26 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
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
Vacuum destabilization from Kaluza-Klein modes in an inflating brane
We discuss the effects from the Kaluza-Klein modes in the brane world
scenario when an interaction between bulk and brane fields is included. We
focus on the bulk inflaton model, where a bulk field drives inflation in
an almost bulk bounded by an inflating brane. We couple to a
brane scalar field representing matter on the brane. The bulk field
is assumed to have a light mode, whose mass depends on the expectation
value of . To estimate the effects from the KK modes, we compute the
1-loop effective potential V_\eff(\phi). With no tuning of the parameters of
the model, the vacuum becomes (meta)stable -- V_\eff(\phi) develops a true
vacuum at a nonzero . In the true vacuum, the light mode of
becomes heavy, degenerates with the KK modes and decays. We comment on some
implications for the bulk inflaton model. Also, we clarify some aspects of the
renormalization procedure in the thin wall approximation, and show that the
fluctuations in the bulk and on the brane are closely related.Comment: 15 pages, 2 eps figures. Notation improved, references adde
A healthy extension of Horava gravity
We propose a natural extension of Horava's model for quantum gravity, which
is free from the notorious pathologies of the original proposal. The new model
endows the scalar graviton mode with a regular quadratic action and remains
power-counting renormalizable. At low energies, it reduces to a
Lorentz-violating scalar-tensor gravity theory. The deviations with respect to
general relativity can be made weak by an appropriate choice of parameters.Comment: 4 pages, no figure
On the Extra Mode and Inconsistency of Horava Gravity
We address the consistency of Horava's proposal for a theory of quantum
gravity from the low-energy perspective. We uncover the additional scalar
degree of freedom arising from the explicit breaking of the general covariance
and study its properties. The analysis is performed both in the original
formulation of the theory and in the Stueckelberg picture. A peculiarity of the
new mode is that it satisfies an equation of motion that is of first order in
time derivatives. At linear level the mode is manifest only around spatially
inhomogeneous and time-dependent backgrounds. We find two serious problems
associated with this mode. First, the mode develops very fast exponential
instabilities at short distances. Second, it becomes strongly coupled at an
extremely low cutoff scale. We also discuss the "projectable" version of
Horava's proposal and argue that this version can be understood as a certain
limit of the ghost condensate model. The theory is still problematic since the
additional field generically forms caustics and, again, has a very low strong
coupling scale. We clarify some subtleties that arise in the application of the
Stueckelberg formalism to Horava's model due to its non-relativistic nature.Comment: Discussion expanded; a figure added; accepted to JHE
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
Primordial fluctuations and non-Gaussianities from multifield DBI Galileon inflation
We study a cosmological scenario in which the DBI action governing the motion
of a D3-brane in a higher-dimensional spacetime is supplemented with an induced
gravity term. The latter reduces to the quartic Galileon Lagrangian when the
motion of the brane is non-relativistic and we show that it tends to violate
the null energy condition and to render cosmological fluctuations ghosts. There
nonetheless exists an interesting parameter space in which a stable phase of
quasi-exponential expansion can be achieved while the induced gravity leaves
non trivial imprints. We derive the exact second-order action governing the
dynamics of linear perturbations and we show that it can be simply understood
through a bimetric perspective. In the relativistic regime, we also calculate
the dominant contribution to the primordial bispectrum and demonstrate that
large non-Gaussianities of orthogonal shape can be generated, for the first
time in a concrete model. More generally, we find that the sign and the shape
of the bispectrum offer powerful diagnostics of the precise strength of the
induced gravity.Comment: 34 pages including 9 figures, plus appendices and bibliography.
Wordings changed and references added; matches version published in JCA
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
Chaotic inflation in modified gravitational theories
We study chaotic inflation in the context of modified gravitational theories.
Our analysis covers models based on (i) a field coupling with
the kinetic energy and a nonmimimal coupling with a
Ricci scalar , (ii) Brans-Dicke (BD) theories, (iii) Gauss-Bonnet (GB)
gravity, and (iv) gravity with a Galileon correction. Dilatonic coupling with
the kinetic energy and/or negative nonminimal coupling are shown to lead to
compatibility with observations of the Cosmic Microwave Background (CMB)
temperature anisotropies for the self-coupling inflaton potential
. BD theory with a quadratic inflaton potential,
which covers Starobinsky's model with the BD
parameter , gives rise to a smaller tensor-to-scalar ratio for
decreasing . In the presence of a GB term coupled to the field
, we express the scalar/tensor spectral indices and as
well as the tensor-to-scalar ratio in terms of two slow-roll parameters and
place bounds on the strength of the GB coupling from the joint data analysis of
WMAP 7yr combined with other observations. We also study the Galileon-like
self-interaction with exponential coupling
. Using a CMB likelihood analysis we put bounds
on the strength of the Galileon coupling and show that the self coupling
potential can in fact be made compatible with observations in the presence of
the exponential coupling with .Comment: 28 pages, 8 figure