1,226 research outputs found
The Poincare' coset models ISO(d-1,1)/R^n and T-duality
We generalize a family of Lagrangians with values in the Poincar\'e group
ISO(d-1,1), which contain the description of spinning strings in flat (d-1)+1
dimensions, by including symmetric terms in the world-sheet coordinates. Then,
by promoting a subgroup H=R^n, n less than or equal to d, which acts
invariantly from the left on the element of ISO(d-1,1), to a gauge symmetry of
the action, we obtain a family of sigma-models. They describe bosonic strings
moving in (generally) curved, and in some cases degenerate, space-times with an
axion field. Further, the space-times of the effective theory admit in general
T-dual geometries. We give explicit results for two non degenerate cases.Comment: LaTeX, 24 pages, no figure
Brane-world black holes and the scale of gravity
A particle in four dimensions should behave like a classical black hole if
the horizon radius is larger than the Compton wavelength or, equivalently, if
its degeneracy (measured by entropy in units of the Planck scale) is large. For
spherically symmetric black holes in 4 + d dimensions, both arguments again
lead to a mass threshold MC and degeneracy scale Mdeg of the order of the
fundamental scale of gravity MG. In the brane-world, deviations from the
Schwarzschild metric induced by bulk effects alter the horizon radius and
effective four-dimensional Euclidean action in such a way that MC \simeq Mdeg
might be either larger or smaller than MG. This opens up the possibility that
black holes exist with a mass smaller than MG and might be produced at the LHC
even if M>10 TeV, whereas effects due to bulk graviton exchanges remain
undetectable because suppressed by inverse powers of MG. Conversely, even if
black holes are not found at the LHC, it is still possible that MC>MG and MG
\simeq 1TeV.Comment: 4 pages, no figur
Gravitational Collapse of a Shell of Quantized Matter
The semi-classical collapse, including lowest order back-reaction, of a thin
shell of self-gravitating quantized matter is illustrated. The conditions for
which self-gravitating matter forms a thin shell are first discussed and an
effective Lagrangian for such matter is obtained. The matter-gravity system is
then quantized, the semi-classical limit for gravitation is taken and the
method of adiabatic invariants is applied to the resulting time dependent
matter Hamiltonian. The governing equations are integrated numerically, for
suitable initial conditions, in order to illustrate the effect of
back-reaction, due to the creation of matter, in slowing down the collapse near
the horizon.Comment: 20 pages, 1 eps figure. Problem with figure fixe
Minimum length effects in black hole physics
We review the main consequences of the possible existence of a minimum
measurable length, of the order of the Planck scale, on quantum effects
occurring in black hole physics. In particular, we focus on the ensuing minimum
mass for black holes and how modified dispersion relations affect the Hawking
decay, both in four space-time dimensions and in models with extra spatial
dimensions. In the latter case, we briefly discuss possible phenomenological
signatures.Comment: 29 pages, 12 figures. To be published in "Quantum Aspects of Black
Holes", ed. X. Calmet (Springer, 2014
Holography and trace anomaly: what is the fate of (brane-world) black holes?
The holographic principle relates (classical) gravitational waves in the bulk
to quantum fluctuations and the Weyl anomaly of a conformal field theory on the
boundary (the brane). One can thus argue that linear perturbations in the bulk
of static black holes located on the brane be related to the Hawking flux and
that (brane-world) black holes are therefore unstable. We try to gain some
information on such instability from established knowledge of the Hawking
radiation on the brane. In this context, the well-known trace anomaly is used
as a measure of both the validity of the holographic picture and of the
instability for several proposed static brane metrics. In light of the above
analysis, we finally consider a time-dependent metric as the (approximate)
representation of the late stage of evaporating black holes which is
characterized by decreasing Hawking temperature, in qualitative agreement with
what is required by energy conservation.Comment: 11 pages, 2 figures, a few comments and references added, accepted
for publication in Phys. Rev.
Computational neurorehabilitation: modeling plasticity and learning to predict recovery
Despite progress in using computational approaches to inform medicine and neuroscience in the last 30 years, there have been few attempts to model the mechanisms underlying sensorimotor rehabilitation. We argue that a fundamental understanding of neurologic recovery, and as a result accurate predictions at the individual level, will be facilitated by developing computational models of the salient neural processes, including plasticity and learning systems of the brain, and integrating them into a context specific to rehabilitation. Here, we therefore discuss Computational Neurorehabilitation, a newly emerging field aimed at modeling plasticity and motor learning to understand and improve movement recovery of individuals with neurologic impairment. We first explain how the emergence of robotics and wearable sensors for rehabilitation is providing data that make development and testing of such models increasingly feasible. We then review key aspects of plasticity and motor learning that such models will incorporate. We proceed by discussing how computational neurorehabilitation models relate to the current benchmark in rehabilitation modeling – regression-based, prognostic modeling. We then critically discuss the first computational neurorehabilitation models, which have primarily focused on modeling rehabilitation of the upper extremity after stroke, and show how even simple models have produced novel ideas for future investigation. Finally, we conclude with key directions for future research, anticipating that soon we will see the emergence of mechanistic models of motor recovery that are informed by clinical imaging results and driven by the actual movement content of rehabilitation therapy as well as wearable sensor-based records of daily activity
Global alignments of parsec-scale AGN radio jets and their polarization planes
A number of works reported on the existence of a large scale alignment of the
polarization plane of extragalactic sources as well as the alignment of
radio-sources structural axes. However, both claims and their interpretation
remain controversial. For the first time we explore the parsec-scale jets
alignments. Additionally, we use archival polarimetric data at different
wavelengths in order to compare relative orientations of the jets and the
polarization planes of their emission. Using the flux density distribution in
very long baseline interferometry (VLBI) radio maps from the Astrogeo database,
we determine the parsec-scale jet orientation for the largest sample of active
galactic nuclei (AGN) to date. Employing the method of parallel transport and a
sample statistics characterizing the jet orientation dispersion among
neighbors, we test whether the identified jets are significantly aligned. We
show that the parsec-scale jets in our sample do not demonstrate any
significant global alignments. Moreover, the jet direction is found to be
weakly correlated with the polarization plane direction at different
frequencies.Comment: 8 pages, 9 figures, accepted by Astronomy and Astrophysic
Hairy black holes by gravitational decoupling
Black holes with hair represented by generic fields surrounding the central
source of the vacuum Schwarzschild metric are examined under the minimal set of
requirements consisting of i) the existence of a well defined event horizon and
ii) the strong or dominant energy condition for the hair outside the horizon.
We develop our analysis by means of the gravitational decoupling approach. We
find that trivial deformations of the seed Schwarzschild vacuum preserve the
energy conditions and provide a new mechanism to evade the no-hair theorem
based on a primary hair associated with the charge generating these
transformations. Under the above conditions i) and ii), this charge
consistently increases the entropy from the minimum value given by the
Schwarzschild geometry. As a direct application, we find a non-trivial
extension of the Reissner-Nordstrom black hole showing a surprisingly simple
horizon. Finally, the non-linear electrodynamics generating this new solution
is fully specified.Comment: 11 pages, 4 figure
Adiabatic Invariant Treatment of a Collapsing Sphere of Quantized Dust
The semiclassical collapse of a sphere of quantized dust is studied. A
Born-Oppenheimer decomposition is performed for the wave function of the system
and the semiclassical limit is considered for the gravitational part. The
method of adiabatic invariants for time dependent Hamiltonians is then employed
to find (approximate) solutions to the quantum dust equations of motions. This
allows us to obtain corrections to the adiabatic approximation of the dust
states associated with the time evolution of the metric. The diverse
non-adiabatic corrections are generally associated with particle (dust)
creation and related fluctuations. The back-reaction due to the dominant
contribution to particle creation is estimated and seen to slow-down the
collapse.Comment: LaTeX, 16 pages, no figures, final version to appear in Class. and
Quantum Gravit
Microfield Dynamics of Black Holes
The microcanonical treatment of black holes as opposed to the canonical
formulation is reviewed and some major differences are displayed. In particular
the decay rates are compared in the two different pictures.Comment: 22 pages, 4 figures, Revtex, Minor change in forma
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