10,360 research outputs found
On the Frame Fixing in Quantum Gravity
We provide a discussion about the necessity to fix the reference frame before
quantizing the gravitational field. Our presentation is based on stressing how
the 3+1-slicing of the space time becomes an ambiguous procedure as referred to
a quantum 4-metric.Comment: 3 pages, no figures, proceeding of the X Marcel Grossmann meeting,
Rio de Janeiro, Brazil, July 20-26, 200
A New Approach in Quantum Gravity and its Cosmological Implications
This work concerns a new reformulation of quantum geometrodynamics, which
allows to overcome a fundamental ambiguity contained in the canonical approach
to quantum gravity: the possibility of performing a (3+1)-slicing of
space-time, when the metric tensor is in a quantum regime. Our formulation
provides also a procedure to solve the problems connected to the so called
"frozen formalism". In particular we fix the reference frame (i.e. the lapse
function and the shift vector) by introducing the so called "kinematical
action"; as a consequence, the new hamiltonian constraints become parabolic, so
arriving to evolutive (Schroedinger-like) equations for the quantum dynamics.
The kinematical action can be interpreted as the action of a pressureless, but,
in general, non geodesic perfect fluid, so in the semi classical limit our
theory leads to the dynamics of the gravitational field coupled to a dust which
represents the material reference frame we have introduced fixing the slicing.
We also investigate the cosmological implications of the presence of the dust,
which, in the WKB limit of a cosmological problem, makes account for a "dark
matter" component and could play, at present time, a dynamical role.Comment: A synthesis of previous works, based on a talk given at the VIII
Italian-Korean meeting (Pescara, Italy), to appear on "Il Nuovo Cimento
Bottom changes in coastal areas with complex shorelines
A model for the sea-bottom change simulations in coastal areas with complex shorelines is proposed. In deep and intermediate water depths, the hydrodynamic quantities are calculated by numerically integrating the contravariant Boussinesq equations, devoid of Christoffel symbols. In the surf zone, the propagation of the breaking waves is simulated by the nonlinear shallow water equations. The momentum equation is solved inside the turbulent boundary layer in order to calculate intrawave hydrodynamic quantities. An integral formulation for the contravariant suspended sediment advection-diffusion equation is proposed and used for the sea-bottom dynamic simulations. The proposed model is applied to the real case study of Pescara harbor (in Italy)
Combining rare events techniques: phase change in Si nanoparticles
We introduce a combined Restrained MD/Parallel Tempering approach to study
the difference in free energy as a function of a set of collective variables
between two states in presence of unknown slow degrees of freedom.
We applied this method to study the relative stability of the amorphous vs
crystalline nanoparticles of size ranging between 0.8 and 1.8 nm as a function
of the temperature. We found that, at variance with bulk systems, at low T
small nanoparticles are amorphous and undergo an amorphous-to-crystalline phase
transition at higher T. On the contrary, large nanoparticles recover the
bulk-like behavior: crystalline at low and amorphous at high T
Finite-range effects in dilute Fermi gases at unitarity
We develop a theoretical method going beyond the contact-interaction
approximation frequently used in mean-field theories of many-fermion systems,
based on the low-energy T-matrix of the pair potential to rigorously define the
effective radius of the interaction. One of the main consequences of our
approach is the possibility to investigate finite-density effects, which are
outside the range of validity of approximations based on delta-like potentials.
We apply our method to the calculation of density dependent properties of an
ultracold gas of 6Li atoms at unitarity, whose two-body interaction potential
is calculated using ab initio quantum chemistry methods. We find that density
effects will be significant in ultracold gases with densities one order of
magnitude higher than those attained in current experiments.Comment: 7 pages, 3 figures. v2: Corrected typos in the main text and in the
reference
Is Torsion a Fundamental Physical Field?
The local Lorentz group is introduced in flat space-time, where the resulting
Dirac and Yang-Mills equations are found, and then generalized to curved
space-time: if matter is neglected, the Lorentz connection is identified with
the contortion field, while, if matter is taken into account, both the Lorentz
connection and the spinor axial current are illustrated to contribute to the
torsion of space-time.Comment: 3 pages, proceedings of the XI Marcel Grossmann meeting on
Relativistic Astrophysics, July 23-29, 2006, Berli
Economic feasibility of small wind turbines for domestic consumers in Egypt based on the new feed-in tariff
This paper provides an overview of the wind power potential at different regions in Egypt, along the Mediterranean and Red Sea, and the Western desert. A further technical and economic assessment is conducted for the electricity generation with 8 different small wind turbines at 17 locations. The annual electricity generation from selected wind turbines is evaluated. The obtained data are presented and discussed investigating the net present value and the payback period analyzing the profitability of selected wind turbines. The dependence of the turbine profitability from the feed-in tariff is specifically addressed
Classical and free Fourth Moment Theorems: universality and thresholds
Let be a centered random variable with unit variance, zero third moment,
and such that . Let denote a normalized
sequence of homogeneous sums of fixed degree , built from independent
copies of . Under these minimal conditions, we prove that converges in
distribution to a standard Gaussian random variable if and only if the
corresponding sequence of fourth moments converges to . The statement is
then extended (mutatis mutandis) to the free probability setting. We shall also
discuss the optimality of our conditions in terms of explicit thresholds, as
well as establish several connections with the so-called universality
phenomenon of probability theory. Both in the classical and free probability
frameworks, our results extend and unify previous Fourth Moment Theorems for
Gaussian and semicircular approximations. Our techniques are based on a fine
combinatorial analysis of higher moments for homogeneous sums.Comment: 26 page
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