1,545 research outputs found
Viscoresistive MHD Configurations of Plasma in Accretion Disks
We present a discussion of two-dimensional magneto-hydrodynamics (MHD)
configurations, concerning the equilibria of accretion disks of a strongly
magnetized astrophysical object. We set up a viscoresistive scenario which
generalizes previous two-dimensional analyses by reconciling the ideal MHD
coupling of the vertical and the radial equilibria within the disk with the
standard mechanism of the angular momentum transport, relying on dissipative
properties of the plasma configuration. The linear features of the considered
model are analytically developed and the non-linear configuration problem is
addressed, by fixing the entire disk profile at the same order of
approximation. Indeed, the azimuthal and electron force balance equations are
no longer automatically satisfied when poloidal currents and matter fluxes are
included in the problem. These additional components of the equilibrium
configuration induce a different morphology of the magnetic flux surface, with
respect to the ideal and simply rotating disk.Comment: 19 pages, 4 figures. To appear on the Proceedings of the Second
Italian-Pakistani Workshop on Relativistic Astrophysic
Chiral Bosons as solutions of the BV master equation 2D chiral gauge theories
We construct the chiral Wess-Zumino term as a solution for the
Batalin-Vilkovisky master equation for anomalous two-dimensional gauge
theories, working in an extended field-antifield space, where the gauge group
elements are introduced as additional degrees of freedom.
We analyze the Abelian and the non-Abelian cases, calculating in both cases
the BRST generator in order to show the physical equivalence between this
chiral solution for the master equation and the usual (non-chiral) one.Comment: 11 pages, TEX dialet, IF/UFRJ-94-
Democratic particle motion for meta-basin transitions in simple glass-formers
We use molecular dynamics computer simulations to investigate the local
motion of the particles in a supercooled simple liquid. Using the concept of
the distance matrix we find that the alpha-relaxation corresponds to a small
number of crossings from one meta-basin to a neighboring one. Each crossing is
very rapid and involves the collective motion of O(40) particles that form a
relatively compact cluster, whereas string-like motions seem not to be relevant
for these transitions. These compact clusters are thus candidates for the
cooperatively rearranging regions proposed long times ago by Adam and Gibbs.Comment: 4 pages, 4 Postscript figure
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
General Relativity as Classical Limit of Evolutionary Quantum Gravity
We analyze the dynamics of the gravitational field when the covariance is
restricted to a synchronous gauge. In the spirit of the Noether theorem, we
determine the conservation law associated to the Lagrangian invariance and we
outline that a non-vanishing behavior of the Hamiltonian comes out. We then
interpret such resulting non-zero ``energy'' of the gravitational field in
terms of a dust fluid. This new matter contribution is co-moving to the slicing
and it accounts for the ``materialization'' of a synchronous reference from the
corresponding gauge condition. Further, we analyze the quantum dynamics of a
generic inhomogeneous Universe as described by this evolutionary scheme,
asymptotically to the singularity. We show how the phenomenology of such a
model overlaps the corresponding Wheeler-DeWitt picture. Finally, we study the
possibility of a Schr\"odinger dynamics of the gravitational field as a
consequence of the correspondence inferred between the ensemble dynamics of
stochastic systems and the WKB limit of their quantum evolution. We demonstrate
that the time dependence of the ensemble distribution is associated with the
first order correction in to the WKB expansion of the energy spectrum.Comment: 23 pages, to appear on Class. Quant. Gra
Non Abelian gauge symmetries induced by the unobservability of extra-dimensions in a Kaluza-Klein approach
In this work we deal with the extension of the Kaluza-Klein approach to a
non-Abelian gauge theory; we show how we need to consider the link between the
n-dimensional model and a four-dimensional observer physics, in order to
reproduce fields equations and gauge transformations in the four-dimensional
picture. More precisely, in fields equations any dependence on
extra-coordinates is canceled out by an integration, as consequence of the
unobservability of extra-dimensions. Thus, by virtue of this extra-dimensions
unobservability, we are able to recast the multidimensional Einstein equations
into the four-dimensional Einstein-Yang-Mills ones, as well as all the right
gauge transformations of fields are induced. The same analysis is performed for
the Dirac equation describing the dynamics of the matter fields and, again, the
gauge coupling with Yang-Mills fields are inferred from the multidimensional
free fields theory, together with the proper spinors transformations.Comment: 5 pages, no figures, to appear in Mod. Phys. Lett.
Quantum mechanics over a q-deformed (0+1)-dimensional superspace
We built up a explicit realization of (0+1)-dimensional q-deformed superspace
coordinates as operators on standard superspace. A q-generalization of
supersymmetric transformations is obtained, enabling us to introduce scalar
superfields and a q-supersymmetric action. We consider a functional integral
based on this action. Integration is implemented, at the level of the
coordinates and at the level of the fields, as traces over the corresponding
representation spaces. Evaluation of these traces lead us to standard
functional integrals. The generation of a mass term for the fermion field
leads, at this level, to an explicitely broken version of supersymmetric
quantum mechanics.Comment: 11 pages, Late
The Averaging Problem in Cosmology and Macroscopic Gravity
The averaging problem in cosmology and the approach of macroscopic gravity to
resolve the problem is discussed. The averaged Einstein equations of
macroscopic gravity are modified on cosmological scales by the macroscopic
gravitational correlation tensor terms as compared with the Einstein equations
of general relativity. This correlation tensor satisfies a system of structure
and field equations. An exact cosmological solution to the macroscopic gravity
equations for a constant macroscopic gravitational connection correlation
tensor for a flat spatially homogeneous, isotropic macroscopic space-time is
presented. The correlation tensor term in the macroscopic Einstein equations
has been found to take the form of either a negative or positive spatial
curvature term. Thus, macroscopic gravity provides a cosmological model for a
flat spatially homogeneous, isotropic Universe which obeys the dynamical law
for either an open or closed Universe.Comment: 8 pages, LaTeX, ws-ijmpa.cls, few style and typo corrections. Based
on the plenary talk given at the Second Stueckelberg Workshop, ICRANet
Coordinating Center, Pescara, Italy, September 3-7, 2007. To appear in
International Journal of Modern Physics A (2008
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