40 research outputs found
Effective 4D propagation of a charged scalar particle in Visser brane world
In this work we extend an analysis due to Visser of the effective propagation
of a neutral scalar particle on a brane world scenario which is a particular
solution of the five dimensional Einstein-Maxwell equations with cosmological
constant having an electric field pointing in the extra spatial dimension. We
determine the dispersion relations of a charged scalar particle to first order
in a perturbative analysis around those of the neutral particle. Since
depending on whether the particle is charged or not the dispersion relations
change, we could collect bulk information, namely the presence of the electric
field, by studying the 4D dynamics of the particles.Comment: 12 pages, 5 figure
An Alternative Canonical Approach to the Ghost Problem in a Complexified Extension of the Pais-Uhlenbeck Oscillator
Our purpose in this paper is to analyze the Pais-Uhlenbeck (PU) oscillator
using complex canonical transformations. We show that starting from a
Lagrangian approach we obtain a transformation that makes the extended PU
oscillator, with unequal frequencies, to be equivalent to two standard second
order oscillators which have the original number of degrees of freedom. Such
extension is provided by adding a total time derivative to the PU Lagrangian
together with a complexification of the original variables further subjected to
reality conditions in order to maintain the required number of degrees of
freedom. The analysis is accomplished at both the classical and quantum levels.
Remarkably, at the quantum level the negative norm states are eliminated, as
well as the problems of unbounded below energy and non-unitary time evolution.
We illustrate the idea of our approach by eliminating the negative norm states
in a complex oscillator. Next, we extend the procedure to the Pais-Uhlenbeck
oscillator. The corresponding quantum propagators are calculated using
Schwinger's quantum action principle. We also discuss the equal frequency case
at the classical level
Brane world corrections to scalar vacuum force in RSII-p
Vacuum force is an interesting low energy test for brane worlds due to its
dependence on field's modes and its role in submillimeter gravity experiments.
In this work we generalize a previous model example: the scalar field vacuum
force between two parallel plates lying in the brane of a Randall-Sundrum
scenario extended by compact dimensions (RSII-). Upon use of Green's
function technique, for the massless scalar field, the 4D force is obtained
from a zero mode while corrections turn out attractive and depend on the
separation between plates as . For the massive scalar field a
quasilocalized mode yields the 4D force with attractive corrections behaving
like . Corrections are negligible w.r.t. 4D force for
radius less than m. Although the case is not
physically viable due to the different behavior in regard to localization for
the massless scalar and electromagnetic fields it yields an useful comparison
between the dimensional regularization and Green's function techniques as we
describe in the discussion.Comment: 14 pages, v2: discussion clarified, reference adde
Loop Variables for compact two-dimensional quantum electrodynamics
Variables parametrized by closed and open curves are defined to reformulate
compact U(1) Quantum Electrodynamics in the circle with a massless fermion
field. It is found that the gauge invariant nature of these variables
accommodates into a regularization scheme for the Hamiltonian and current
operators that is specially well suited for the study of the compact case. The
zero mode energy spectrum, the value of the axial anomaly and the anomalous
commutators this model presents are hence determined in a manifestly gauge
invariant manner. Contrary to the non compact case, the zero mode spectrum is
not equally spaced and consequently the theory does not lead to the spectrum of
a free scalar boson. All the states are invariant under large gauge
transformations. In particular, that is the case for the vacuum, and
consequently the -dependence does not appear.Comment: 24 pages, 1 figure, to be published in Phys. Rev.
Quantum gravity corrections to neutrino propagation
Massive spin-1/2 fields are studied in the framework of loop quantum gravity
by considering a state approximating, at a length scale much greater
than Planck length cm, a spin-1/2 field in flat
spacetime. The discrete structure of spacetime at yields corrections
to the field propagation at scale . Next, Neutrino Bursts (GeV) accompaning Gamma Ray Bursts that have travelled
cosmological distances, l.y., are considered. The dominant
correction is helicity independent and leads to a time delay w.r.t. the speed
of light, , of order s. To next order in
the correction has the form of the Gambini and Pullin effect
for photons. Its contribution to time delay is comparable to that caused by the
mass term. Finally, a dependence is
found for a two-flavour neutrino oscillation length.Comment: RevTeX, 5pp, no figures. Notation of a sum in Eq.(2) improved. Slight
modifications in redaction. Final version to appear in Phys. Rev. Let
Comments on the kinematical structure of loop quantum cosmology
We comment on the presence of spurious observables and on a subtle violation
of irreducibility in loop quantum cosmology.Comment: 7 page
Quantum Gravity Hamiltonian for Manifolds with Boundary
In canonical quantum gravity, when space is a compact manifold with boundary
there is a Hamiltonian given by an integral over the boundary. Here we compute
the action of this `boundary Hamiltonian' on observables corresponding to open
Wilson lines in the new variables formulation of quantum gravity. In cases
where the boundary conditions fix the metric on the boundary (e.g., in the
asymptotically Minkowskian case) one can obtain a finite result, given by a
`shift operator' generating translations of the Wilson line in the direction of
its tangent vector. A similar shift operator serves as the Hamiltonian
constraint in Morales-T\'ecotl and Rovelli's work on quantum gravity coupled to
Weyl spinors. This suggests the appearance of an induced field theory of Weyl
spinors on the boundary, analogous to that considered in Carlip's work on the
statistical mechanics of the 2+1-dimensional black hole.Comment: 17 pages in LaTeX format, vastly improved versio
Casimir force in brane worlds: coinciding results from Green's and Zeta function approaches
Casimir force encodes the structure of the field modes as vacuum fluctuations
and so it is sensitive to the extra dimensions of brane worlds. Now, in flat
spacetimes of arbitrary dimension the two standard approaches to the Casimir
force, Green's function and zeta function, yield the same result, but for brane
world models this was only assumed. In this work we show both approaches yield
the same Casimir force in the case of Universal Extra Dimensions and
Randall-Sundrum scenarios with one and two branes added by p compact
dimensions. Essentially, the details of the mode eigenfunctions that enter the
Casimir force in the Green's function approach get removed due to their
orthogonality relations with a measure involving the right hyper-volume of the
plates and this leaves just the contribution coming from the Zeta function
approach. The present analysis corrects previous results showing a difference
between the two approaches for the single brane Randall-Sundrum; this was due
to an erroneous hyper-volume of the plates introduced by the authors when using
the Green's function. For all the models we discuss here, the resulting Casimir
force can be neatly expressed in terms of two four dimensional Casimir force
contributions: one for the massless mode and the other for a tower of massive
modes associated with the extra dimensions.Comment: 30 pages, title, abstract and discussion have change