1,163 research outputs found
Standard General Relativity from Chern-Simons Gravity
Chern-Simons models for gravity are interesting because they provide with a
truly gauge-invariant action principle in the fiber-bundle sense. So far, their
main drawback has largely been the perceived remoteness from standard General
Relativity, based on the presence of higher powers of the curvature in the
Lagrangian (except, remarkably, for three-dimensional spacetime). Here we
report on a simple model that suggests a mechanism by which standard General
Relativity in five-dimensional spacetime may indeed emerge at a special
critical point in the space of couplings, where additional degrees of freedom
and corresponding "anomalous" Gauss-Bonnet constraints drop out from the
Chern-Simons action. To achieve this result, both the Lie algebra g and the
symmetric g-invariant tensor that define the Chern-Simons Lagrangian are
constructed by means of the Lie algebra S-expansion method with a suitable
finite abelian semigroup S. The results are generalized to arbitrary odd
dimensions, and the possible extension to the case of eleven-dimensional
supergravity is briefly discussed.Comment: 6 pages, no figures; v2: published versio
Thermodynamics from a scaling Hamiltonian
There are problems with defining the thermodynamic limit of systems with
long-range interactions; as a result, the thermodynamic behavior of these types
of systems is anomalous. In the present work, we review some concepts from both
extensive and nonextensive thermodynamic perspectives. We use a model, whose
Hamiltonian takes into account spins ferromagnetically coupled in a chain via a
power law that decays at large interparticle distance as for
. Here, we review old nonextensive scaling. In addition, we
propose a new Hamiltonian scaled by that
explicitly includes symmetry of the lattice and dependence on the size, , of
the system. The new approach enabled us to improve upon previous results. A
numerical test is conducted through Monte Carlo simulations. In the model,
periodic boundary conditions are adopted to eliminate surface effects.Comment: 12 pages, 2 figures, submitted for publication to Phys. Rev.
Euler Chern Simons Gravity from Lovelock Born Infeld Gravity
In the context of a gauge theoretical formulation, higher dimensional gravity
invariant under the AdS group is dimensionally reduced to Euler-Chern-Simons
gravity. The dimensional reduction procedure of Grignani-Nardelli [Phys. Lett.
B 300, 38 (1993)] is generalized so as to permit reducing D-dimensional Lanczos
Lovelock gravity to d=D-1 dimensions.Comment: 6 pages, no figures, accepted for publication in Phys. Lett.
Higher Dimensional Gravity, Propagating Torsion and AdS Gauge Invariance
The most general theory of gravity in d-dimensions which leads to second
order field equations for the metric has [(d-1)/2] free parameters. It is shown
that requiring the theory to have the maximum possible number of degrees of
freedom, fixes these parameters in terms of the gravitational and the
cosmological constants. In odd dimensions, the Lagrangian is a Chern-Simons
form for the (A)dS or Poincare groups. In even dimensions, the action has a
Born-Infeld-like form. Torsion may occur explicitly in the Lagrangian in the
parity-odd sector and the torsional pieces respect local (A)dS symmetry for
d=4k-1 only. These torsional Lagrangians are related to the Chern-Pontryagin
characters for the (A)dS group. The additional coefficients in front of these
new terms in the Lagrangian are shown to be quantized.Comment: 10 pages, two columns, no figures, title changed in journal, final
version to appear in Class. Quant. Gra
Osteocondroma intra-raquideo con afectación neurológica
Se presenta el caso de un varón de 16 años con lumbalgia consecutiva a
traumatismo vertebral, sin hallazgo s radiológicos, que no mejoraba con tratamient o
conservador, y que a las 5 semanas desarrolló un cuadro de paraparesia e incontinencia
de esfínteres indicativo de afectación del cono medular. En el estudio mielográfico
se objetivó bloqueo completo a nivel de Ll. La tomografía axial mostró una imagen sugestiva
de osteocondroma intraraquídeo, que s e confirmó quirúrgicamente . Dos año s
tras la intervención, se logró la total recuperación neurológica. Se destaca la rareza de
la lesión, la dificultad diagnóstica, y la probable etiología traumática de la tumoración.A 16-year-old man with no improvement of low back pain after vertebral
trauma conservatively treated and without radiological findings is presented. Five weeks
after trauma, the patient developed paraparesia and fecal and urinary incontinence
indicating compression of the medullary conus. A complet e stop of the contrast at
Ll level wa s found in the myelographi c study. The CT-Scan showed an image suggesting
osteochondroma whic h wa s confirmed a r surgery. Complet e neurogical recover y
w a s achieved 2 year s after surgical treatment. The rare character of the lesion, the dificulty
for diagnosis and the probable traumatic etiology of this tumor is discussed
Transgression forms and extensions of Chern-Simons gauge theories
A gauge invariant action principle, based on the idea of transgression forms,
is proposed. The action extends the Chern-Simons form by the addition of a
boundary term that makes the action gauge invariant (and not just
quasi-invariant). Interpreting the spacetime manifold as cobordant to another
one, the duplication of gauge fields in spacetime is avoided. The advantages of
this approach are particularly noticeable for the gravitation theory described
by a Chern-Simons lagrangian for the AdS group, in which case the action is
regularized and finite for black hole geometries in diverse situations. Black
hole thermodynamics is correctly reproduced using either a background field
approach or a background-independent setting, even in cases with asymptotically
nontrivial topologies. It is shown that the energy found from the thermodynamic
analysis agrees with the surface integral obtained by direct application of
Noether's theorem.Comment: 28 pages, no figures. Minor changes in the introduction, final
comments and reference
Even-dimensional topological gravity from Chern-Simons gravity
It is shown that the topological action for gravity in 2n-dimensions can be
obtained from the 2n+1-dimensional Chern-Simons gravity genuinely invariant
under the Poincare group. The 2n-dimensional topological gravity is described
by the dynamics of the boundary of a 2n+1-dimensional Chern-Simons gravity
theory with suitable boundary conditions. The field , which is
necessary to construct this type of topological gravity in even dimensions, is
identified with the coset field associated with the non-linear realizations of
the Poincare group ISO(d-1,1)
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