1,075 research outputs found
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
On Matter Coupling in 5D Kaluza-Klein Model
We analyze some unphysical features of the geodesic approach to matter
coupling in a compactified Kaluza-Klein scenario, like the q/m puzzle and the
huge massive modes. We propose a new approach, based on Papapetrou multipole
expansion, that provides a new equation for the motion of a test particle. We
show how this equation provides right couplings and does not generate huge
massive modes.Comment: 4 pages, to appear in Proceedings of the II Stueckelberg Workshop -
Int. J. Mod. Phys.
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
Covariant Formulation of the Invariant Measure for the Mixmaster Dynamics
We provide a Hamiltonian analysis of the Mixmaster Universe dynamics showing
the covariant nature of its chaotic behavior with respect to any choice of time
variable. We construct the appropriate invariant measure for the system (which
relies on the existence of an ``energy-like'' constant of motion) without
fixing the time gauge, i.e. the corresponding lapse function. The key point in
our analysis consists of introducing generic Misner-Chitr\'e-like variables
containing an arbitrary function, whose specification allows one to set up the
same dynamical scheme in any time gauge.Comment: 11 pages, 1 figur
Mixmaster Chaoticity as Semiclassical Limit of the Canonical Quantum Dynamics
Within a cosmological framework, we provide a Hamiltonian analysis of the
Mixmaster Universe dynamics on the base of a standard Arnowitt-Deser-Misner
approach, showing how the chaotic behavior characterizing the evolution of the
system near the cosmological singularity can be obtained as the semiclassical
limit of the canonical quantization of the model in the same dynamical
representation. The relation between this intrinsic chaotic behavior and the
indeterministic quantum dynamics is inferred through the coincidence between
the microcanonical probability distribution and the semiclassical quantum one.Comment: 9 pages, 1 figur
Mixed diffusive-convective relaxation of a broad beam of energetic particles in cold plasma
We revisit the applications of quasi-linear theory as a paradigmatic model
for weak plasma turbulence and the associated bump-on-tail problem. The work,
presented here, is built around the idea that large-amplitude or strongly
shaped beams do not relax through diffusion only and that there exists an
intermediate time scale where the relaxations are convective (ballistic-like).
We cast this novel idea in the rigorous form of a self-consistent nonlinear
dynamical model, which generalizes the classic equations of the quasi-linear
theory to "broad" beams with internal structure. We also present numerical
simulation results of the relaxation of a broad beam of energetic particles in
cold plasma. These generally demonstrate the mixed diffusive-convective
features of supra-thermal particle transport; and essentially depend on
nonlinear wave-particle interactions and phase-space structures. Taking into
account modes of the stable linear spectrum is crucial for the self-consistent
evolution of the distribution function and the fluctuation intensity spectrum.Comment: 25 pages, 15 figure
On the Gravitational Collapse of a Gas Cloud in Presence of Bulk Viscosity
We analyze the effects induced by the bulk viscosity on the dynamics
associated to the extreme gravitational collapse. Aim of the work is to
investigate whether the presence of viscous corrections to the evolution of a
collapsing gas cloud influence the fragmentation process. To this end we study
the dynamics of a uniform and spherically symmetric cloud with corrections due
to the negative pressure contribution associated to the bulk viscosity
phenomenology. Within the framework of a Newtonian approach (whose range of
validity is outlined), we extend to the viscous case either the Lagrangian,
either the Eulerian motion of the system and we treat the asymptotic evolution
in correspondence to a viscosity coefficient of the form ( being the cloud density and ). We show how,
in the adiabatic-like behavior of the gas (i.e. when the politropic index takes
values ), density contrasts acquire, asymptotically, a
vanishing behavior which prevents the formation of sub-structures. We can
conclude that in the adiabatic-like collapse the top down mechanism of
structures formation is suppressed as soon as enough strong viscous effects are
taken into account. Such a feature is not present in the isothermal-like (i.e.
) collapse because the sub-structures formation is yet present
and outlines the same behavior as in the non-viscous case. We emphasize that in
the adiabatic-like collapse the bulk viscosity is also responsible for the
appearance of a threshold scale beyond which perturbations begin to increase.Comment: 13 pages, no figur
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
The Jeans Instability in Presence of Viscous Effects
An analysis of the gravitational instability in presence of dissipative
effects is addressed. In particular, the standard Jeans Mechanism and the
generalization in treating the Universe expansion are both analyzed when bulk
viscosity affects the first-order Newtonian dynamics. As results, the
perturbation evolution is founded to be damped by dissipative processes and the
top-down mechanism of structure fragmentation is suppressed. In such a scheme,
the value of the Jeans Mass remains unchanged also in presence of viscosity.Comment: 13 pages, 2 figure
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