86 research outputs found
Chaotic Inflation on the Brane with Induced Gravity
We study the slow-roll inflationary dynamics in a self-gravitating induced
gravity braneworld model with bulk cosmological constant. For E \gg
M_{5}^{3}/M^{2}_{2} we find important corrections to the four-dimensional
Friedmann equation which bring the standard chaotic inflationary scenario in
closer agreement with recent observations. For \lambda/M^{3}_{5} \ll E \ll
M^{3}_{5}/M^{2}_{4} we find five-dimensional corrections to the Friedmann
equation, which give the known Randall-Sundrum results of the inflationary
parameters.Comment: LaTex file, 21 pages, 3 figures, the energy scales are redefined, new
results on the chaotic inflation, typos are corrected. Accepted for
publication in JCA
Bi-paracontact structures and Legendre foliations
We study almost bi-paracontact structures on contact manifolds. We prove that
if an almost bi-paracontact structure is defined on a contact manifold
, then under some natural assumptions of integrability, carries
two transverse bi-Legendrian structures. Conversely, if two transverse
bi-Legendrian structures are defined on a contact manifold, then admits an
almost bi-paracontact structure. We define a canonical connection on an almost
bi-paracontact manifold and we study its curvature properties, which resemble
those of the Obata connection of an anti-hypercomplex (or complex-product)
manifold. Further, we prove that any contact metric manifold whose Reeb vector
field belongs to the -nullity distribution canonically carries an
almost bi-paracontact structure and we apply the previous results to the theory
of contact metric -spaces.Comment: To appear on: Kodai Mathematical Journa
BTZ Solutions on Codimension-2 Braneworlds
We consider five-dimensional gravity with a Gauss-Bonnet term in the bulk and
an induced gravity term on a 2-brane of codimension-2. We show that this system
admits BTZ-like black holes on the 2-brane which are extended into the bulk
with regular horizons.Comment: 6 pages. Talk given at Recent Developments in Gravity (NEB XIII),
Thessaloniki, Greece, June 4-6, 2008
Anisotropic Extra Dimensions
We consider the scenario where in a five-dimensional theory, the extra
spatial dimension has different scaling than the other four dimensions. We find
background maximally symmetric solutions, when the bulk is filled with a
cosmological constant and at the same time it has a three-brane embedded in it.
These background solutions are reminiscent of Randall-Sundrum warped metrics,
with bulk curvature depending on the parameters of the breaking of
diffeomorphism invariance. Subsequently, we consider the scalar perturbation
sector of the theory and show that it has certain pathologies and the striking
feature that in the limit where the diffeomorphism invariance is restored,
there remain ghost scalar mode(s) in the spectrum.Comment: 12 pages, no figures, comments and references added, published
versio
Cosmological Evolution of a Purely Conical Codimension-2 Brane World
We study the cosmological evolution of isotropic matter on an infinitely thin
conical codimension-two brane-world. Our analysis is based on the boundary
dynamics of a six-dimensional model in the presence of an induced gravity term
on the brane and a Gauss-Bonnet term in the bulk. With the assumption that the
bulk contains only a cosmological constant Lambda_B, we find that the isotropic
evolution of the brane-universe imposes a tuned relation between the energy
density and the brane equation of state. The evolution of the system has fixed
points (attractors), which correspond to a final state of radiation for
Lambda_B=0 and to de Sitter state for Lambda_B>0. Furthermore, considering
anisotropic matter on the brane, the tuning of the parameters is lifted, and
new regions of the parametric space are available for the cosmological
evolution of the brane-universe. The analysis of the dynamics of the system
shows that, the isotropic fixed points remain attractors of the system, and for
values of Lambda_B which give acceptable cosmological evolution of the equation
of state, the line of isotropic tuning is a very weak attractor. The initial
conditions, in this case, need to be fine tuned to have an evolution with
acceptably small anisotropy.Comment: 20 pages, 4 figures, typo correcte
Mirage Cosmology of U(1) Gauge Field on Unstable D3 Brane Universe
An unstable -brane universe governed by the DBI action of the tachyon
field minimally coupled to a U(1) gauge boson is examined. The cosmological
evolution of this coupled system, is further analyzed, in terms of the
expansion rate of the inflating brane, which is highly affected by the presence
of the tachyonic and gauge field charges. We show, that the minimal coupling
makes the effective brane density less divergent. However, for some sectors of
the theory the tachyon is not able to regulate it in an efficient fashion.
Also, a detailed analysis of the dependance of the effective brane density on
the scale factor of the universe is performed, which leads to various
cosmological models.Comment: ReVTeX format 20 pages; v2 1 figure added, one additional paragraph
with extra comments added, enlarged list of references, version to appear in
JHE
AdS/CFT Correspondence and the Reheating of the Brane-Universe
We present a mechanism for exit from inflation and reheating using the
AdS/CFT correspondence. A cosmological evolution is induced on a probe D3-brane
as it moves in a black D-brane background of type-0 string theory. If the
tachyon field is non zero, inflation is induced on the brane-universe, with the
equation of state parameter in the range -1<w<-1/3 depending on the position of
the probe brane in the bulk. As the probe brane approaches the horizon of the
background black hole, the inflation rate decreases and the value of w gets
larger. At some critical distance away from the horizon, inflation ends. When
the brane-universe reaches the horizon, the conformal invariance is restored,
the background geometry becomes AdS{5}X S{5}, and the brane-universe feels the
CFT thermal radiation and reheats.Comment: LaTex file, 18 pages, 2 figures, accepted for publication in JHE
Instability of brane cosmological solutions with flux compactifications
We discuss the stability of the higher-dimensional de Sitter (dS) brane
solutions with two-dimensional internal space in the Einstein-Maxwel theory. We
show that an instability appears in the scalar-type perturbations with respect
to the dS spacetime. We derive a differential relation which has the very
similar structure to the ordinary laws of thermodynamics as an extension of the
work for the six-dimensional model [20]. In this relation, the area of dS
horizon (integrated over the two internal dimensions) exactly behaves as the
thermodynamical entropy. The dynamically unstable solutions are in the
thermodynamically unstable branch. An unstable dS compactification either
evolves toward a stable configuration or two-dimensional internal space is
decompactified. These dS brane solutions are equivalent to the accelerating
cosmological solutions in the six-dimensional Einstein-Maxwell-dilaton theory
via dimensional reduction. Thus, if the seed higher-dimensional solution is
unstable, the corresponding six-dimensional solution is also unstable. From the
effective four-dimensional point of view, a cosmological evolution from an
unstable cosmological solution in higher dimensions may be seen as a process of
the transition from the initial cosmological inflation to the current dark
energy dominated Universe.Comment: 11 pages, 3 figures, references added, to appear in CQ
Braneworlds in six dimensions: new models with bulk scalars
Six dimensional bulk spacetimes with 3-- and 4--branes are constructed using
certain non--conventional bulk scalars as sources. In particular, we
investigate the consequences of having the phantom (negative kinetic energy)
and the Brans--Dicke scalar in the bulk while obtaining such solutions. We find
geometries with 4--branes with a compact on--brane dimension (hybrid
compactification) which may be assumed to be small in order to realize a
3--brane world. On the other hand, we also construct, with similar sources,
bulk spacetimes where a 3--brane is located at a conical singularity.
Furthermore, we investigate the issue of localization of matter fields (scalar,
fermion, graviton, vector) on these 3-- and 4--branes and conclude with
comments on our six dimensional models.Comment: 24 pages, 1 figure, Replaced to match version published in Class.
Quant. Gra
Irreversible Time Flow in a Two-Dimensional Dilaton Black Hole with Matter
We show that an exact solution of two-dimensional dilaton gravity with matter discovered previously exhibits an irreversible temporal flow towards flat space with a vanishing cosmological constant. This time flow is induced by the back reaction of matter on the space-time geometry. We demonstrate that the system is not in equilibrium if the cosmological constant is non-zero, whereas the solution with zero cosmological constant is stable. The flow of the system towards this stable end-point is derived from the renormalization-group flow of the Zamolodchikov function. This behaviour is interpreted in terms of non-critical Liouville string, with the Liouville field identified as the target time
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