5,010 research outputs found
Remarks on the consistency of minimal deviations from General Relativity
We study the consequences of the modification of the phase space structure of
General Relativity imposed by breaking the full diffeomorphism invariance but
retaining the time foliation preserving diffeomorphisms. We examine the
different sectors in phase space that satisfy the new structure of constraints.
For some sectors we find an infinite tower of constraints. In spite of that, we
also show that these sectors allow for solutions, among them some well known
families of black hole and cosmologies which fulfill all the constraints. We
raise some physical concerns on the consequences of an absolute Galilean time,
on the thermodynamical pathologies of such models and on their unusual vacuum
structure.Comment: latex 28 pages, 1 figure. Added comments and a reference. Text
improved
BV analysis for covariant and non-covariant actions
The equivalence between the covariant and the non-covariant version of a
constrained system is shown to hold after quantization in the framework of the
field-antifield formalism. Our study covers the cases of Electromagnetism and
Yang-Mills fields and sheds light on some aspects of the Faddeev-Popov method,
for both the coratiant and non-covariant approaches, which had not been fully
clarified in the literature.Comment: 21 pages, preprint # UTTG-02-93, UB-ECM-PF 93/5. To appear in Phys.
Rev.
Quantum Interference and Decoherence in Single-Molecule Junctions: How Vibrations Induce Electrical Current
Quantum interference effects and decoherence mechanisms in single-molecule
junctions are analyzed employing a nonequilibrium Green's function approach.
Electrons tunneling through quasi-degenerate states of a nanoscale molecular
junction exhibit interference effects. We show that electronic-vibrational
coupling, inherent to any molecular junction, strongly quenches such
interference effects. As a result, the electrical current can be significantly
larger than without electronic-vibrational coupling. The analysis reveals that
the quenching of quantum interference is particularly pronounced if the
junction is vibrationally highly excited, e.g. due to current-induced
nonequilibrium effects in the resonant transport regime.Comment: 11 pages, 4 figure
The influence of magnetic field geometry on magnetars X-ray spectra
Nowadays, the analysis of the X-ray spectra of magnetically powered neutron
stars or magnetars is one of the most valuable tools to gain insight into the
physical processes occurring in their interiors and magnetospheres. In
particular, the magnetospheric plasma leaves a strong imprint on the observed
X-ray spectrum by means of Compton up-scattering of the thermal radiation
coming from the star surface. Motivated by the increased quality of the
observational data, much theoretical work has been devoted to develop Monte
Carlo (MC) codes that incorporate the effects of resonant Compton scattering in
the modeling of radiative transfer of photons through the magnetosphere. The
two key ingredients in this simulations are the kinetic plasma properties and
the magnetic field (MF) configuration. The MF geometry is expected to be
complex, but up to now only mathematically simple solutions (self-similar
solutions) have been employed. In this work, we discuss the effects of new,
more realistic, MF geometries on synthetic spectra. We use new force-free
solutions in a previously developed MC code to assess the influence of MF
geometry on the emerging spectra. Our main result is that the shape of the
final spectrum is mostly sensitive to uncertain parameters of the
magnetospheric plasma, but the MF geometry plays an important role on the
angle-dependence of the spectra.Comment: 6 pages, 4 figures To appear in Proceedings of II Iberian Nuclear
Astrophysics Meeting held in Salamanca, September 22-23, 201
Generally covariant theories: the Noether obstruction for realizing certain space-time diffeomorphisms in phase space
Relying on known results of the Noether theory of symmetries extended to
constrained systems, it is shown that there exists an obstruction that prevents
certain tangent-space diffeomorphisms to be projectable to phase-space, for
generally covariant theories. This main result throws new light on the old fact
that the algebra of gauge generators in the phase space of General Relativity,
or other generally covariant theories, only closes as a soft algebra and not a
a Lie algebra.
The deep relationship between these two issues is clarified. In particular,
we see that the second one may be understood as a side effect of the procedure
to solve the first. It is explicitly shown how the adoption of specific
metric-dependent diffeomorphisms, as a way to achieve projectability, causes
the algebra of gauge generators (constraints) in phase space not to be a Lie
algebra --with structure constants-- but a soft algebra --with structure {\it
functions}.Comment: 22 pages, version to be published in Classical & Quantum Gravit
Evolutionary Laws, Initial Conditions, and Gauge Fixing in Constrained Systems
We describe in detail how to eliminate nonphysical degrees of freedom in the
Lagrangian and Hamiltonian formulations of a constrained system. Two important
and distinct steps in our method are the fixing of ambiguities in the dynamics
and the determination of inequivalent initial data. The Lagrangian discussion
is novel, and a proof is given that the final number of degrees of freedom in
the two formulations agrees. We give applications to reparameterization
invariant theories, where we prove that one of the constraints must be
explicitly time dependent. We illustrate our procedure with the examples of
trajectories in spacetime and with spatially homogeneous cosmological models.
Finally, we comment briefly on Dirac's extended Hamiltonian technique.Comment: 23 pages; plain TeX. To appear: Classical & Quantum Gravit
Optimization of the e-e- option for the ILC
The e-e- running mode is one of the interesting physics options at the
International Linear Collider (ILC). The luminosity for e-e- collisions is
reduced by the beam-beam effects. The resulting beamstrahlung energy loss and
beam-beam deflection angles as function of the vertical transverse offset are
different compared to the e+e- collisions. In this paper, the dependence of
these observables with the offset for different beam sizes has been analyzed to
optimize performances for the e-e- mode, taking into account the requirements
of the beam-beam deflection based intra-train feedback system. A first study of
the implications for the final focus and extraction line optics is also
presented for the cases of the 20 mrad and 2 mrad ILC base line crossing angle
geometries
On the equivalence of the Einstein-Hilbert and the Einstein-Palatini formulations of general relativity for an arbitrary connection
In the framework of the Einstein-Palatini formalism, even though the
projective transformation connecting the arbitrary connection with the Levi
Civita connection has been floating in the literature for a long time and
perhaps the result was implicitly known in the affine gravity community, yet as
far as we know Julia and Silva were the first to realise its gauge character.
We rederive this result by using the Rosenfeld-Dirac-Bergmann approach to
constrained Hamiltonian systems and do a comprehensive self contained analysis
establishing the equivalence of the Einstein-Palatini and the metric
formulations without having to impose the gauge choice that the connection is
symmetric. We also make contact with the the Einstein-Cartan theory when the
matter Lagrangian has fermions.Comment: 18 pages. Slight change in the title and wording of some sections to
emphasize the main results. References added. Matches published versio
Population Synthesis of Isolated Neutron Stars with magneto-rotational evolution II: from radio-pulsars to magnetars
Population synthesis studies constitute a powerful method to reconstruct the
birth distribution of periods and magnetic fields of the pulsar population.
When this method is applied to populations in different wavelengths, it can
break the degeneracy in the inferred properties of initial distributions that
arises from single-band studies. In this context, we extend previous works to
include -ray thermal emitting pulsars within the same evolutionary model as
radio-pulsars. We find that the cumulative distribution of the number of X-ray
pulsars can be well reproduced by several models that, simultaneously,
reproduce the characteristics of the radio-pulsar distribution. However, even
considering the most favourable magneto-thermal evolution models with fast
field decay, log-normal distributions of the initial magnetic field
over-predict the number of visible sources with periods longer than 12 s. We
then show that the problem can be solved with different distributions of
magnetic field, such as a truncated log-normal distribution, or a binormal
distribution with two distinct populations. We use the observational lack of
isolated NSs with spin periods P>12 s to establish an upper limit to the
fraction of magnetars born with B > 10^{15} G (less than 1\%). As future
detections keep increasing the magnetar and high-B pulsar statistics, our
approach can be used to establish a severe constraint on the maximum magnetic
field at birth of NSs.Comment: 12 pages, 11 figures, 5 table
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