110 research outputs found
General structure of the solutions of the Hamiltonian constraints of gravity
A general framework for the solutions of the constraints of pure gravity is
constructed. It provides with well defined mathematical criteria to classify
their solutions in four classes. Complete families of solutions are obtained in
some cases. A starting point for the systematic study of the solutions of
Einstein gravity is suggested.Comment: 17 pages, LaTeX, submitted to International J. of Geom. Meth. in
Modern Physics. Added comments in the last sectio
Test Matter in a Spacetime with Nonmetricity
Examples in which spacetime might become non-Riemannian appear above Planck
energies in string theory or, in the very early universe, in the inflationary
model. The simplest such geometry is metric-affine geometry, in which {\it
nonmetricity} appears as a field strength, side by side with curvature and
torsion. In matter, the shear and dilation currents couple to nonmetricity, and
they are its sources. After reviewing the equations of motion and the Noether
identities, we study two recent vacuum solutions of the metric-affine gauge
theory of gravity. We then use the values of the nonmetricity in these
solutions to study the motion of the appropriate test-matter. As a
Regge-trajectory like hadronic excitation band, the test matter is endowed with
shear degrees of freedom and described by a world spinor.Comment: 14 pages, file in late
The Husain-Kuchar Model: Time Variables and Non-degenerate Metrics
We study the Husain-Kuchar model by introducing a new action principle
similar to the self-dual action used in the Ashtekar variables approach to
Quantum Gravity. This new action has several interesting features; among them,
the presence of a scalar time variable that allows the definition of geometric
observables without adding new degrees of freedom, the appearance of a natural
non-degenerate four-metric and the possibility of coupling ordinary matter.Comment: LaTeX, 22 pages, accepted for publication in Phys. Rev.
Non-Linear Affine Embedding of the Dirac Field from the Multiplicity-Free SL(4,R) Unirreps
The correspondence between the linear multiplicity-free unirreps of SL(4, R)
studied by Ne'eman and {\~{S}}ija{\~{c}}ki and the non-linear realizations of
the affine group is worked out. The results obtained clarify the inclusion of
spinorial fields in a non-linear affine gauge theory of gravitation.Comment: 13 pages, plain TeX, macros include
Growth hormone and aging
Les alteracions vasculars i degeneratives del sistema nerviós central (SNC) són dues de les
causes més comunes de malaltia i de mort entre la gent gran; ambdues es correlacionen amb
l'edat, amb la deficiència en GH, i poden afectar les funcions fisiològiques de la població d'edat
avançada.
Amb la finalitat de clarificar els efectes de la GH en el metabolisme, en els vasos i en el SNC,
hem dut a terme un estudi in vivo utilitzant rates vellesWistar tractades crònicament amb GH.
Les rates velles varen presentar un augment en el pes de greix i una disminució de l'índex
específic de gravetat (SGI) (p < 0,05) en comparar-les amb les rates adultes no tractades. La
GH va reduir el pes en greix (p < 0,05), i va mostrar també una tendència a augmentar l'SGI.
Es va analitzar també la resposta de diverses substàncies vasoactives en els anells aòrtics, i es
va demostrar una disminució de la vasodilatació per acetilcolina i isoprenalina (p < 0,05) en
els animals vells. La contracció induïda per acetilcolina+L-NAME era més alta en els animals
vells que en els adults. L'administració de GH millorava les respostes vasodilatadores (p < 0,05)
mentre que tendia a reduïr les respostes vasoconstrictores. L'àrea aòrtica mitja augmentava
també en les rates velles, mentre que la GH reduïa aquest paràmetre (p < 0,05).
Les poblacions neuronals es reduïen en els hipocamps de les rates velles en comparar-les
amb les joves. Aquesta reducció estava asociada a un augment dels nucleosomes i a una reducció
de Bcl2 en el cervell. Les caspases 3 i 9 també varen augmentar. El tractament amb GH va
augmentar significativament el nombre de neurones i va reduir els nucleosomes i les caspases
i augmentar el Bcl2. En conclusió, el tractament perGHindueix l'aparició d'efectes beneficiosos
en la composició del cos i ha restablert també les funcions cerebrals i vasculars en les rates velles.Vascular and degenerative alterations of the central nervous system (CNS) are two of the
most common reasons for illness and death in elderly people; they exhibit an age-related GH
deficiency that can affect their physiological functions.
A study was conducted under chronic in vivo conditions using old Wistar rats, in order to
clarify the effects of GH on the metabolism, vessels, and the CNS. The old rats showed an increased
fat weight and a decreased Specific Gravity Index (SGI) (p < 0.05), as compared to the
adult animals. GH reduced the fat weight (p < 0.05) and tended to increase the SGI (N.S.). The
response to several vasoactive substances in aortic rings showed impaired vasodilatation to
Acetylcholine and Isoprenaline (p < 0.05) in the old animals. Contraction, induced by Acetylcholine+
L-NAME, was higher in the old rats than in the adults. GH administration improved
the vasodilatory responses (p < 0.05) and tended to reduce the constrictory responses. The aortic
media area was increased in the old rats, and GH reduced this parameter (p < 0.05).
The neuronal populations were reduced in the hippocampi of the old rats as compared to the
young ones. This reduction was associated with an increase in nucleosomes and a reduction
in Bcl2 in the brain. An increase was also detected in caspases 3 and 9. GH treatment was able
to significantly enhance the number of neurons by reducing the nucleosomes and the caspases
and by increasing Bcl2. In conclusion,GHtreatment was able to show beneficial effects on body
composition and was able to restore both vascular and brain functions in the old rats
Hamiltonian Poincar\'e Gauge Theory of Gravitation
We develop a Hamiltonian formalism suitable to be applied to gauge theories
in the presence of Gravitation, and to Gravity itself when considered as a
gauge theory. It is based on a nonlinear realization of the Poincar\'e group,
taken as the local spacetime group of the gravitational gauge theory, with
as the classification subgroup. The Wigner--like rotation induced by
the nonlinear approach singularizes out the role of time and allows to deal
with ordinary vectors. We apply the general results to the
Einstein--Cartan action. We study the constraints and we obtain Einstein's
classical equations in the extremely simple form of time evolution equations of
the coframe. As a consequence of our approach, we identify the
gauge--theoretical origin of the Ashtekar variables.Comment: 38 pages, plainTe
Black Holes with Weyl Charge and Non-Riemannian Waves
A simple modification to Einstein's theory of gravity in terms of a
non-Riemannian connection is examined. A new tensor-variational approach yields
field equations that possess a covariance similar to the gauge covariance of
electromagnetism. These equations are shown to possess solutions analogous to
those found in the Einstein-Maxwell system. In particular one finds
gravi-electric and gravi-magnetic charges contributing to a spherically
symmetric static Reissner-Nordstr\"om metric. Such Weyl ``charges'' provide a
source for the non-Riemannian torsion and metric gradient fields instead of the
electromagnetic field. The theory suggests that matter may be endowed with
gravitational charges that couple to gravity in a manner analogous to
electromagnetic couplings in an electromagnetic field. The nature of
gravitational coupling to spinor matter in this theory is also investigated and
a solution exhibiting a plane-symmetric gravitational metric wave coupled via
non-Riemannian waves to a propagating spinor field is presented.Comment: 18 pages Plain Tex (No Figures), Classical and Quantum Gravit
Dark Matter Gravitational Interactions
We argue that the conjectured dark mater in the Universe may be endowed with
a new kind of gravitational charge that couples to a short range gravitational
interaction mediated by a massive vector field. A model is constructed that
assimilates this concept into ideas of current inflationary cosmology. The
model is also consistent with the observed behaviour of galactic rotation
curves according to Newtonian dynamics. The essential idea is that stars
composed of ordinary (as opposed to dark matter) experience Newtonian forces
due to the presence of an all pervading background of massive gravitationally
charged cold dark matter. The novel gravitational interactions are predicted to
have a significant influence on pre-inflationary cosmology. The precise details
depend on the nature of a gravitational Proca interaction and the description
of matter. A gravitational Proca field configuration that gives rise to
attractive forces between dark matter charges of like polarity exhibits
homogeneous isotropic eternal cosmologies that are free of cosmological
curvature singularities thus eliminating the horizon problem associated with
the standard big-bang scenario. Such solutions do however admit dense hot
pre-inflationary epochs each with a characteristic scale factor that may be
correlated with the dark matter density in the current era of expansion. The
model is based on a theory in which a modification of Einsteinian gravity at
very short distances can be expressed in terms of the gradient of the Einstein
metric and the torsion of a non-Riemannian connection on the bundle of linear
frames over spacetime. Indeed we demonstrate that the genesis of the model
resides in a remarkable simplification that occurs when one analyses the
variational equations associated with a broad class of non-Riemannian actions.Comment: 40 pages, 4 Postscript figure
Towards complete integrability of two dimensional Poincar\'e gauge gravity
It is shown that gravity on the line can be described by the two dimensional
(2D) Hilbert-Einstein Lagrangian supplemented by a kinetic term for the coframe
and a translational {\it boundary} term. The resulting model is equivalent to a
Yang-Mills theory of local {\it translations} and frozen Lorentz gauge degrees.
We will show that this restricted Poincar\'e gauge model in 2 dimensions is
completely integrable. {\it Exact} wave, charged black hole, and `dilaton'
solutions are then readily found. In vacuum, the integrability of the {\it
general} 2D Poincar\'e gauge theory is formally proved along the same line of
reasoning.Comment: 35 pages, report Cologne-thp-1993-H
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