4,273 research outputs found
Higher dimensional gravity invariant under the Poincare group
It is shown that the Stelle-West Grignani-Nardelli-formalism allows, both
when odd dimensions and when even dimensions are considered, constructing
actions for higher dimensional gravity invariant under local Lorentz rotations
and under local Poincar\`{e} translations. It is also proved that such actions
have the same coefficients as those obtained by Troncoso and Zanelli in ref.
Class. Quantum Grav. 17 (2000) 4451.Comment: 7 pages, Latex, accepted in Phys. Rev.
Köhler disease: an infrequent or underdiagnosed cause of child's limping?
Köhler disease (KD) is the osteochondrosis of the tarsal navicular bone of the young children, which is a self-limited condition. Typically the child reports pain with mechanical characteristics in the medial region of the foot, however, intermittent or continuous limping may be the only clinical manifestation, which delays the diagnosis and consequently exposes to unnecessary tests and treatments. We describe a bilateral KD whose clinical manifestation was unilateral limping with 4 months evolution. The radiologic evaluation showed flattening, sclerosis and irregular rarefaction of both navicular bones. Claudication persisted for four weeks, after which, the child remains asymptomatic.info:eu-repo/semantics/publishedVersio
Coexistence of Quantum Theory and Special Relativity in signaling scenarios
The coexistence between Quantum Mechanics and Special Relativity is usually
formulated in terms of the no-signaling condition. Several authors have even
suggested that this condition should be included between the basic postulates
of Quantum Theory. However, there are several scenarios where signaling is, in
principle, possible: based on previous results and the analysis of the relation
between unitarity and signaling we present an example of a two-particle
interferometric arrangement for which the dynamics is, in principle, compatible
with superluminal transmission of information. This type of non-locality is not
in the line of Bell's theorem, but closer in spirit to the one-particle
acausality studied by Hegerfeldt and others. We analyze in this paper the
meaning of this non-locality and how to preserve the coexistence of the two
fundamental theories in this signaling scenario.Comment: See also the comment by G C Hegerfeldt in the online version of the
journal, including more reference
Supergravity with cosmological constant and the AdS group
It is shown that the supersymmetric extension of the Stelle-West formalism
permits the construction of an action for -dimensional N=1 supergravity
with cosmological constant genuinely invariant under the Since the
action is invariant under the supersymmetric extension of the group, the
supersymmetry algebra closes off shell without the need for auxiliary fields.
The limit case , i.e. -dimensional N=1 supergravity invariant
under the Poincar\'{e} supergroup is also discussed.Comment: 10 page
X-ray time variability across the atoll source states of 4U 1636--53
We have studied the rapid X-ray time variability in 149 pointed observations
with the \textit{Rossi X-ray Timing Explorer} (RXTE)'s Proportional Counter
Array of the atoll source 4U~1636--53 in the banana state and, for the first
time with RXTE, in the island state. We compare the frequencies of the
variability components of 4U~1636--53 with those in other atoll and Z-sources
and find that 4U~1636--53 follows the universal scheme of correlations
previously found for other atoll sources at (sometimes much) lower
luminosities. Our results on the hectohertz QPO suggest that the mechanism that
sets its frequency differs from that for the other components, while the
amplitude setting mechanism is common. A previously proposed interpretation of
the narrow low-frequency QPO frequencies in different sources in terms of
harmonic mode switching is not supported by our data, nor by some previous data
on other sources and the frequency range that this QPO covers is found not to
be related to spin, angular momentum or luminosity.Comment: 16 pages, 13 figures, accepted for publication in Ap
A comment on "The Cauchy problem of f(R)- gravity", Class. Quantum Grav., 24, 5667 (2007), arXiv:0709.4414
A critical comment on [N. Lanahan--Tremblay and V. Faraoni, 2007, {\it Class.
Quantum Grav.}, {\bf 24}, 5667, arXiv:0709.4414] is given discussing the
well-formulation of the Chauchy problem for -gravity in metric-affine
theories.Comment: 3 page
Symmetries and conservation laws in the Gunther k-symplectic formalism of field theory
This paper is devoted to studying symmetries of k-symplectic Hamiltonian and
Lagrangian first-order classical field theories. In particular, we define
symmetries and Cartan symmetries and study the problem of associating
conservation laws to these symmetries, stating and proving Noether's theorem in
different situations for the Hamiltonian and Lagrangian cases. We also
characterize equivalent Lagrangians, which lead to an introduction of
Lagrangian gauge symmetries, as well as analyzing their relation with Cartan
symmetries.Comment: 29 page
Galactic periodicity and the oscillating G model
We consider the model involving the oscillation of the effective
gravitational constant that has been put forward in an attempt to reconcile the
observed periodicity in the galaxy number distribution with the standard
cosmological models. This model involves a highly nonlinear dynamics which we
analyze numerically. We carry out a detailed study of the bound that
nucleosynthesis imposes on this model. The analysis shows that for any assumed
value for (the total energy density) one can fix the value of
(the baryonic energy density) in such a way as to
accommodate the observational constraints coming from the
primordial abundance. In particular, if we impose the inflationary value
the resulting baryonic energy density turns out to be . This result lies in the very narrow range allowed by the observed values of the primordial
abundances of the other light elements. The remaining fraction of
corresponds to dark matter represented by a scalar field.Comment: Latex file 29 pages with no figures. Please contact M.Salgado for
figures. A more careful study of the model appears in gr-qc/960603
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