2,606 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
On the classification of type D spacetimes
We give a classification of the type D spacetimes based on the invariant
differential properties of the Weyl principal structure. Our classification is
established using tensorial invariants of the Weyl tensor and, consequently,
besides its intrinsic nature, it is valid for the whole set of the type D
metrics and it applies on both, vacuum and non-vacuum solutions. We consider
the Cotton-zero type D metrics and we study the classes that are compatible
with this condition. The subfamily of spacetimes with constant argument of the
Weyl eigenvalue is analyzed in more detail by offering a canonical expression
for the metric tensor and by giving a generalization of some results about the
non-existence of purely magnetic solutions. The usefulness of these results is
illustrated in characterizing and classifying a family of Einstein-Maxwell
solutions. Our approach permits us to give intrinsic and explicit conditions
that label every metric, obtaining in this way an operational algorithm to
detect them. In particular a characterization of the Reissner-Nordstr\"{o}m
metric is accomplished.Comment: 29 pages, 0 figure
Spin-Charge Separation in Two Dimensions - A Numerical Study
The question of spin-charge separation in two-dimensional lattices has been
addressed by numerical simulations of the motion of one hole in a half-filled
band. The calculations have been performed on finite clusters with Hubbard and
t-J models. By comparing the time evolution of spin and charge polarisation
currents in one and two dimensions, evidence in favor of spin-charge separation
in two dimensions is presented. In contrast with this, spin-charge separation
is absent in a highly doped, metallic, system.Comment: RevTeX 3.0, 10 Pages, 6 PostScript Figures (on request
Kerr-Schild Symmetries
We study continuous groups of generalized Kerr-Schild transformations and the
vector fields that generate them in any n-dimensional manifold with a
Lorentzian metric. We prove that all these vector fields can be intrinsically
characterized and that they constitute a Lie algebra if the null deformation
direction is fixed. The properties of these Lie algebras are briefly analyzed
and we show that they are generically finite-dimensional but that they may have
infinite dimension in some relevant situations. The most general vector fields
of the above type are explicitly constructed for the following cases: any
two-dimensional metric, the general spherically symmetric metric and
deformation direction, and the flat metric with parallel or cylindrical
deformation directions.Comment: 15 pages, no figures, LaTe
Some results on homoclinic and heteroclinic connections in planar systems
Consider a family of planar systems depending on two parameters and
having at most one limit cycle. Assume that the limit cycle disappears at some
homoclinic (or heteroclinic) connection when We present a method
that allows to obtain a sequence of explicit algebraic lower and upper bounds
for the bifurcation set The method is applied to two quadratic
families, one of them is the well-known Bogdanov-Takens system. One of the
results that we obtain for this system is the bifurcation curve for small
values of , given by . We obtain
the new three terms from purely algebraic calculations, without evaluating
Melnikov functions
Tumor transfection after systemic injection of DNA lipid nanocapsules
With the goal of generating an efficient vector for systemic gene delivery, a new kind of nanocarrier consisting of lipid nanocapsules encapsulating DOTAP/DOPE lipoplexes (DNA LNCs) was pegylated by the post-insertion of amphiphilic and flexible polymers. The aim of this surface modification was to create a long-circulating vector, able to circulate in the blood stream and efficient in transfecting tumoral cells after passive targeting by enhanced permeability and retention effect (EPR effect). PEG conformation, electrostatic features, and hydrophylicity are known to be important factors able to influence the pharmacokinetic behaviour of vectors. In this context, the surface structure characteristics of the newly pegylated DNA LNCs were studied by measuring electrophoretic mobility as a function of ionic strength in order to establish a correlation between surface properties and in vivo performance of the vector. Finally, thanks to this PEGylation, gene expression was measured up to 84-fold higher in tumor compared to other tested organs after intravenous injection. The present results indicate that PEGylated DNA LNCs are promising carriers for an efficient cancer gene therapy
I feel it in my finger: Measurement device affects cardiac interoceptive accuracy
This is the final version. Available on open access from Elsevier via the DOI in this recordIn recent years, measures of cardiac interoceptive accuracy have been heavily scrutinised. The focus has been on potentially confounding physiological and psychological factors; little research has examined whether the device used to record objective heartbeats may influence cardiac interoceptive accuracy. The present studies assessed whether the device employed influences heartbeat counting (HCT) accuracy and the location from which heartbeats are perceived. In Study One, participants completed the HCT using a hard-clip finger pulse oximeter, electrocardiogram (ECG) and a smartphone application. In Study Two, an ECG, hard-clip and soft-clip oximeter were compared. Moderate-strong correlations were observed across devices, however, mean HCT accuracy and confidence varied as a function of device. Increased sensation in the finger when using a hard-clip pulse oximeter was related to increased accuracy relative to ECG. Results suggest that the device employed can influence HCT performance, and argue against comparing, or combining, scores obtained using different devices.Economic and Social Research Council (ESRC)Baily Thomas TrustFonds de Recherche QuĂ©bec â Sant
Null cone preserving maps, causal tensors and algebraic Rainich theory
A rank-n tensor on a Lorentzian manifold V whose contraction with n arbitrary
causal future directed vectors is non-negative is said to have the dominant
property. These tensors, up to sign, are called causal tensors, and we
determine their general properties in dimension N. We prove that rank-2 tensors
which map the null cone on itself are causal. It is known that, to any tensor A
on V there is a corresponding ``superenergy'' (s-e) tensor T{A} which always
has the dominant property. We prove that, conversely, any symmetric rank-2
tensor with the dominant property can be written in a canonical way as a sum of
N s-e tensors of simple forms. We show that the square of any rank-2 s-e tensor
is proportional to the metric if N<5, and that this holds for the s-e tensor of
any simple form for arbitrary N. Conversely, we prove that any symmetric rank-2
tensor T whose square is proportional to the metric must be, up to sign, the
s-e of a simple p-form, and that the trace of T determines the rank p of the
form. This generalises, both with respect to N and the rank p, the classical
algebraic Rainich conditions, which are necessary and sufficient conditions for
a metric to originate in some physical field, and has a geometric
interpretation: the set of s-e tensors of simple forms is precisely the set of
tensors which preserve the null cone and its time orientation. It also means
that all involutory Lorentz transformations (LT) can be represented as s-e
tensors of simple forms, and that any rank-2 s-e tensor is the sum of at most N
conformally involutory LT. Non-symmetric null cone preserving maps are shown to
have a causal symmetric part and are classified according to the null
eigenvectors of the skew-symmetric part. We thus obtain a complete
classification of all conformal LT and singular null cone preserving maps on V.Comment: 36 pages, no figures, LaTeX fil
Nuclear reactions in the Sun after SNO and KamLAND
In this brief review we discuss the possibility of studying the solar
interior by means of neutrinos, in the light of the enormous progress of
neutrino physics in the last few years. The temperature near the solar center
can be extracted from Boron neutrino experiments as: . The energy production rate in the Sun from pp chain and CNO cycle, as
deduced from neutrino measurements, agrees with the observed solar luminosity
to about twenty per cent. Progress in extracting astrophysical information from
solar neutrinos requires improvement in the measurements of \\ and .Comment: To appear in the Proceedings of Beyond the Desert '03, Fourth
International Conference on Physics Beyond the Standard Model, Schloss
Ringberg, Germany, June 9-14, 200
On the Papapetrou field in vacuum
In this paper we study the electromagnetic fields generated by a Killing
vector field in vacuum space-times (Papapetrou fields). The motivation of this
work is to provide new tools for the resolution of Maxwell's equations as well
as for the search, characterization, and study of exact solutions of Einstein's
equations. The first part of this paper is devoted to an algebraic study in
which we give an explicit and covariant procedure to construct the principal
null directions of a Papapetrou field. In the second part, we focus on the main
differential properties of the principal directions, studying when they are
geodesic, and in that case we compute their associated optical scalars. With
this information we get the conditions that a principal direction of the
Papapetrou field must satisfy in order to be aligned with a multiple principal
direction of the Weyl tensor in the case of algebraically special vacuum
space-times. Finally, we illustrate this study using the Kerr, Kasner and pp
waves space-times.Comment: 24 pages, LaTeX2e, IOP style. To appear in Classical and Quantum
Gravit
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