3,845 research outputs found
Gravitational backreaction of anti-D branes in the warped compactification
We derive a low-energy effective theory for gravity with anti-D branes, which
are essential to get de Sitter solutions in the type IIB string warped
compactification, by taking account of gravitational backreactions of anti-D
branes. In order to see the effects of the self-gravity of anti-D branes, a
simplified model is studied where a 5-dimensional anti-de Sitter ({\it AdS})
spacetime is realized by the bulk cosmological constant and the 5-form flux,
and anti-D branes are coupled to the 5-form field by Chern-Simon terms. The
{\it AdS} spacetime is truncated by introducing UV and IR cut-off branes like
the Randall-Sundrum model. We derive an effective theory for gravity on the UV
brane and reproduce the familiar result that the tensions of the anti-D branes
give potentials suppressed by the forth-power of the warp factor at the
location of the anti-D branes. However, in this simplified model, the potential
energy never inflates the UV brane, although the anti-D-branes are inflating.
The UV brane is dominated by dark radiation coming from the projection of the
5-dimensional Weyl tensor, unless the moduli fields for the anti-D branes are
stabilized. We comment on the possibility of avoiding this problem in a
realistic string theory compactification.Comment: typos corrected, 11 pages, 3 figure
Induced gravity on intersecting brane-worlds Part I: Maximally symmetric solutions
We explore models of intersecting brane-worlds with induced gravity terms on
codimension one branes and on their intersection. Maximally symmetric solutions
for the branes and the intersection are found. We find new self-accelerating
solutions. In a 6d spacetime, the solutions realize the see-saw modification of
gravity where the UV scale of the modification to 4d gravity is determined by
6d Planck scale given by eV and the IR scale of the
modification is determined by GeV where
is present-day Hubble scale. We find that it is increasingly difficult to
construct phenomenologically viable models in higher-dimensional spacetime due
to the necessity to have the lower value for the fundamental Planck scale to
realize the late time acceleration. It is found that the system also admits
self-tuning solutions where the tension at the intersection does not change the
geometry of the intersection. The induced gravity terms can avoid the necessity
to compactify the extra dimensions. Finally, we discuss the possibility to have
ordinary matter at the intersection, without introducing any regularisation,
using the induced gravity terms.Comment: 16 pages, some mistakes in the identification of the higher
codimensional singular structure corrected. Main results unchange
Coupled boundary and bulk fields in anti-de Sitter
We investigate the dynamics of a boundary field coupled to a bulk field with
a linear coupling in an anti-de Sitter bulk spacetime bounded by a Minkowski
(Randall-Sundrum) brane. An instability criterion for the coupled boundary and
bulk system is found. There exists a tachyonic bound state when the coupling is
above a critical value, determined by the masses of the brane and bulk fields
and AdS curvature scale. This bound state is normalizable and localised near
the brane, and leads to a tachonic instability of the system on large scales.
Below the critical coupling, there is no tachyonic state and no bound state.
Instead, we find quasi-normal modes which describe stable oscillations, but
with a finite decay time. Only if the coupling is tuned to the critical value
does there exist a massless stable bound state, as in the case of zero coupling
for massless fields. We discuss the relation to gravitational perturbations in
the Randall-Sundrum brane-world.Comment: 12 pages, 4 figures, revtex
More on ghosts in DGP model
It is shown by an explicit calculation that the excitations about the
self-accelerating cosmological solution of the Dvali--Gabadaze--Porrati model
contain a ghost mode. This raises serious doubts about viability of this
solution. Our analysis reveals the similarity between the quadratic theory for
the perturbations around the self-accelerating Universe and an Abelian gauge
model with two Stueckelberg fields.Comment: Revtex, 9 pages, no figure
Characterising Vainshtein Solutions in Massive Gravity
We study static, spherically symmetric solutions in a recently proposed
ghost-free model of non-linear massive gravity. We focus on a branch of
solutions where the helicity-0 mode can be strongly coupled within certain
radial regions, giving rise to the Vainshtein effect. We truncate the analysis
to scales below the gravitational Compton wavelength, and consider the weak
field limit for the gravitational potentials, while keeping all non-linearities
of the helicity-0 mode. We determine analytically the number and properties of
local solutions which exist asymptotically on large scales, and of local
(inner) solutions which exist on small scales. We find two kinds of asymptotic
solutions, one of which is asymptotically flat, while the other one is not, and
also two types of inner solutions, one of which displays the Vainshtein
mechanism, while the other exhibits a self-shielding behaviour of the
gravitational field. We analyse in detail in which cases the solutions match in
an intermediate region. The asymptotically flat solutions connect only to inner
configurations displaying the Vainshtein mechanism, while the non
asymptotically flat solutions can connect with both kinds of inner solutions.
We show furthermore that there are some regions in the parameter space where
global solutions do not exist, and characterise precisely in which regions of
the phase space the Vainshtein mechanism takes place.Comment: 21 pages, 7 figures, published versio
Inflaton perturbations in brane-world cosmology with induced gravity
We study cosmological perturbations in the brane models with an induced
Einstein-Hilbert term on a brane. We consider an inflaton confined to a de
Sitter brane in a five-dimensional Minkowski spacetime. Inflaton fluctuations
excite Kaluza-Klein modes of bulk metric perturbations with mass and where is an
integer. There are two branches ( branches) of solutions for the
background spacetime. In the branch, which includes the self-accelerating
universe, a resonance appears for a mode with due to a spin-0
perturbation with . The self-accelerating universe has a distinct
feature because there is also a helicity-0 mode of spin-2 perturbations with
. In the branch, which can be thought as the Randall-Sundrum
type brane-world with the high energy quantum corrections, there is no
resonance. At high energies, we analytically confirm that four-dimensional
Einstein gravity is recovered, which is related to the disappearance of van
Dam-Veltman-Zakharov discontinuity in de Sitter spacetime.
On sufficiently small scales, we confirm that the lineariaed gravity on the
brane is well described by the Brans-Dicke theory with in
branch and in branch, respectively, which confirms the
existence of the ghost in branch. We also study large scale perturbations.
In branch, the resonance induces a non-trivial anisotropic stress on the
brane via the projection of Weyl tensor in the bulk, but no instability is
shown to exist on the brane.Comment: 20 pages, 4 figure
Scalar cosmological perturbations in the Gauss-Bonnet braneworld
We study scalar cosmological perturbations in a braneworld model with a bulk
Gauss-Bonnet term. For an anti-de Sitter bulk, the five-dimensional
perturbation equations share the same form as in the Randall-Sundrum model,
which allows us to obtain metric perturbations in terms of a master variable.
We derive the boundary conditions for the master variable from the generalized
junction conditions on the brane. We then investigate several limiting cases in
which the junction equations are reduced to a feasible level. In the low energy
limit, we confirm that the standard result of four-dimensional Einstein gravity
is reproduced on large scales, whereas on small scales we find that the
perturbation dynamics is described by the four-dimensional Brans-Dicke theory.
In the high energy limit, all the non-local contributions drop off from the
junction equations, leaving a closed system of equations on the brane. We show
that, for inflation models driven by a scalar field on the brane, the
Sasaki-Mukhanov equation holds on the high energy brane in its original
four-dimensional form.Comment: 18 pages, v2: minor changes, reference added, v3: comments and
references added, accepted for publication in JCA
Primordial perturbations from slow-roll inflation on a brane
In this paper we quantise scalar perturbations in a Randall-Sundrum-type
model of inflation where the inflaton field is confined to a single brane
embedded in five-dimensional anti-de Sitter space-time. In the high energy
regime, small-scale inflaton fluctuations are strongly coupled to metric
perturbations in the bulk and gravitational back-reaction has a dramatic effect
on the behaviour of inflaton perturbations on sub-horizon scales. This is in
contrast to the standard four-dimensional result where gravitational
back-reaction can be neglected on small scales. Nevertheless, this does not
give rise to significant particle production, and the correction to the power
spectrum of the curvature perturbations on super-horizon scales is shown to be
suppressed by a slow-roll parameter. We calculate the complete first order
slow-roll corrections to the spectrum of primordial curvature perturbations.Comment: 23 pages, 10 figure
Diffuse Hard X-ray Sources Discovered with the ASCA Galactic Plane Survey
We found diffuse hard X-ray sources, G11.0+0.0, G25.5+0.0, and G26.6-0.1 in
the ASCA Galactic plane survey data. The X-ray spectra are featureless with no
emission line, and are fitted with both models of a thin thermal plasma in
non-equilibrium ionization and a power-law function. The source distances are
estimated to be 1-8 kpc, using the best-fit NH values on the assumption that
the mean density in the line of sight is 1 H cm^-3. The source sizes and
luminosities are then 4.5-27 pc and (0.8-23)x10^33 ergs/s. Although the source
sizes are typical to supernova remnants (SNR) with young to intermediate ages,
the X-ray luminosity, plasma temperature, and weak emission lines in the
spectra are all unusual. This suggests that these objects are either shell-like
SNRs dominated by X-ray synchrotron emission, like SN 1006, or, alternatively,
plerionic SNRs. The total number of these classes of SNRs in our Galaxy is also
estimated.Comment: 17 pages, 9 figures; to appear in Ap
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