480 research outputs found
Space-time extensions II
The global extendibility of smooth causal geodesically incomplete spacetimes
is investigated. Denote by one of the incomplete non-extendible causal
geodesics of a causal geodesically incomplete spacetime . First, it
is shown that it is always possible to select a synchronised family of causal
geodesics and an open neighbourhood of a final segment
of in such that is comprised by members of ,
and suitable local coordinates can be defined everywhere on
provided that does not terminate either on a tidal force tensor
singularity or on a topological singularity. It is also shown that if, in
addition, the spacetime, , is globally hyperbolic, and the
components of the curvature tensor, and its covariant derivatives up to order
are bounded on , and also the line integrals of the
components of the -order covariant derivatives are finite along the
members of ---where all the components are meant to be registered with
respect to a synchronised frame field on ---then there exists a
extension so that for each , which
is inextendible in , the image, , is
extendible in . Finally, it is also proved that
whenever does terminate on a topological singularity
cannot be generic.Comment: 42 pages, no figures, small changes to match the published versio
Stationary Black Holes as Holographs
Smooth spacetimes possessing a (global) one-parameter group of isometries and
an associated Killing horizon in Einstein's theory of gravity are investigated.
No assumption concerning the asymptotic structure is made, thereby, the
selected spacetimes may be considered as generic distorted stationary black
holes. First, spacetimes of arbitrary dimension, , with matter
satisfying the dominant energy condition and allowing non-zero cosmological
constant are investigated. In this part complete characterisation of the
topology of the event horizon of ``distorted'' black holes is given. It is
shown that the topology of the event horizon of ``distorted'' black holes is
allowed to possess a much larger variety than that of the isolated black hole
configurations. In the second part, 4-dimensional (non-degenerate) electrovac
distorted black hole spacetimes are considered. It is shown that the spacetime
geometry and the electromagnetic field are uniquely determined in the black
hole region once the geometry of the bifurcation surface and one of the
electromagnetic potentials are specified there. Conditions guaranteeing the
same type of determinacy, in a neighbourhood of the event horizon, on the {\it
domain of outer communication} side are also investigated. In particular, they
are shown to be satisfied in the analytic case.Comment: 31 pages, LaTeX; typos corrected, new references added, to appear in
CQ
Ellipsoidal shapes in general relativity: general definitions and an application
A generalization of the notion of ellipsoids to curved Riemannian spaces is
given and the possibility to use it in describing the shapes of rotating bodies
in general relativity is examined. As an illustrative example, stationary,
axisymmetric perfect-fluid spacetimes with a so-called confocal inside
ellipsoidal symmetry are investigated in detail under the assumption that the
4-velocity of the fluid is parallel to a time-like Killing vector field. A
class of perfect-fluid metrics representing interior NUT-spacetimes is obtained
along with a vacuum solution with a non-zero cosmological constant.Comment: Latex, 22 pages, Revised version accepted in Class. Quantum. Grav.,
references adde
Scratching resistance of SiC-rich nano-coatings produced by noble gas ion mixing
SiC-rich nano-layers were produced at room temperature by applying ion beam mixing of various C/Si multilayer structures using argon and xenon ions with energy in the range of 40–120 keV and fluences between 0.25 and 3 × 1016 ions/cm2. The mechanical behavior of the layers was characterized by scratch test. The scratching resistance of the ion mixed samples has been measured by standard scratch test applying an atomic-force microscope with a diamond-coated tip (radius < 15 nm) and they were compared to that measured on Si single crystal. The applied load varied in the range of 4–18 μN. The scratching resistance of the samples correlated with the effective areal density of the SiC; with increasing effective areal density the scratch depth decreases. Above sufficiently high effective areal density of SiC the scratch resistance (hardness) of the produced layer was somewhat higher than that of single crystal silicon. Previously it has been shown that such layers have excellent corrosion resistive properties as well. These findings allow to tune and design the mechanical and chemical properties of the SiC protective coatings
Dynamic scaling of fronts in the quantum XX chain
The dynamics of the transverse magnetization in the zero-temperature XX chain
is studied with emphasis on fronts emerging from steplike initial magnetization
profiles. The fronts move with fixed velocity and display a staircase like
internal structure whose dynamic scaling is explored both analytically and
numerically. The front region is found to spread with time sub-diffusively with
the height and the width of the staircase steps scaling as t^(-1/3) and t^1/3,
respectively. The areas under the steps are independent of time, thus the
magnetization relaxes in quantized "steps" of spin-flips.Comment: 4 pages, 3 eps figures, RevTe
Substance p immunoreactivity exhibits frequent colocalization with kisspeptin and neurokinin B in the human infundibular region.
Neurons synthesizing neurokinin B (NKB) and kisspeptin (KP) in the hypothalamic arcuate nucleus represent important upstream regulators of pulsatile gonadotropin-releasing hormone (GnRH) neurosecretion. In search of neuropeptides co-expressed in analogous neurons of the human infundibular nucleus (Inf), we have carried out immunohistochemical studies of the tachykinin peptide Substance P (SP) in autopsy samples from men (21-78 years) and postmenopausal (53-83 years) women. Significantly higher numbers of SP-immunoreactive (IR) neurons and darker labeling were observed in the Inf of postmenopausal women than in age-matched men. Triple-immunofluorescent studies localized SP immunoreactivity to considerable subsets of KP-IR and NKB-IR axons and perikarya in the infundibular region. In postmenopausal women, 25.1% of NKB-IR and 30.6% of KP-IR perikarya contained SP and 16.5% of all immunolabeled cell bodies were triple-labeled. Triple-, double- and single-labeled SP-IR axons innervated densely the portal capillaries of the infundibular stalk. In quadruple-labeled sections, these axons formed occasional contacts with GnRH-IR axons. Presence of SP in NKB and KP neurons increases the functional complexity of the putative pulse generator network. First, it is possible that SP modulates the effects of KP and NKB in axo-somatic and axo-dendritic afferents to GnRH neurons. Intrinsic SP may also affect the activity and/or neuropeptide release of NKB and KP neurons via autocrine/paracrine actions. In the infundibular stalk, SP may influence the KP and NKB secretory output via additional autocrine/paracrine mechanisms or regulate GnRH neurosecretion directly. Finally, possible co-release of SP with KP and NKB into the portal circulation could underlie further actions on adenohypophysial gonadotrophs
Black Hole Entropy in the presence of Chern-Simons Terms
We derive a formula for the black hole entropy in theories with gravitational
Chern-Simons terms, by generalizing Wald's argument which uses the Noether
charge. It correctly reproduces the entropy of three-dimensional black holes in
the presence of Chern-Simons term, which was previously obtained via indirect
methods.Comment: v2: 12 pages, added reference
New Dynamic Monte Carlo Renormalization Group Method
The dynamical critical exponent of the two-dimensional spin-flip Ising model
is evaluated by a Monte Carlo renormalization group method involving a
transformation in time. The results agree very well with a finite-size scaling
analysis performed on the same data. The value of is
obtained, which is consistent with most recent estimates
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