1,233 research outputs found
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
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
Oscillatory regime in the Multidimensional Homogeneous Cosmological Models Induced by a Vector Field
We show that in multidimensional gravity vector fields completely determine
the structure and properties of singularity. It turns out that in the presence
of a vector field the oscillatory regime exists in all spatial dimensions and
for all homogeneous models. By analyzing the Hamiltonian equations we derive
the Poincar\'e return map associated to the Kasner indexes and fix the rules
according to which the Kasner vectors rotate. In correspondence to a
4-dimensional space time, the oscillatory regime here constructed overlap the
usual Belinski-Khalatnikov-Liftshitz one.Comment: 9 pages, published on Classical and Quantum Gravit
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
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
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
Lifshitz fermionic theories with z=2 anisotropic scaling
We construct fermionic Lagrangians with anisotropic scaling z=2, the natural
counterpart of the usual z=2 Lifshitz field theories for scalar fields. We
analyze the issue of chiral symmetry, construct the Noether axial currents and
discuss the chiral anomaly giving explicit results for two-dimensional case. We
also exploit the connection between detailed balance and the dynamics of
Lifshitz theories to find different z=2 fermionic Lagrangians and construct
their supersymmetric extensions.Comment: Typos corrected, comment adde
Backreaction of Cosmological Perturbations in Covariant Macroscopic Gravity
The problem of corrections to Einstein's equations arising from averaging of
inhomogeneities ("backreaction") in the cosmological context, has gained
considerable attention recently. We present results of analysing cosmological
perturbation theory in the framework of Zalaletdinov's fully covariant
Macroscopic Gravity. We show that this framework can be adapted to the setting
of cosmological perturbations in a manner which is free from gauge related
ambiguities. We derive expressions for the backreaction which can be readily
applied in \emph{any} situation (not necessarily restricted to the linear
perturbations considered here) where the \emph{metric} can be brought to the
perturbed FLRW form. In particular these expressions can be employed in toy
models studying nonlinear structure formation, and possibly also in N-body
simulations. Additionally, we present results of example calculations which
show that the backreaction remains negligible well into the matter dominated
era.Comment: 17 pages, 5 figures, revtex4; v2 -- typos fixed, 1 reference updated,
to appear in Phys Rev
Performance of the ARPA-SMR limited-area ensemble prediction system: two flood cases
The performance of the ARPA-SMR Limited-area Ensemble Prediction System (LEPS), generated by nesting a limited-area model on selected members of the ECMWF targeted ensemble, is evaluated for two flood events that occurred during September 1992. The predictability of the events is studied for forecast times ranging from 2 to 4 days. The extent to which floods localised in time and space can be forecast at high resolution in probabilistic terms was investigated. Rainfall probability maps generated by both LEPS and ECMWF targeted ensembles are compared for different precipitation thresholds in order to assess the impact of enhanced resolution. At all considered forecast ranges, LEPS performs better, providing a more accurate description of the event with respect to the spatio-temporal location, as well as its intensity. In both flood cases, LEPS probability maps turn out to be a very valuable tool to assist forecasters to issue flood alerts at different forecast ranges. It is also shown that at the shortest forecast range, the deterministic prediction provided by the limited area model, when run in a higher-resolution configuration, provides a very accurate rainfall pattern and a good quantitative estimate of the total rainfall deployed in the flooded regions
Definition of a time variable with Entropy of a perfect fluid in Canonical Quantum Gravity
The Brown-Kuchar mechanism is applied in the case of General Relativity
coupled with the Schutz model for a perfect fluid. Using the canonical
formalism and manipulating the set of modified constraints one is able to
recover the definition of a time evolution operator, i.e. a physical
Hamiltonian, expressed as a functional of gravitational variables and the
entropy.Comment: 13 pages, no figures. To be published in Classical and Quantum
Gravity. v2: major change
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