1,904 research outputs found
SU(2) gauge theory of gravity with topological invariants
The most general gravity Lagrangian in four dimensions contains three
topological densities, namely Nieh-Yan, Pontryagin and Euler, in addition to
the Hilbert-Palatini term. We set up a Hamiltonian formulation based on this
Lagrangian. The resulting canonical theory depends on three parameters which
are coefficients of these terms and is shown to admit a real SU(2) gauge
theoretic interpretation with a set of seven first-class constraints. Thus, in
addition to the Newton's constant, the theory of gravity contains three
(topological) coupling constants, which might have non-trivial imports in the
quantum theory.Comment: Based on a talk at Loops-11, Madrid, Spain; To appear in Journal of
Physics: Conference Serie
Dark Matter Prediction from Canonical Quantum Gravity with Frame Fixing
We show how, in canonical quantum cosmology, the frame fixing induces a new
energy density contribution having features compatible with the (actual) cold
dark matter component of the Universe. First we quantize the closed
Friedmann-Robertson-Walker (FRW) model in a sinchronous reference and determine
the spectrum of the super-Hamiltonian in the presence of ultra-relativistic
matter and a perfect gas contribution. Then we include in this model small
inhomogeneous (spherical) perturbations in the spirit of the Lemaitre-Tolman
cosmology. The main issue of our analysis consists in outlining that, in the
classical limit, the non-zero eigenvalue of the super-Hamiltonian can make
account for the present value of the dark matter critical parameter.
Furthermore we obtain a direct correlation between the inhomogeneities in our
dark matter candidate and those one appearing in the ultra-relativistic matter.Comment: 5 pages, to appear on Modern Physics Letters
Dystonia: sparse synapses for D2 receptors in striatum of a DYT1 knock-out mouse model
Dystonia pathophysiology has been partly linked to downregulation and dysfunction of dopamine D2 receptors in striatum. We aimed to investigate the possible morpho-structural correlates of D2 receptor downregulation in the striatum of a DYT1 Tor1a mouse model. Adult control Tor1a+/+ and mutant Tor1a+/− mice were used. The brains were perfused and free-floating sections of basal ganglia were incubated with polyclonal anti-D2 antibody, followed by secondary immune-fluorescent antibody. Confocal microscopy was used to detect immune-fluorescent signals. The same primary antibody was used to evaluate D2 receptor expression by western blot. The D2 receptor immune-fluorescence appeared circumscribed in small disks (~0.3–0.5 μm diameter), likely representing D2 synapse aggregates, densely distributed in the striatum of Tor1a+/+ mice. In the Tor1a+/− mice the D2 aggregates were significantly smaller (μm2 2.4 ± SE 0.16, compared to μm2 6.73 ± SE 3.41 in Tor1a+/+) and sparse, with ~30% less number per microscopic field, value correspondent to the amount of reduced D2 expression in western blotting analysis. In DYT1 mutant mice the sparse and small D2 synapses in the striatum may be insufficient to “gate” the amount of presynaptic dopamine release diffusing in peri-synaptic space, and this consequently may result in a timing and spatially larger nonselective sphere of influence of dopamine action
Genetic therapies for inherited neuromuscular disorders
Inherited neuromuscular disorders encompass a broad group of genetic conditions, and the discovery of these underlying genes has expanded greatly in the past three decades. The discovery of such genes has enabled more precise diagnosis of these disorders and the development of specific therapeutic approaches that target the genetic basis and pathophysiological pathways. Such translational research has led to the approval of two genetic therapies by the US Food and Drug Administration: eteplirsen for Duchenne muscular dystrophy and nusinersen for spinal muscular atrophy, which are both antisense oligonucleotides that modify pre-mRNA splicing. In this Review we aim to discuss new genetic therapies and ongoing clinical trials for Duchenne muscular dystrophy, spinal muscular atrophy, and other less common childhood neuromuscular disorders
Quantum realizations of Hilbert-Palatini second-class constraints
In a classical theory of gravity, the Barbero-Immirzi parameter ()
appears as a topological coupling constant through the Lagrangian density
containing the Hilbert-Palatini term and the Nieh-Yan invariant. In a quantum
framework, the topological interpretation of can be captured through a
rescaling of the wavefunctional representing the Hilbert-Palatini theory, as in
the case of the QCD vacuum angle. However, such a rescaling cannot be realized
for pure gravity within the standard (Dirac) quantization procedure where the
second-class constraints of Hilbert-Palatini theory are eliminated beforehand.
Here we present a different treatment of the Hilbert-Palatini second-class
constraints in order to set up a general rescaling procedure (a) for gravity
with or without matter and (b) for any choice of gauge (e.g. time gauge). The
analysis is developed using the Gupta-Bleuler and the coherent state
quantization methods.Comment: Published versio
Flujo de partículas en un ambiente costero Antártico de aguas someras: un estudio con trampas de sedimento
Sediment trap arrays were deployed at Potter Cove, King George Island, Antarctica, between December 1991 and August 1992 and in the summer seasons of 1993-1994, and 1995. The sampling sites reached 30 m depth, and traps were placed during the different seasons at various distances from the sea bottom (0.1 to 25 m), some being buried in the sediments. Daily sedimentation rates of total particulate matter (TPM) and chlorophyll-a were estimated. Water column temperature and salinity as well as pigment and TPM concentration were also measured and related to traps´ results. Water column data evidenced processes in relation with phytoplankton dynamics, stormy events and particles containing fresh water runoff. Similar trends were observed in the traps located at 25 m, 1 m and sometimes even in those located to around 0.1 m from sea bottom, although traps buried in the sediments mainly reflected resuspension events. Chlorophyll a fluxes were higher in the bottom traps, but the sedimented organic fraction of the TPM (particulate organic matter, POM) was higher in the traps located remotely from the bottom, being significantly lower in the buried traps (i.e. POM represented 50% of TPM at 1 m and 9% POM in the buried trap on November 23 in 1993, although a great variability among sampling dates was also observed). The significance of these dynamics for the food availability for the macrozoobenthic organisms present in the area is discussed.Entre diciembre de 1991 y agosto de 1992 y durante las campañas de verano de 1993-1994 y 1995 se estudió la tasa de sedimentación de partículas en Caleta Potter, Isla King George - 25 de Mayo, Antártida, mediante el empleo de trampas de sedimento. La profundidad en los sitios de muestreo fue de 30 m. Las trampas fueron colocadas a distintas distancias del fondo (0,1 a 25 m), habiéndose enterrado algunas en los sedimentos. Se estimaron las tasas diarias de sedimentación para el total de material particulado y los pigmentos. Se midieron además la temperatura y la salinidad, así como la concentración de material particulado total y pigmentos en la columna de agua. Estos parámetros fueron relacionados con aquellos de las trampas de sedimento. Los resultados de la columna de agua reflejaron procesos relacionados con la dinámica del fitoplancton, eventos de tormentas y aportes terrígenos de agua dulce. Esto fue válido para las trampas colocadas a 25 m, 1 m y en ocasiones aún para aquellas colocadas a 0,1 m, si bien los datos obtenidos de las trampas enterradas en el sedimento se relacionan con episodios de resuspensión. Los flujos de clorofila a fueron mayores en las trampas más cercanas al fondo, pero la fracción orgánica del material particulado sedimentada fue significativamente menor en las trampas enterradas. Por ejemplo, el porcentaje de materia orgánica en las trampas del 23 de noviembre de 1993 fue de 50% a 1 m y del 9% en la trampa enterrada en los sedimentos, si bien pudo observarse una gran variabilidad entre fechas de muestreo. Se discute la importancia de esta dinámica para la nutrición de los organismos del macrozoobentos presentes en el área
Gravity from a fermionic condensate of a gauge theory
The most prominent realization of gravity as a gauge theory similar to the
gauge theories of the standard model comes from enlarging the gauge group from
the Lorentz group to the de Sitter group. To regain ordinary Einstein-Cartan
gravity the symmetry must be broken, which can be accomplished by known
quasi-dynamic mechanisms. Motivated by symmetry breaking models in particle
physics and condensed matter systems, we propose that the symmetry can
naturally be broken by a homogenous and isotropic fermionic condensate of
ordinary spinors. We demonstrate that the condensate is compatible with the
Einstein-Cartan equations and can be imposed in a fully de Sitter invariant
manner. This lends support, and provides a physically realistic mechanism for
understanding gravity as a gauge theory with a spontaneously broken local de
Sitter symmetry.Comment: 16 page
Generalized Chern-Simons Modified Gravity in First-Order Formalism
We propose a generalization of Chern-Simons (CS) modified gravity in
first-order formalism. CS modified gravity action has a term that comes from
the chiral anomaly which is Pontryagin invariant. First-order CS modified
gravity is a torsional theory and in a space-time with torsion the chiral
anomaly includes a torsional topological term called Nieh-Yan invariant. We
generalize the CS modified gravity by adding the Nieh-Yan term to the action
and find the effective theory. We compare the generalized theory with the
first-order CS modified gravity and comment on the similarities and
differences.Comment: 8 pages, an author added, new paragraphs, comments and references
added, published in Gen. Relativ. Gravi
E-Voting in an ubicomp world: trust, privacy, and social implications
The advances made in technology have unchained the user from the desktop into interactions where access is anywhere, anytime. In addition, the introduction of ubiquitous computing (ubicomp) will see further changes in how we interact with technology and also socially. Ubicomp evokes a near future in which humans will be surrounded by “always-on,” unobtrusive, interconnected intelligent objects where information is exchanged seamlessly. This seamless exchange of information has vast social implications, in particular the protection and management of personal information. This research project investigates the concepts of trust and privacy issues specifically related to the exchange of e-voting information when using a ubicomp type system
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