448 research outputs found
Cosmic Acceleration Data and Bulk-Brane Energy Exchange
We consider a braneworld model with bulk-brane energy exchange. This allows
for crossing of the w=-1 phantom divide line without introducing phantom energy
with quantum instabilities. We use the latest SnIa data included in the Gold06
dataset to provide an estimate of the preferred parameter values of this
braneworld model. We use three fitting approaches which provide best fit
parameter values and hint towards a bulk energy component that behaves like
relativistic matter which is propagating in the bulk and is moving at a speed v
along the fifth dimension, while the bulk-brane energy exchange component
corresponds to negative pressure and signifies energy flowing from the bulk
into the brane. We find that the best fit effective equation of state parameter
marginally crosses the phantom divide line w=-1. Thus, we have
demonstrated both the ability of this class of braneworld models to provide
crossing of the phantom divide and also that cosmological data hint towards
natural values for the model parameters.Comment: 12 pages, 2 figures, added comments, references update
Localizing gravity on thick branes: a solution for massive KK modes of the Schroedinger equation
We generate scalar thick brane configurations in a 5D Riemannian space time
which describes gravity coupled to a self-interacting scalar field. We also
show that 4D gravity can be localized on a thick brane which does not
necessarily respect Z_2-symmetry, generalizing several previous models based on
the Randall-Sundrum system and avoiding the restriction to orbifold geometries
as well as the introduction of the branes in the action by hand. We begin by
obtaining a smooth brane configuration that preserves 4D Poincar'e invariance
and violates reflection symmetry along the fifth dimension. The extra dimension
can have either compact or extended topology, depending on the values of the
parameters of the solution. In the non-compact case, our field configuration
represents a thick brane with positive energy density centered at y=c_2,
whereas in the compact case we get pairs of thick branes. We recast as well the
wave equations of the transverse traceless modes of the linear fluctuations of
the classical solution into a Schroedinger's equation form with a volcano
potential of finite bottom. We solve Schroedinger equation for the massless
zero mode m^2=0 and obtain a single bound wave function which represents a
stable 4D graviton and is free of tachyonic modes with m^2<0. We also get a
continuum spectrum of Kaluza-Klein (KK) states with m^2>0 that are suppressed
at y=c_2 and turn asymptotically into plane waves. We found a particular case
in which the Schroedinger equation can be solved for all m^2>0, giving us the
opportunity of studying analytically the massive modes of the spectrum of KK
excitations, a rare fact when considering thick brane configurations.Comment: 8 pages in latex. We corrected signs in the field equations, the
expressions for the scalar field and the self-interacting potential. Due to
the fact that no changes are introduced in the warp factor, the physics of
the system remains the sam
Brane Cosmology with a Non-Minimally Coupled Bulk-Scalar Field
We consider the cosmological evolution of a brane in the presence of a bulk
scalar field coupled to the Ricci scalar through a term f(\phi)R. We derive the
generalized Friedmann equation on the brane in the presence of arbitrary brane
and bulk-matter, as well as the scalar field equation, allowing for a general
scalar potential V(phi). We focus on a quadratic form of the above non-minimal
coupling and obtain a class of late-time solutions for the scale factor and the
scalar field on the brane that exhibit accelerated expansion for a range of the
non-minimal coupling parameter.Comment: 15 page
Braneworld models with a non-minimally coupled phantom bulk field: a simple way to obtain the -1-crossing at late times
We investigate general braneworld models, with a non-minimally coupled
phantom bulk field and arbitrary brane and bulk matter contents. We show that
the effective dark energy of the brane-universe acquires a dynamical nature, as
a result of the non-minimal coupling which provides a mechanism for an indirect
"bulk-brane interaction" through gravity. For late-time cosmological evolution
and without resorting to special ansatzes or to specific areas of the parameter
space, we show that the -1-crossing of its equation-of-state parameter is
general and can be easily achieved. As an example we provide a simple, but
sufficiently general, approximate analytical solution, that presents the
crossing behavior.Comment: 11 pages, 2 figure
Remarks on the Scalar Graviton Decoupling and Consistency of Horava Gravity
Recently Horava proposed a renormalizable gravity theory with higher
derivatives by abandoning the Lorenz invariance in UV. But there have been
confusions regarding the extra scalar graviton mode and the consistency of the
Horava model. I reconsider these problems and show that, in the Minkowski
vacuum background, the scalar graviton mode can be consistency decoupled from
the usual tensor graviton modes by imposing the (local) Hamiltonian as well as
the momentum constraints.Comment: Some clarifications regarding the projectable case added, Typos
corrected, Comments (Footnote No.9, Note Added) added, References updated,
Accepted in CQ
The phase portrait of a matter bounce in Horava-Lifshitz cosmology
The occurrence of a bounce in FRW cosmology requires modifications of general
relativity. An example of such a modification is the recently proposed
Horava-Lifshitz theory of gravity, which includes a ``dark radiation'' term
with a negative coefficient in the analog of the Friedmann equation. This paper
describes a phase space analysis of models of this sort with the aim of
determining to what extent bouncing solutions can occur. A simplification,
valid in the relevant region, allows a reduction of the dimension of phase
space so that visualization in three dimensions is possible. It is found that a
bounce is possible, but not generic in models under consideration. Apart from
previously known bouncing solutions some new ones are also described. Other
interesting solutions found include ones which describe a novel sort of
oscillating universes.Comment: 14 pages, 8 figure
The Cosmological Constant and Horava-Lifshitz Gravity
Horava-Lifshitz theory of gravity with detailed balance is plagued by the
presence of a negative bare (or geometrical) cosmological constant which makes
its cosmology clash with observations. We argue that adding the effects of the
large vacuum energy of quantum matter fields, this bare cosmological constant
can be approximately compensated to account for the small observed (total)
cosmological constant. Even though we cannot address the fine-tuning problem in
this way, we are able to establish a relation between the smallness of observed
cosmological constant and the length scale at which dimension 4 corrections to
the Einstein gravity become significant for cosmology. This scale turns out to
be approximately 5 times the Planck length for an (almost) vanishing observed
cosmological constant and we therefore argue that its smallness guarantees that
Lorentz invariance is broken only at very small scales. We are also able to
provide a first rough estimation for the infrared values of the parameters of
the theory and .Comment: 9 pages, Late
DGP Cosmology with a Non-Minimally Coupled Scalar Field on the Brane
We construct a DGP inspired braneworld scenario where a scalar field
non-minimally coupled to the induced Ricci curvature is present on the brane.
First we investigate the status of gravitational potential with non-minimal
coupling and observational constraints on this non-minimal model. Then we
further deepen the idea of embedding of FRW cosmology in this non-minimal
setup. Cosmological implications of this scenario are examined with details and
the quintessence and late-time expansion of the universe within this framework
are examined. Some observational constraints imposed on this non-minimal
scenario are studied and relation of this model with dark radiation formalism
is determined with details.Comment: 26 pages, 3 eps figure
Detailed balance condition and ultraviolet stability of scalar field in Horava-Lifshitz gravity
Detailed balance and projectability conditions are two main assumptions when
Horava recently formulated his theory of quantum gravity - the Horava-Lifshitz
(HL) theory. While the latter represents an important ingredient, the former
often believed needs to be abandoned, in order to obtain an ultraviolet stable
scalar field, among other things. In this paper, because of several attractive
features of this condition, we revisit it, and show that the scalar field can
be stabilized, if the detailed balance condition is allowed to be softly
broken. Although this is done explicitly in the non-relativistic general
covariant setup of Horava-Melby-Thompson with an arbitrary coupling constant
, generalized lately by da Silva, it is also true in other versions of
the HL theory. With the detailed balance condition softly breaking, the number
of independent coupling constants can be still significantly reduced. It is
remarkable to note that, unlike other setups, in this da Silva generalization,
there exists a master equation for the linear perturbations of the scalar field
in the flat Friedmann-Robertson-Walker background.Comment: Some typos are corrected. To appear in JCA
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