29 research outputs found
Wightman function and vacuum densities for a Z_2-symmetric thick brane in AdS spacetime
Positive frequency Wightman function, vacuum expectation values of the field
square and the energy-momentum tensor induced by a Z_{2}-symmetric brane with
finite thickness located on (D+1)- dimensional AdS background are evaluated for
a massive scalar field with general curvature coupling parameter. For the
general case of static plane symmetric interior structure the expectation
values in the region outside the brane are presented as the sum of free AdS and
brane induced parts. For a conformally coupled massless scalar the brane
induced part in the vacuum energy-momentum tensor vanishes. In the limit of
strong gravitational fields the brane induced parts are exponentially
suppressed for points not too close to the brane boundary. As an application of
general results a special model is considered in which the geometry inside the
brane is a slice of the Minkowski spacetime orbifolded along the direction
perpendicular to the brane. For this model the Wightman function, vacuum
expectation values of the field square and the energy-momentum tensor inside
the brane are evaluated as well and their behavior is discussed in various
asymptotic regions of the parameters. It is shown that for both minimally and
conformally coupled scalar fields the interior vacuum forces acting on the
brane boundaries tend to decrease the brane thickness.Comment: 25 pages, 6 figures, discussion adde
Dynamics of domain walls intersecting black holes
Previous studies concerning the interaction of branes and black holes
suggested that a small black hole intersecting a brane may escape via a
mechanism of reconnection. Here we consider this problem by studying the
interaction of a small black hole and a domain wall composed of a scalar field
and simulate the evolution of this system when the black hole acquires an
initial recoil velocity. We test and confirm previous results, however, unlike
the cases previously studied, in the more general set-up considered here, we
are able to follow the evolution of the system also during the separation, and
completely illustrate how the escape of the black hole takes place.Comment: 4 pages, 6 figure
Vacuum densities for a thick brane in AdS spacetime
For a massive scalar field with general curvature coupling parameter we
evaluate Wightman function, vacuum expectation values of the field square and
the energy-momentum tensor induced by a -symmetric brane with finite
thickness located on -dimensional AdS bulk. For the general case of
static plane symmetric interior structure the expectation values in the region
outside the brane are presented as the sum of free AdS and brane induced parts.
For a conformally coupled massless scalar the brane induced part in the vacuum
energy-momentum tensor vanishes. In the limit of strong gravitational fields
the brane induced parts are exponentially suppressed for points not too close
to the brane boundary. As an application of general results a special model is
considered in which the geometry inside the brane is a slice of the Minkowski
spacetime orbifolded along the direction perpendicular to the brane. For this
model the Wightman function, vacuum expectation values of the field square and
the energy-momentum tensor inside the brane are evaluated. It is shown that for
both minimally and conformally coupled scalar fields the interior vacuum forces
acting on the brane boundaries tend to decrease the brane thickness.Comment: 12 pages, 2 figures, talk presented at QFEXT07, Leipzig, September
17-21, 200
Massless scalar fields and infrared divergences in the inflationary brane world
We study the quantum effects induced by bulk scalar fields in a model with a
de Sitter (dS) brane in a flat bulk (the Vilenkin-Ipser-Sikivie model) in more
than four dimensions. In ordinary dS space, it is well known that the stress
tensor in the dS invariant vacuum for an effectively massless scalar
(m_\eff^2=m^2+\xi {\cal R}=0 with the Ricci scalar) is infrared
divergent except for the minimally coupled case. The usual procedure to tame
this divergence is to replace the dS invariant vacuum by the Allen Follaci (AF)
vacuum. The resulting stress tensor breaks dS symmetry but is regular.
Similarly, in the brane world context, we find that the dS invariant vacuum
generates \tmn divergent everywhere when the lowest lying mode becomes
massless except for massless minimal coupling case. A simple extension of the
AF vacuum to the present case avoids this global divergence, but \tmn remains
to be divergent along a timelike axis in the bulk. In this case, singularities
also appear along the light cone emanating from the origin in the bulk,
although they are so mild that \tmn stays finite except for non-minimal
coupling cases in four or six dimensions. We discuss implications of these
results for bulk inflaton models. We also study the evolution of the field
perturbations in dS brane world. We find that perturbations grow linearly with
time on the brane, as in the case of ordinary dS space. In the bulk, they are
asymptotically bounded.Comment: 20 pages. References adde
Imperfect Dark Energy from Kinetic Gravity Braiding
We introduce a large class of scalar-tensor models with interactions
containing the second derivatives of the scalar field but not leading to
additional degrees of freedom. These models exhibit peculiar features, such as
an essential mixing of scalar and tensor kinetic terms, which we have named
kinetic braiding. This braiding causes the scalar stress tensor to deviate from
the perfect-fluid form. Cosmology in these models possesses a rich
phenomenology, even in the limit where the scalar is an exact Goldstone boson.
Generically, there are attractor solutions where the scalar monitors the
behaviour of external matter. Because of the kinetic braiding, the position of
the attractor depends both on the form of the Lagrangian and on the external
energy density. The late-time asymptotic of these cosmologies is a de Sitter
state. The scalar can exhibit phantom behaviour and is able to cross the
phantom divide with neither ghosts nor gradient instabilities. These features
provide a new class of models for Dark Energy. As an example, we study in
detail a simple one-parameter model. The possible observational signatures of
this model include a sizeable Early Dark Energy and a specific equation of
state evolving into the final de-Sitter state from a healthy phantom regime.Comment: 41 pages, 7 figures. References and some clarifying language added.
This version was accepted for publication in JCA
Quantum self-consistency of brane models
Continuing on our previous work, we consider a class of higher dimensional
brane models with the topology of , where
is a one-parameter compact manifold and two branes of codimension 1 are located
at the orbifold fixed points. We consider a set-up where such a solution arises
from Einstein-Yang-Mills theory and evaluate the one-loop effective potential
induced by gauge fields and by a generic bulk scalar field. We show that this
type of brane models resolves the gauge hierarchy between the Planck and
electroweak scales through redshift effects due to the warp factor . The value of is then fixed by minimizing the effective potential. We
find that, as in the Randall Sundrum case, the gauge field contribution to the
effective potential stabilises the hierarchy without fine-tuning as long as the
laplacian on has a zero eigenvalue. Scalar fields can
stabilise the hierarchy depending on the mass and the non-minimal coupling. We
also address the quantum self-consistency of the solution, showing that the
classical brane solution is not spoiled by quantum effects.Comment: 10 page
Boundary Terms and Junction Conditions for Generalized Scalar-Tensor Theories
We compute the boundary terms and junction conditions for Horndeski's
panoptic class of scalar-tensor theories, and write the bulk and boundary
equations of motion in explicitly second order form. We consider a number of
special subclasses, including galileon theories, and present the corresponding
formulae. Our analysis opens up of the possibility of studying tunnelling
between vacua in generalized scalar-tensor theories, and braneworld dynamics.
The latter follows because our results are independent of spacetime dimension.Comment: 13 pages, Equation corrected. Thanks to Tsutomu Kobayashi for
informing us of the typ
The Imperfect Fluid behind Kinetic Gravity Braiding
We present a standard hydrodynamical description for non-canonical scalar
field theories with kinetic gravity braiding. In particular, this picture
applies to the simplest galileons and k-essence. The fluid variables not only
have a clear physical meaning but also drastically simplify the analysis of the
system. The fluid carries charges corresponding to shifts in field space. This
shift-charge current contains a spatial part responsible for diffusion of the
charges. Moreover, in the incompressible limit, the equation of motion becomes
the standard diffusion equation. The fluid is indeed imperfect because the
energy flows neither along the field gradient nor along the shift current. The
fluid has zero vorticity and is not dissipative: there is no entropy
production, the energy-momentum is exactly conserved, the temperature vanishes
and there is no shear viscosity. Still, in an expansion around a perfect fluid
one can identify terms which correct the pressure in the manner of bulk
viscosity. We close by formulating the non-trivial conditions for the
thermodynamic equilibrium of this imperfect fluid.Comment: 23 pages plus appendices. New version includes extended discussion on
diffusion and dynamics in alternative frames, as well as additional
references. v3 reflects version accepted for publication in JHEP: minor
comments added regarding suitability to numerical approache
Supersymmetric Aether
It has been suggested by Groot Nibbelink and Pospelov that Lorentz invariance
can be an emergent symmetry of low-energy physics provided the theory enjoys a
non-relativistic version of supersymmetry. We construct a model that realizes
the latter symmetry dynamically: it breaks Lorentz invariance but leaves the
supersymmetry generators intact. The model is a supersymmetric extension of the
dynamical aether theory of Jacobson and Mattingly. It shows rich dynamics and
possesses a family of inequivalent vacua realizing different symmetry breaking
patterns. In particular, we find stable vacua that break spontaneously spatial
isotropy. Supersymmetry breaking terms give masses to fermionic and bosonic
partners of the aether field. We comment on the coupling of the model to
supergravity and on the implications for Horava gravity.Comment: 21 pages, no figure
Lorentz violation, Gravity, Dissipation and Holography
We reconsider Lorentz Violation (LV) at the fundamental level. We show that
Lorentz Violation is intimately connected with gravity and that LV couplings in
QFT must always be fields in a gravitational sector. Diffeomorphism invariance
must be intact and the LV couplings transform as tensors under coordinate/frame
changes. Therefore searching for LV is one of the most sensitive ways of
looking for new physics, either new interactions or modifications of known
ones. Energy dissipation/Cerenkov radiation is shown to be a generic feature of
LV in QFT. A general computation is done in strongly coupled theories with
gravity duals. It is shown that in scale invariant regimes, the energy
dissipation rate depends non-triviallly on two characteristic exponents, the
Lifshitz exponent and the hyperscaling violation exponent.Comment: LateX, 51 pages, 9 figures. (v2) References and comments added.
Misprints correcte