47 research outputs found
Gauss-Bonnet Chameleon Mechanism of Dark Energy
As a model of the current accelerated expansion of the universe, we consider
a model of the scalar-Einstein-Gauss-Bonnet gravity. This model includes the
propagating scalar modes, which might give a large correction to the Newton
law. In order to avoid this problem, we propose an extension of the Chameleon
mechanism where the scalar mode becomes massive due to the coupling with the
Gauss-Bonnet term. Since the Gauss-Bonnet invariant does not vanish near the
earth or in the Solar System, even in the vacuum, the scalar mode is massive
even in the vacuum and the correction to the Newton law could be small. We also
discuss about the possibility that the model could describe simultaneously the
inflation in the early universe, in addition to the current accelerated
expansion.Comment: LaTeX 11 pages, no figur
Quadratic Curvature Gravity with Second Order Trace and Massive Gravity Models in Three Dimensions
The quadratic curvature lagrangians having metric field equations with second
order trace are constructed relative to an orthonormal coframe. In
dimensions, pure quadratic curvature lagrangian having second order trace
constructed contains three free parameters in the most general case. The fourth
order field equations of some of these models, in arbitrary dimensions, are
cast in a particular form using the Schouten tensor. As a consequence, the
field equations for the New massive gravity theory are related to those of the
Topologically massive gravity. In particular, the conditions under which the
latter is "square root" of the former are presented.Comment: 24 pages, to appear in GR
Brane World Cosmological Perturbations
We consider a brane world and its gravitational linear perturbations. We
present a general solution of the perturbations in the bulk and find the
complete perturbed junction conditions for generic brane dynamics. We also
prove that (spin 2) gravitational waves in the great majority of cases can only
arise in connection with a non-vanishing anisotropic stress. This has far
reaching consequences for inflation in the brane world. Moreover, contrary to
the case of the radion, perturbations are stable.Comment: 16 pages, one figur
Aspects of Scalar Field Dynamics in Gauss-Bonnet Brane Worlds
The Einstein-Gauss-Bonnet equations projected from the bulk to brane lead to
a complicated Friedmann equation which simplifies to in the
asymptotic regimes. The Randall-Sundrum (RS) scenario corresponds to
whereas & give rise to high energy Gauss-Bonnet (GB) regime and
the standard GR respectively. Amazingly, while evolving from RS regime to high
energy GB limit, one passes through a GR like region which has important
implications for brane world inflation. For tachyon GB inflation with
potentials investigated in this paper, the scalar to
tensor ratio of perturbations is maximum around the RS region and is
generally suppressed in the high energy regime for the positive values of .
The ratio is very low for at all energy scales relative to GB inflation
with ordinary scalar field. The models based upon tachyon inflation with
polynomial type of potentials with generic positive values of turn out to
be in the observational contour bound at all energy scales varying
from GR to high energy GB limit. The spectral index improves for the
lower values of and approaches its scale invariant limit for in the
high energy GB regime. The ratio also remains small for large negative
values of , however, difference arises for models close to scale invariance
limit. In this case, the tensor to scale ratio is large in the GB regime
whereas it is suppressed in the intermediate region between RS and GB. Within
the frame work of patch cosmologies governed by , the behavior
of ordinary scalar field near cosmological singularity and the nature of
scaling solutions are distinguished for the values of .Comment: 15 pages, 10 eps figures; appendix on various scales in GB brane
world included and references updated; final version to appear in PR
The unification of inflation and late-time acceleration in the frame of -essence
By using the formulation of the reconstruction, we explicitly construct
models of -essence, which unify the inflation in the early universe and the
late accelerating expansion of the present universe by a single scalar field.
Due to the higher derivative terms, the solution describing the unification can
be stable in the space of solutions, which makes the restriction for the
initial condition relaxed. The higher derivative terms also eliminate tachyon.
Therefore we can construct a model describing the time development, which
cannot be realized by a usual inflaton or quintessence models of the canonical
scalar field due to the instability or the existence of tachyon. We also
propose a mechanism of the reheating by the quantum effects coming from the
variation of the energy density of the scalar field.Comment: LaTeX, 13 pages, 10 figure
Stable phantom-divide crossing in two scalar models with matter
We construct cosmological models with two scalar fields, which has the
structure as in the ghost condensation model or k-essence model. The models can
describe the stable phantom crossing, which should be contrasted with one
scalar tensor models, where the infinite instability occurs at the crossing the
phantom divide. We give a general formulation of the reconstruction in terms of
the e-foldings N by including the matter although in the previous two scalar
models, which are extensions of the scalar tensor model, it was difficult to
give a formulation of the reconstruction when we include matters. In the
formulation of the reconstruction, we start with a model with some arbitrary
functions, and find the functions which generates the history in the expansion
of the universe. We also give general arguments for the stabilities of the
models and the reconstructed solution. The viability of a model is also
investigated by comparing the observational data.Comment: 12 pages, 1 figur
(Non)-singular brane-world cosmology induced by quantum effects in d5 dilatonic gravity
5d dilatonic gravity (bosonic sector of gauged supergravity) with non-trivial
bulk potential and with surface terms (boundary cosmological constant and trace
anomaly induced effective action for brane quantum matter) is considered. For
constant bulk potential and maximally SUSY Yang-Mills theory (CFT living on the
brane) the inflationary brane-world is constructed. The bulk is singular
asymptotically AdS space with non-constant dilaton and dilatonic de Sitter or
hyperbolic brane is induced by quantum matter effects. At the same time,
dilaton on the brane is determined dynamically. This all is natural realization
of warped compactification in AdS/CFT correspondence. For fine-tuned toy
example of non-constant bulk potential we found the non-singular dilatonic
brane-world where bulk again represents asymptotically AdS space and de Sitter
brane (inflationary phase of observable Universe) is induced exclusively by
quantum effects. The radius of the brane and dilaton are determined
dynamically. The analytically solvable example of exponential bulk potential
leading to singular asymptotically AdS dilatonic bulk space with de Sitter (or
hyperbolic) brane is also presented.In all cases under discussion the gravity
on the brane is trapped via Randall-Sundrum scenario. It is shown that
qualitatively the same types of brane-worlds occur when quantum brane matter is
described by dilaton coupled spinors.Comment: LaTeX file 28 pages and two eps files, few misprints are correcte
Future of the universe in modified gravitational theories: Approaching to the finite-time future singularity
We investigate the future evolution of the dark energy universe in modified
gravities including gravity, string-inspired scalar-Gauss-Bonnet and
modified Gauss-Bonnet ones, and ideal fluid with the inhomogeneous equation of
state (EoS). Modified Friedmann-Robertson-Walker (FRW) dynamics for all these
theories may be presented in universal form by using the effective ideal fluid
with an inhomogeneous EoS without specifying its explicit form. We construct
several examples of the modified gravity which produces accelerating
cosmologies ending at the finite-time future singularity of all four known
types by applying the reconstruction program. Some scenarios to resolve the
finite-time future singularity are presented. Among these scenarios, the most
natural one is related with additional modification of the gravitational action
in the early universe. In addition, late-time cosmology in the non-minimal
Maxwell-Einstein theory is considered. We investigate the forms of the
non-minimal gravitational coupling which generates the finite-time future
singularities and the general conditions for this coupling in order that the
finite-time future singularities cannot emerge. Furthermore, it is shown that
the non-minimal gravitational coupling can remove the finite-time future
singularities or make the singularity stronger (or weaker) in modified gravity.Comment: 25 pages, no figure, title changed, accepted in JCA
Comments on orientifold projection in the conifold and SO x USp duality cascade
We study the O3-plane in the conifold. On the D3-brane world-volume we obtain
SO x USp gauge theory that exhibits a duality cascade phenomenon. The
orientifold projection is determined on the type IIB string side, and
corresponds to that of O4-plane on the dual type IIA side. We show that SUGRA
solutions of Klebanov-Tseytlin and Klebanov-Strassler survive under the
projection. We also investigate the orientifold projection in the generalized
conifolds, and verify desired features of the O4-projection in the type IIA
picture.Comment: 1+27 pages, 9 figures, references added; version to appear in Phys.
Rev.
Dilatonic quantum multi-brane-worlds
d5 dilatonic gravity action with surface counterterms motivated by AdS/CFT
correspondence and with contributions of brane quantum CFTs is considered
around AdS-like bulk. The effective equations of motion are constructed. They
admit two (outer and inner) or multi-brane solutions where brane CFTs may be
different. The role of quantum brane CFT is in inducing of complicated brane
dilatonic gravity. For exponential bulk potentials the number of AdS-like bulk
spaces is found in analytical form.The correspondent flat or curved (de Sitter
or hyperbolic) dilatonic two branes are created, as a rule, thanks to quantum
effects. The observable early Universe may correspond to inflationary brane.
The found dilatonic quantum two brane-worlds usually contain the naked
singularity but in couple explicit examples the curvature is finite and horizon
(corresponding to wormhole-like space) appears.Comment: LaTeX file, 25 pages, discussion is enlarge