134 research outputs found
A physical application of Kerr-Schild groups
The present work deals with the search of useful physical applications of
some generalized groups of metric transformations. We put forward different
proposals and focus our attention on the implementation of one of them.
Particularly, the results show how one can control very efficiently the kind of
spacetimes related by a Generalized Kerr-Schild (GKS) Ansatz through
Kerr-Schild groups. Finally a preliminar study regarding other generalized
groups of metric transformations is undertaken which is aimed at giving some
hints in new Ans\"atze to finding useful solutions to Einstein's equations.Comment: 18 page
Bi-conformal vector fields and their applications
We introduce the concept of bi-conformal transformation, as a generalization
of conformal ones, by allowing two orthogonal parts of a manifold with metric
\G to be scaled by different conformal factors. In particular, we study their
infinitesimal version, called bi-conformal vector fields. We show the
differential conditions characterizing them in terms of a "square root" of the
metric, or equivalently of two complementary orthogonal projectors. Keeping
these fixed, the set of bi-conformal vector fields is a Lie algebra which can
be finite or infinite dimensional according to the dimensionality of the
projectors. We determine (i) when an infinite-dimensional case is feasible and
its properties, and (ii) a normal system for the generators in the
finite-dimensional case. Its integrability conditions are also analyzed, which
in particular provides the maximum number of linearly independent solutions. We
identify the corresponding maximal spaces, and show a necessary geometric
condition for a metric tensor to be a double-twisted product. More general
``breakable'' spaces are briefly considered. Many known symmetries are
included, such as conformal Killing vectors, Kerr-Schild vector fields,
kinematic self-similarity, causal symmetries, and rigid motions.Comment: Replaced version with some changes in the terminology and a new
theorem. To appear in Classical and Quantum Gravit
Covariant Perturbations of Schwarzschild Black Holes
We present a new covariant and gauge-invariant perturbation formalism for
dealing with spacetimes having spherical symmetry (or some preferred spatial
direction) in the background, and apply it to the case of gravitational wave
propagation in a Schwarzschild black hole spacetime. The 1+3 covariant approach
is extended to a `1+1+2 covariant sheet' formalism by introducing a radial unit
vector in addition to the timelike congruence, and decomposing all covariant
quantities with respect to this. The background Schwarzschild solution is
discussed and a covariant characterisation is given. We give the full
first-order system of linearised 1+1+2 covariant equations, and we show how, by
introducing (time and spherical) harmonic functions, these may be reduced to a
system of first-order ordinary differential equations and algebraic constraints
for the 1+1+2 variables which may be solved straightforwardly. We show how both
the odd and even parity perturbations may be unified by the discovery of a
covariant, frame- and gauge-invariant, transverse-traceless tensor describing
gravitational waves, which satisfies a covariant wave equation equivalent to
the Regge-Wheeler equation for both even and odd parity perturbations. We show
how the Zerilli equation may be derived from this tensor, and derive a similar
transverse traceless tensor equivalent to this equation. The so-called
`special' quasinormal modes with purely imaginary frequency emerge naturally.
The significance of the degrees of freedom in the choice of the two frame
vectors is discussed, and we demonstrate that, for a certain frame choice, the
underlying dynamics is governed purely by the Regge-Wheeler tensor. The two
transverse-traceless Weyl tensors which carry the curvature of gravitational
waves are discussed.Comment: 23 pages, 1 figure, Revtex 4. Submitted to Classical and Quantum
Gravity. Revised version is significantly streamlined with an important error
corrected which simplifies the presentatio
PROBIOGAS:Promotion of Biogas for Electricity and Heat Production in EU-Countries. Economic and Environmental Benefits of Biogas from Centralised Co-digestion
The Raychaudhuri equations: a brief review
We present a brief review on the Raychaudhuri equations. Beginning with a
summary of the essential features of the original article by Raychaudhuri and
subsequent work of numerous authors, we move on to a discussion of the
equations in the context of alternate non--Riemannian spacetimes as well as
other theories of gravity, with a special mention on the equations in
spacetimes with torsion (Einstein--Cartan--Sciama--Kibble theory). Finally, we
give an overview of some recent applications of these equations in General
Relativity, Quantum Field Theory, String Theory and the theory of relativisitic
membranes. We conclude with a summary and provide our own perspectives on
directions of future research.Comment: 35 pages, two figures, to appear in the special issue of Pramana
dedicated to the memory of A. K. Raychaudhur
Decoherent Histories Approach to the Arrival Time Problem
We use the decoherent histories approach to quantum theory to compute the
probability of a non-relativistic particle crossing during an interval of
time. For a system consisting of a single non-relativistic particle, histories
coarse-grained according to whether or not they pass through spacetime regions
are generally not decoherent, except for very special initial states, and thus
probabilities cannot be assigned. Decoherence may, however, be achieved by
coupling the particle to an environment consisting of a set of harmonic
oscillators in a thermal bath. Probabilities for spacetime coarse grainings are
thus calculated by considering restricted density operator propagators of the
quantum Brownian motion model. We also show how to achieve decoherence by
replicating the system times and then projecting onto the number density of
particles that cross during a given time interval, and this gives an
alternative expression for the crossing probability. The latter approach shows
that the relative frequency for histories is approximately decoherent for
sufficiently large , a result related to the Finkelstein-Graham-Hartle
theorem.Comment: 42 pages, plain Te
Impaired Small-World Network Efficiency and Dynamic Functional Distribution in Patients with Cirrhosis
Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome and a major complication of liver cirrhosis. Dysmetabolism of the brain, related to elevated ammonia levels, interferes with intercortical connectivity and cognitive function. For evaluation of network efficiency, a ‘small-world’ network model can quantify the effectiveness of information transfer within brain networks. This study aimed to use small-world topology to investigate abnormalities of neuronal connectivity among widely distributed brain regions in patients with liver cirrhosis using resting-state functional magnetic resonance imaging (rs-fMRI). Seventeen cirrhotic patients without HE, 9 with minimal HE, 9 with overt HE, and 35 healthy controls were compared. The interregional correlation matrix was obtained by averaging the rs-fMRI time series over all voxels in each of the 90 regions using the automated anatomical labeling model. Cost and correlation threshold values were then applied to construct the functional brain network. The absolute and relative network efficiencies were calculated; quantifying distinct aspects of the local and global topological network organization. Correlations between network topology parameters, ammonia levels, and the severity of HE were determined using linear regression and ANOVA. The local and global topological efficiencies of the functional connectivity network were significantly disrupted in HE patients; showing abnormal small-world properties. Alterations in regional characteristics, including nodal efficiency and nodal strength, occurred predominantly in the association, primary, and limbic/paralimbic regions. The degree of network organization disruption depended on the severity of HE. Ammonia levels were also significantly associated with the alterations in local network properties. Results indicated that alterations in the rs-fMRI network topology of the brain were associated with HE grade; and that focal or diffuse lesions disturbed the functional network to further alter the global topology and efficiency of the whole brain network. These findings provide insights into the functional changes in the human brain in HE
The Effect of Repetitive Transcranial Magnetic Stimulation on Gamma Oscillatory Activity in Schizophrenia
Gamma (γ) oscillations (30-50 Hz) have been shown to be excessive in patients with schizophrenia (SCZ) during working memory (WM). WM is a cognitive process that involves the online maintenance and manipulation of information that is mediated largely by the dorsolateral prefrontal cortex (DLPFC). Repetitive transcranial magnetic stimulation (rTMS) represents a non-invasive method to stimulate the cortex that has been shown to enhance cognition and γ oscillatory activity during WM.We examined the effect of 20 Hz rTMS over the DLPFC on γ oscillatory activity elicited during the N-back task in 24 patients with SCZ compared to 22 healthy subjects. Prior to rTMS, patients with SCZ elicited excessive γ oscillatory activity compared to healthy subjects across WM load. Active rTMS resulted in the reduction of frontal γ oscillatory activity in patients with SCZ, while potentiating activity in healthy subjects in the 3-back, the most difficult condition. Further, these effects on γ oscillatory activity were found to be specific to the frontal brain region and were absent in the parieto-occipital brain region.We suggest that this opposing effect of rTMS on γ oscillatory activity in patients with SCZ versus healthy subjects may be related to homeostatic plasticity leading to differential effects of rTMS on γ oscillatory activity depending on baseline differences. These findings provide important insights into the neurophysiological mechanisms underlying WM deficits in SCZ and demonstrated that rTMS can modulate γ oscillatory activity that may be a possible avenue for cognitive potentiation in this disorder
The social dimension of globalization: A review of the literature
With globalization affecting so many inter-connected areas, it is difficult to grasp its full impact. This literature review of over 120 sources considers the impact of globalization on wages and taxes, poverty, inequality, insecurity, child labour, gender, and migration. Opening with some stylized facts concerning globalization in 1985-2002, the authors then highlight recent findings on these areas, reporting on controversies and on emerging consensus where it exists. There follows a review of national and international policy responses designed to make globalization more sustainable and equitable and to deliver decent jobs, security and a voice in decision-making
Tracking rates of ecotone migration due to salt-water encroachment using fossil mollusks in coastal South Florida
- …