373 research outputs found
Dark energy domination in the Virgocentric flow
The standard \LambdaCDM cosmological model implies that all celestial bodies
are embedded in a perfectly uniform dark energy background, represented by
Einstein's cosmological constant, and experience its repulsive antigravity
action. Can dark energy have strong dynamical effects on small cosmic scales as
well as globally? Continuing our efforts to clarify this question, we focus now
on the Virgo Cluster and the flow of expansion around it. We interpret the
Hubble diagram, from a new database of velocities and distances of galaxies in
the cluster and its environment, using a nonlinear analytical model which
incorporates the antigravity force in terms of Newtonian mechanics. The key
parameter is the zero-gravity radius, the distance at which gravity and
antigravity are in balance. Our conclusions are: 1. The interplay between the
gravity of the cluster and the antigravity of the dark energy background
determines the kinematical structure of the system and controls its evolution.
2. The gravity dominates the quasi-stationary bound cluster, while the
antigravity controls the Virgocentric flow, bringing order and regularity to
the flow, which reaches linearity and the global Hubble rate at distances \ga
15 Mpc. 3. The cluster and the flow form a system similar to the Local Group
and its outflow. In the velocity-distance diagram, the cluster-flow structure
reproduces the group-flow structure with a scaling factor of about 10; the
zero-gravity radius for the cluster system is also 10 times larger. The phase
and dynamical similarity of the systems on the scales of 1-30 Mpc suggests that
a two-component pattern may be universal for groups and clusters: a
quasi-stationary bound central component and an expanding outflow around it,
due to the nonlinear gravity-antigravity interplay with the dark energy
dominating in the flow component.Comment: 7 pages, 2 figures, Astronomy and Astrophysics (accepted
The Hall instability of weakly ionized, radially stratified, rotating disks
Cool weakly ionized gaseous rotating disk, are considered by many models as
the origin of the evolution of protoplanetary clouds. Instabilities against
perturbations in such disks play an important role in the theory of the
formation of stars and planets. Thus, a hierarchy of successive fragmentations
into smaller and smaller pieces as a part of the Kant-Laplace theory of
formation of the planetary system remains valid also for contemporary
cosmogony. Traditionally, axisymmetric magnetohydrodynamic (MHD), and recently
Hall-MHD instabilities have been thoroughly studied as providers of an
efficient mechanism for radial transfer of angular momentum, and of density
radial stratification. In the current work, the Hall instability against
nonaxisymmetric perturbations in compressible rotating fluids in external
magnetic field is proposed as a viable mechanism for the azimuthal
fragmentation of the protoplanetary disk and thus perhaps initiating the road
to planet formation. The Hall instability is excited due to the combined effect
of the radial stratification of the disk and the Hall electric field, and its
growth rate is of the order of the rotation period.Comment: 15 pages, 2 figure
Sources of Radiation in the Early Universe: The Equation of Radiative Transfer and Optical Distances
We have derived the radiative-transfer equation for a point source with a
specified intensity and spectrum, originating in the early Universe between the
epochs of annihilation and recombination, at redshifts z_\s =10^8\div 10^4.
The direct radiation of the source is separated from the diffuse radiation it
produces. Optical distances from the source for Thomson scattering and
bremsstrahlung absorption at the maximum of the thermal background radiation
are calculated as a function of the redshift z.The distances grow sharply with
decreasing z, approaching asymptotic values, the absorption distance increasing
more slowly and reaching their limiting values at lower z. For the adopted z
values, the optical parameters of the Universe can be described in a flat model
with dusty material and radiation, and radiative transfer can be treated in a
grey approximation.Comment: 14 pages, 2 figure
Polygonal Structures in the Gaseous Disk: Numerical Simulations
The results of numerical simulations of a gaseous disk in the potential of a
stellar spiral density wave are presented. The conditions under which
straightened spiral arm segments (rows) form in the gas component are studied.
These features of the spiral structure were identified in a series of works by
A.D. Chernin with coauthors. Gas-dynamic simulations have been performed for a
wide range of model parameters: the pitch angle of the spiral pattern, the
amplitude of the stellar spiral density wave, the disk rotation speed, and the
temperature of the gas component. The results of 2D- and 3D-disk simulations
are compared. The rows in the numerical simulations are shown to be an
essentially nonstationary phenomenon. A statistical analysis of the
distribution of geometric parameters for spiral patterns with rows in the
observed galaxies and the constructed hydrodynamic models shows good agreement.
In particular, the numerical simulations and observations of galaxies give
for the average angles between straight segments.Comment: 22 pages, 10 figure
Collapse of cycloidal electron flows induced by misalignments in a magnetically insulated diode
The effect of a slight misalignment in the magnetic field on a magnetically insulated diode is investigated. It is found that a slight tilt in the magnetic field, with a minute component along the dc electric field, completely destabilizes the cycloidal electron flow in the crossed-field gap. The final state consists of the classical Brillouin flow superimposed by a turbulent background, together with a slow electron drift across the gap. This disruption of the cycloidal flow is quite insensitive to the emission current density, and is due to the accumulation of space charge in the gap caused by the magnetic misalignment. This result was obtained from a one-dimensional simulation code. It reinforces the notion that the turbulent, near Brillouin-like states are generic in ALL vacuum crossed-field devices. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69778/2/PHPAEN-5-6-2447-1.pd
The very local Hubble flow: computer simulations of dynamical history
The phenomenon of the very local ( Mpc) Hubble flow is studied on the
basis of the data of recent precision observations. A set of computer
simulations is performed to trace the trajectories of the flow galaxies back in
time to the epoch of the formation of the Local Group. It is found that the
`initial conditions' of the flow are drastically different from the linear
velocity-distance relation. The simulations enable also to recognize the major
trends of the flow evolution and identify the dynamical role of universal
antigravity produced by cosmic vacuum.Comment: LaTeX, 10 pages, 4 figures, accepted for publication in A&
Resistive destabilization of cycloidal electron flow and universality of (nearâ) Brillouin flow in a crossedâfield gap
It is shown that a small amount of dissipation, caused by current flow in a lossy external circuit, can produce a disruption of steadyâstate cycloidal electron flow in a crossedâfield gap, leading to the establishment of a turbulent steady state that is close to, but not exactly, Brillouin flow. This disruption, which has nothing to do with a diocotron or cyclotron instability, is fundamentally caused by the failure of a subset of the emitted electrons to return to the cathode surface as a result of resistive dissipation. This mechanism was revealed in particle simulations, and was confirmed by an analytic theory. These nearâBrillouin states differ in several interesting respects from classic Brillouin flow, the most important of which is the presence of a microsheath and a timeâvarying potential minimum very close to the cathode surface. They are essentially identical to that produced when (i) injected current exceeds a certain critical value [P. J. Christenson and Y. Y. Lau, Phys. Plasmas 1, 3725 (1994)] or (ii) a small rf electric field is applied to the gap [P. J. Christenson and Y. Y. Lau, Phys. Rev. Lett. 76, 3324 (1996)]. It is speculated that such nearâBrillouin states are generic in vacuum crossedâfield devices, due to the ease with which the cycloidal equilibrium can be disrupted. Another novel aspect of this paper is the introduction of transformations by which the nonlinear, coupled partial differential equations in the Eulerian description (equation of motion, continuity equation, Poisson equation, and the circuit equation) are reduced to an equivalent system of very simple linear ordinary differential equations. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71350/2/PHPAEN-3-12-4455-1.pd
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