530 research outputs found
Dark energy and key physical parameters of clusters of galaxies
We study physics of clusters of galaxies embedded in the cosmic dark energy
background. Under the assumption that dark energy is described by the
cosmological constant, we show that the dynamical effects of dark energy are
strong in clusters like the Virgo cluster. Specifically, the key physical
parameters of the dark mater halos in clusters are determined by dark energy:
1) the halo cut-off radius is practically, if not exactly, equal to the
zero-gravity radius at which the dark matter gravity is balanced by the dark
energy antigravity; 2) the halo averaged density is equal to two densities of
dark energy; 3) the halo edge (cut-off) density is the dark energy density with
a numerical factor of the unity order slightly depending on the halo profile.
The cluster gravitational potential well in which the particles of the dark
halo (as well as galaxies and intracluster plasma) move is strongly affected by
dark energy: the maximum of the potential is located at the zero-gravity radius
of the cluster.Comment: 8 pages, 1 figur
Zeldovich flow on cosmic vacuum background: new exact nonlinear analytical solution
A new exact nonlinear Newtonian solution for a plane matter flow superimposed
on the isotropic Hubble expansion is reported. The dynamical effect of cosmic
vacuum is taken into account. The solution describes the evolution of nonlinear
perturbations via gravitational instability of matter and the termination of
the perturbation growth by anti-gravity of vacuum at the epoch of transition
from matter domination to vacuum domination. On this basis, an `approximate' 3D
solution is suggested as an analog of the Zeldovich ansatz.Comment: 9 pages, 1 figure
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
Energy composition of the Universe: time-independent internal symmetry
The energy composition of the Universe, as emerged from the Type Ia supernova
observations and the WMAP data, looks preposterously complex, -- but only at
the first glance. In fact, its structure proves to be simple and regular. An
analysis in terms of the Friedmann integral enables to recognize a remarkably
simple time-independent covariant robust recipe of the cosmic mix: the
numerical values of the Friedmann integral for vacuum, dark matter, baryons and
radiation are approximately identical. The identity may be treated as a
symmetry relation that unifies cosmic energies into a regular set, a quartet,
with the Friedmann integral as its common genuine time-independent physical
parameter. Such cosmic internal (non-geometrical) symmetry exists whenever
cosmic energies themselves exist in nature. It is most natural for a finite
Universe suggested by the WMAP data. A link to fundamental theory may be found
under the assumption about a special significance of the electroweak energy
scale in both particle physics and cosmology. A freeze-out model developed on
this basis demonstrates that the physical nature of new symmetry might be due
to the interplay between electroweak physics and gravity at the cosmic age of a
few picoseconds. The big `hierarchy number' of particle physics represents the
interplay in the model. This number quantifies the Friedmann integral and gives
also a measure to some other basic cosmological figures and phenomena
associated with new symmetry. In this way, cosmic internal symmetry provides a
common ground for better understanding of old and recent problems that
otherwise seem unrelated; the coincidence of the observed cosmic densities, the
flatness of the co-moving space, the initial perturbations and their amplitude,
the cosmic entropy are among them.Comment: 32 page
Effect Of Energy Spread On The Single Bunch Dipole Beam breakup instability In A High-energy Rf Linac
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
On Friedmann integrals coincidence
Expansion of the two-component universe with arbitrary spatial curvature has
been considered. It has been shown that the Friedmann integrals of the almost
flat universe do not coincide.Comment: 2 pages, LaTeX2e, 1 eps figure, to appear in Astronomy Letter
Two-fluid matter-quintessence FLRW models: energy transfer and the equation of state of the universe
Recent observations support the view that the universe is described by a FLRW
model with , , and at the present epoch. There are several theoretical suggestions for
the cosmological component and for the particular form of the energy
transfer between this dark energy and matter. This gives a strong motive for a
systematic study of general properties of two-fluid FLRW models. We consider a
combination of one perfect fluid, which is quintessence with negative pressure
(), and another perfect fluid, which is a mixture of
radiation and/or matter components with positive pressure (), which define the associated one-fluid model (). We introduce a useful classification which contains 4 classes of
models defined by the presence or absence of energy transfer and by the
stationarity ( and ) or/and non stationarity (
or time dependent) of the equations of state. It is shown that, for
given and , the energy transfer defines and, therefore, the
total gravitating mass and dynamics of the model. We study important examples
of two-fluid FLRW models within the new classification. The behaviour of the
energy content, gravitating mass, pressure, and the energy transfer are given
as functions of the scale factor. We point out three characteristic scales,
, and , which separate periods of time in which
quintessence energy, pressure and gravitating mass dominate. Each sequence of
the scales defines one of 6 evolution types
A graph of dark energy significance on different spatial and mass scales
The current cosmological paradigm sees the formation and evolution of the
cosmic large-scale structure as governed by the gravitational attraction of the
Dark Matter (DM) and the repulsion of the Dark Energy (DE).
We characterize the relative importance of uniform and constant dark energy,
as given by the Lambda term in the standard LCDM cosmology, in galaxy systems
of different scales, from groups to superclusters.
An instructive "Lambda significance graph" is introduced where the matter-DE
density ratio /rho_Lambda for different galaxy systems is plotted
against the radius R. This presents gravitation and DE dominated regions and
shows directly the zero velocity radius, the zero-gravity radius, and the
Einstein-Straus radius for any fixed value of mass.
Example galaxy groups and clusters from the local universe illustrate the use
of the Lambda significance graph. These are generally located deep in the
gravity-dominated region /rho_Lambda > 2, being virialized. Extended
clusters and main bodies of superclusters can reach down near the border line
between gravity-dominated and DE dominated regions /rho_Lambda = 2. The
scale--mass relation from the standard 2-point correlation function intersects
this balance line near the correlation lenght.
The log /rho_Lambda vs. log R diagram is a useful and versatile way to
characterize the dynamical state of systems of galaxies within the Lambda
dominated expanding universe.Comment: 4 pages, 2 figure
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