781 research outputs found
Redshift drift in axially symmetric quasi-spherical Szekeres models
Models of inhomogeneous universes constructed with exact solutions of
Einstein's General Relativity have been proposed in the literature with the aim
of reproducing the cosmological data without any need for a dark energy
component. Besides large scale inhomogeneity models spherically symmetric
around the observer, Swiss-cheese models have also been studied. Among them,
Swiss-cheeses where the inhomogeneous patches are modeled by different
particular Szekeres solutions have been used for reproducing the apparent
dimming of the type Ia supernovae (SNIa). However, the problem of fitting such
models to the SNIa data is completely degenerate and we need other constraints
to fully characterize them. One of the tests which is known to be able to
discriminate between different cosmological models is the redshift-drift. This
drift has already been calculated by different authors for
Lema\^itre-Tolman-Bondi (LTB) models. We compute it here for one particular
axially symmetric quasi-spherical Szekeres (QSS) Swiss-cheese which has
previously been shown to reproduce to a good accuracy the SNIa data, and we
compare the results to the drift in the CDM model and in some LTB
models that can be found in the literature. We show that it is a good
discriminator between them. Then, we discuss our model's remaining degrees of
freedom and propose a recipe to fully constrain them.Comment: 15 pages, 7 figures, minor changes in title, text, figures and
references; conclusions unchanged, this version matches the published versio
The hodograph method applicability in the problem of long-scale nonlinear dynamics of a thin vortex filament near a flat boundary
Hamiltonian dynamics of a thin vortex filament in ideal incompressible fluid
near a flat fixed boundary is considered at the conditions that at any point of
the curve determining shape of the filament the angle between tangent vector
and the boundary plane is small, also the distance from a point on the curve to
the plane is small in comparison with the curvature radius. The dynamics is
shown to be effectively described by a nonlinear system of two
(1+1)-dimensional partial differential equations. The hodograph transformation
reduces that system to a single linear differential equation of the second
order with separable variables. Simple solutions of the linear equation are
investigated at real values of spectral parameter when the filament
projection on the boundary plane has shape of a two-branch spiral or a smoothed
angle, depending on the sign of .Comment: 9 pages, revtex4, 6 eps-figure
Fuzzy Control Based Renewable Energy Sources for DC Microgrid Applications using FPGA Platform with EMS
The main objective of this proposed system is to provide uninterruptible power supply to the load.Ă‚Â This proposed system mainly deals with the Energy Management System (EMS) of the DC microgrid systems, using the fuzzy logic control.Ă‚Â This proposed system consists of the power sources, which obtains its power from the PV panels, Wind turbine, and fuel cells stack.Ă‚Â The EMS incorporates the fuzzy control that is responsible for the Energy Management and Battery Management.Ă‚Â The fuzzy maintains the State of Charge (SoC) parameters of the battery.Ă‚Â The fuzzy logic implementation of this system was done by using the Field Programmable Gate Array (FPGA)
Exactly solvable model of wormhole supported by phantom energy
We have found a simple exact solution of spherically-symmetrical Einstein
equations describing a wormhole for an inhomogeneous distribution of the
phantom energy. The equation of state is linear but highly anisotropic: while
the radial pressure is negative, the transversal one is positive. At infinity
the spacetime is not asymptotically flat and possesses on each side of the
bridge a regular cosmological Killing horizon with an infinite area,
impenetrable for any particles. This horizon does not arise if the wormhole
region is glued to the Schwarzschild region. In doing so, the wormhole can
enclose an arbitrary amount of the phantom energy. The configuration under
discussion has a limit in which the phantom energy turns into the string dust,
the areal radius tends to the constant. In this limit, the strong gravitational
mass defect is realized in that the gravitational active mass is finite and
constant while the proper mass integrated over the total manifold is infinite.Comment: 6 pages. Two references added, typos corrected. Accepted for
publication in Phys. Rev. D as Rapid Communicatio
Nonlinear interfacial waves in a constant-vorticity planar flow over variable depth
Exact Lagrangian in compact form is derived for planar internal waves in a
two-fluid system with a relatively small density jump (the Boussinesq limit
taking place in real oceanic conditions), in the presence of a background shear
current of constant vorticity, and over arbitrary bottom profile. Long-wave
asymptotic approximations of higher orders are derived from the exact
Hamiltonian functional in a remarkably simple way, for two different
parametrizations of the interface shape.Comment: revtex, 4.5 pages, minor corrections, summary added, accepted to JETP
Letter
Current-sheet formation in incompressible electron magnetohydrodynamics
The nonlinear dynamics of axisymmetric, as well as helical, frozen-in vortex
structures is investigated by the Hamiltonian method in the framework of ideal
incompressible electron magnetohydrodynamics. For description of current-sheet
formation from a smooth initial magnetic field, local and nonlocal nonlinear
approximations are introduced and partially analyzed that are generalizations
of the previously known exactly solvable local model neglecting electron
inertia. Finally, estimations are made that predict finite-time singularity
formation for a class of hydrodynamic models intermediate between that local
model and the Eulerian hydrodynamics.Comment: REVTEX4, 5 pages, no figures. Introduction rewritten, new material
and references adde
The Birkhoff Theorem in Multidimensional Gravity
The validity conditions for the extended Birkhoff theorem in multidimensional
gravity with internal spaces are formulated, with no restriction on
space-time dimensionality and signature. Examples of matter sources and
geometries for which the theorem is valid are given. Further generalization of
the theorem is discussed.Comment: 8 page
Gravitational Collapse of Dust with a Cosmological Constant
The recent analysis of Markovic and Shapiro on the effect of a cosmological
constant on the evolution of a spherically symmetric homogeneous dust ball is
extended to include the inhomogeneous and degenerate cases. The histories are
shown by way of effective potential and Penrose-Carter diagrams.Comment: 2 pages, 2 figures (png), revtex. To appear in Phys. Rev.
Regular black holes with flux tube core
We consider a class of black holes for which the area of the two-dimensional
spatial cross-section has a minimum on the horizon with respect to a
quasiglobal (Krusckal-like) coordinate. If the horizon is regular, one can
generate a tubelike counterpart of such a metric and smoothly glue it to a
black hole region. The resulting composite space-time is globally regular, so
all potential singuilarities under the horizon of the original metrics are
removed. Such a space-time represents a black hole without an apparent horizon.
It is essential that the matter should be non-vacuum in the outer region but
vacuumlike in the inner one. As an example we consider the noninteracting
mixture of vacuum fluid and matter with a linear equation of state and scalar
phantom fields. This approach is extended to distorted metrics, with the
requirement of spherical symmetry relaxed.Comment: 15 pages. 2 references adde
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