1,763 research outputs found
Squeezing of toroidal accretion disks
Accretion disks around very compact objects such as very massive Black hole
can grow according to thick toroidal models. We face the problem of defining
how does change the thickness of a toroidal accretion disk spinning around a
Schwarzschild Black hole under the influence of a toroidal magnetic field and
by varying the fluid angular momentum. We consider both an hydrodynamic and a
magnetohydrodynamic disk based on the Polish doughnut thick model. We show that
the torus thickness remains basically unaffected but tends to increase or
decrease slightly depending on the balance of the magnetic, gravitational and
centrifugal effects which the disk is subjected to.Comment: 6 pages, 17 figures, to appear in EP
Gravity in presence of fermions as a SU(2) gauge theory
The Hamiltonian formulation of the Holst action in presence of a massless
fermion field with a non-minimal Lagrangian is performed without any
restriction on the local Lorentz frame. It is outlined that the phase space
structure does not resemble that one of a background independent Lorentz gauge
theory, as some additional constraints are present. Proper phase space
coordinates are introduced, such that SU(2) connections can be defined and the
vanishing of conjugate momenta to boost variables is predicted. Finally, it is
demonstrated that for a particular value of the non-minimal parameter the
kinematics coincides with that one of a background independent SU(2) gauge
theory and the Immirzi parameter becomes the coupling constant of such an
interaction between fermions and the gravitational field.Comment: 5 pages, accepted for publication in Phys. Rev.
Minisuperspace Model for Revised Canonical Quantum Gravity
We present a reformulation of the canonical quantization of gravity, as
referred to the minisuperspace; the new approach is based on fixing a Gaussian
(or synchronous) reference frame and then quantizing the system via the
reconstruction of a suitable constraint; then the quantum dynamics is re-stated
in a generic coordinates system and it becomes dependent on the lapse function.
The analysis follows a parallelism with the case of the non-relativistic
particle and leads to the minisuperspace implementation of the so-called {\em
kinematical action} as proposed in \cite{M02} (here almost coinciding also with
the approach presented in \cite{KT91}). The new constraint leads to a
Schr\"odinger equation for the system. i.e. to non-vanishing eigenvalues for
the super-Hamiltonian operator; the physical interpretation of this feature
relies on the appearance of a ``dust fluid'' (non-positive definite) energy
density, i.e. a kind of ``materialization'' of the reference frame. As an
example of minisuperspace model, we consider a Bianchi type IX Universe, for
which some dynamical implications of the revised canonical quantum gravity are
discussed. We also show how, on the classical limit, the presence of the dust
fluid can have relevant cosmological issues. Finally we upgrade our analysis by
its extension to the generic cosmological solution, which is performed in the
so-called long-wavelength approximation. In fact, near the Big-Bang, we can
neglect the spatial gradients of the dynamical variables and arrive to
implement, in each space point, the same minisuperspace paradigm valid for the
Bianchi IX model.Comment: 16 pages, no figures, to appear on International Journal of Modern
Physics
Cosmological implications of an evolutionary quantum gravity
The cosmological implications of an evolutionary quantum gravity are analyzed
in the context of a generic inhomogeneous model. The Schr\"{o}dinger problem is
formulated and solved in the presence of a scalar field, an ultrarelativistic
matter and a perfect gas regarded as the dust-clock. Considering the actual
phenomenology, it is shown how the evolutionary approach overlaps the
Wheeler-DeWitt one.Comment: 4 pages; to appear in the proceedings of the II Stueckelberg
Workshop, Int.J.Mod.Phys.A, references adde
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