes,
on the four-dimensional hypersurfaces orthogonal to the extra dimension.
Regarding the extra dimension we find that it is {\em growing} in size if the
universe is speeding up its expansion. We also get an estimate for the relative
change of the extra dimension over time. This estimate could have important
observational implications, notably for the time variation of rest mass,
electric charge and the gravitational "constant". Our results extend previous
ones in the literature.Comment: Few comments added, references updated. To appear in Int. J. of Mod.
Phys.
Colliding Bubble Worlds
We consider a cosmological model in which our Universe is a spherically
symmetric bubble wall in 5-dimensional anti-de Sitter spacetime. We argue that
the bubble on which we live will undergo collisions with other similar bubbles
and estimate the spectrum of such collisions. The collision rate is found to be
independent of the age of our Universe. Collisions with small bubbles provide
an experimental signature of this scenario, while collisions with larger
bubbles would be catastrophic.Comment: 7 pages, no figure
Asymmetric embedding in brane cosmology
We derive a system of cosmological equations for a braneworld with induced
curvature which is a junction between several bulk spaces. The permutation
symmetry of the bulk spaces is not imposed, and the values of the fundamental
constants, and even the signatures of the extra dimension, may be different on
different sides of the brane. We then consider the usual partial case of two
asymmetric bulk spaces and derive an exact closed system of scalar equations on
the brane. We apply this result to the cosmological evolution on such a brane
and describe its various partial cases.Comment: 10 page
Giant planet migration, disk evolution, and the origin of transitional disks
We present models of giant planet migration in evolving protoplanetary disks.
Our disks evolve subject to viscous transport of angular momentum and
photoevaporation, while planets undergo Type II migration. We use a Monte Carlo
approach, running large numbers of models with a range in initial conditions.
We find that relatively simple models can reproduce both the observed radial
distribution of extra-solar giant planets, and the lifetimes and accretion
histories of protoplanetary disks. The use of state-of-the-art photoevaporation
models results in a degree of coupling between planet formation and disk
clearing, which has not been found previously. Some accretion across planetary
orbits is necessary if planets are to survive at radii <~1.5AU, and if planets
of Jupiter mass or greater are to survive in our models they must be able to
form at late times, when the disk surface density in the formation region is
low. Our model forms two different types of "transitional" disks, embedded
planets and clearing disks, which show markedly different properties. We find
that the observable properties of these systems are broadly consistent with
current observations, and highlight useful observational diagnostics. We
predict that young transition disks are more likely to contain embedded giant
planets, while older transition disks are more likely to be undergoing disk
clearing.Comment: 13 pages, 9 figures. Accepted for publication in Ap
Closed System of Equations on a Brane
We obtain a generic closed system of equations on a brane that describes its
inner evolution and give a method for extending solutions on the brane to the
bulk. We also discuss the cosmological implications of the closed system of
equations obtained. We consider bulk spaces with both spacelike and timelike
extra dimension, with and without the symmetry of reflection relative to
the brane.Comment: 11 pages, revtex. Substantially revised version to be published in
PL
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