1,876 research outputs found
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
The Cosmological Constant as an Eigenvalue of a Sturm-Liouville Problem and its Renormalization
We discuss the case of massive gravitons and their relation with the
cosmological constant, considered as an eigenvalue of a Sturm-Liouville
problem. A variational approach with Gaussian trial wave functionals is used as
a method to study such a problem. We approximate the equation to one loop in a
Schwarzschild background and a zeta function regularization is involved to
handle with divergences. The regularization is closely related to the
subtraction procedure appearing in the computation of Casimir energy in a
curved background. A renormalization procedure is introduced to remove the
infinities together with a renormalization group equation.Comment: 8 pages, Talk given at "QFEXT'05", the 7-th workshop on quantum field
theory under the influence of external conditions, Barcelona, Spain, Sept.
5-9, 200
Tunneling into Extra Dimension and High-Energy Violation of Lorentz Invariance
We consider a class of models with infinite extra dimension, where bulk space
does not possess SO(1,3) invariance, but Lorentz invariance emerges as an
approximate symmetry of the low-energy effective theory. In these models, the
maximum attainable speeds of the graviton, gauge bosons and scalar particles
are automatically equal to each other and smaller than the maximum speed in the
bulk. Additional fine-tuning is needed in order to assure that the maximum
attainable speed of fermions takes the same value. A peculiar feature of our
scenario is that there are no truly localized modes. All four-dimensional
particles are resonances with finite widths. The latter depends on the energy
of the particle and is naturally small at low energies.Comment: 21 pages, references and comments added, final version to appear in
JHE
Low-energy effects in brane worlds: Liennard-Wiechert potentials and Hydrogen Lamb shift
Testing extra dimensions at low-energies may lead to interesting effects. In
this work a test point charge is taken to move uniformly in the 3-dimensional
subspace of a (3+)-brane embedded in a (3++1)-space with compact and
one warped infinite spatial extra dimensions. We found that the electromagnetic
potentials of the point charge match standard Liennard-Wiechert's at large
distances but differ from them close to it. These are finite at the position of
the charge and produce finite self-energies. We also studied a localized
Hydrogen atom and take the deviation from the standard Coulomb potential as a
perturbation. This produces a Lamb shift that is compared with known
experimental data to set bounds for the parameter of the model. This work
provides details and extends results reported in a previous Letter.Comment: Manuscript (LaTeX) and 2 figure files (eps format) used by the
manuscript LaTeX fil
Positronium oscillations to Mirror World revisited
We present a calculation of the branching ratio of orthopositronium decay
into an invisible mode, which is done in the context of Mirror World models,
where ordinary positronium can disappear from our world due to oscillation into
its mirror twin. In this revision we clarify some formulas and approximations
used previously, correct them at some places, add new effects relevant for a
feasible experiment and finally perform a combined analysis. We include into
consideration various effects due to external magnetic and electric fields,
collisions with cavity walls and scattering off gas atoms in the cavity.
Oscillations of the Rydberg positroniums are also considered. To perform a
numerical estimates in a realistic case we wrote computer code, which can be
adopted in any experimental setup. Its work is illustrated with an example of a
planned positronium experiment within the AEgIS project.Comment: 23 pages, 4 figures, typos corrected, references added, published
versio
Quantum Horizons of the Standard Model Landscape
The long-distance effective field theory of our Universe--the Standard Model
coupled to gravity--has a unique 4D vacuum, but we show that it also has a
landscape of lower-dimensional vacua, with the potential for moduli arising
from vacuum and Casimir energies. For minimal Majorana neutrino masses, we find
a near-continuous infinity of AdS3xS1 vacua, with circumference ~20 microns and
AdS3 length 4x10^25 m. By AdS/CFT, there is a CFT2 of central charge c~10^90
which contains the Standard Model (and beyond) coupled to quantum gravity in
this vacuum. Physics in these vacua is the same as in ours for energies between
10^-1 eV and 10^48 GeV, so this CFT2 also describes all the physics of our
vacuum in this energy range. We show that it is possible to realize
quantum-stabilized AdS vacua as near-horizon regions of new kinds of quantum
extremal black objects in the higher-dimensional space--near critical black
strings in 4D, near-critical black holes in 3D. The violation of the
null-energy condition by the Casimir energy is crucial for these horizons to
exist, as has already been realized for analogous non-extremal 3D black holes
by Emparan, Fabbri and Kaloper. The new extremal 3D black holes are
particularly interesting--they are (meta)stable with an entropy independent of
hbar and G_N, so a microscopic counting of the entropy may be possible in the
G_N->0 limit. Our results suggest that it should be possible to realize the
larger landscape of AdS vacua in string theory as near-horizon geometries of
new extremal black brane solutions.Comment: 44 pages, 9 figure
Effective String Theory Revisited
We revisit the effective field theory of long relativistic strings such as
confining flux tubes in QCD. We derive the Polchinski-Strominger interaction by
a calculation in static gauge. This interaction implies that a non-critical
string which initially oscillates in one direction gets excited in orthogonal
directions as well. In static gauge no additional term in the effective action
is needed to obtain this effect. It results from a one-loop calculation using
the Nambu-Goto action. Non-linearly realized Lorentz symmetry is manifest at
all stages in dimensional regularization. We also explain that independent of
the number of dimensions non-covariant counterterms have to be added to the
action in the commonly used zeta-function regularization.Comment: 21 pages, 4 figures, v2: typo corrected, references added, published
versio
Blown-up p-Branes and the Cosmological Constant
We consider a blown-up 3-brane, with the resulting geometry R^(3,1) \times
S^(N-1), in an infinite-volume bulk with N > 2 extra dimensions. The action on
the brane includes both an Einstein term and a cosmological constant. Similar
setups have been proposed both to reproduce 4-d gravity on the brane, and to
solve the cosmological constant problem. Here we obtain a singularity-free
solution to Einstein's equations everywhere in the bulk and on the brane, which
allows us to address these question explicitely. One finds, however, that the
proper volume of S^(N-1) and the cosmological constant on the brane have to be
fine-tuned relatively to each other, thus the cosmological constant problem is
not solved. Moreover the scalar propagator on the brane behaves 4-dimensionally
over a phenomenologically acceptable range only if the warp factor on the brane
is huge, which aggravates the Weak Scale - Planck Scale hierarchy problem.Comment: 21 pages, no figure
Drastic effects of damping mechanisms on the third-order optical nonlinearity
We have investigated the optical response of superradiant atoms, which
undergoes three different damping mechanisms: radiative dissipation
(), dephasing (), and nonradiative dissipation
(). Whereas the roles of and are equivalent in
the linear susceptibility, the third-order nonlinear susceptibility drastically
depends on the ratio of and : When , the third-order susceptibility is essentially that of a single atom.
Contrarily, in the opposite case of , the third-order
susceptibility suffers the size-enhancement effect and becomes proportional to
the system size.Comment: 5pages, 2figure
Bosonic String Quantization in Static Gauge
The bosonic string in D dimensional Minkowski space-time is quantized in
static gauge. It is shown that the system can be described by D-1 massless free
fields constrained on the surface L_m = 0, for m \neq 0, where L_m are the
generators of conformal transformations. The free fields are quantized and the
physical states are selected by the conditions L_m|phys>=0, for m>0. The
Poincar\'e group generators on the physical Hilbert space are constructed and
the critical dimension D=26 is recovered from the commutation relations of the
boost operators. The equivalence with the covariant quantization is
established. A possible generalization to the AdS string dynamics is discussed.Comment: 10 page
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