30 research outputs found
The multi-stream flows and the dynamics of the cosmic web
A new numerical technique to identify the cosmic web is proposed. It is based
on locating multi-stream flows, i.e. the places where the velocity field is
multi-valued. The method is local in Eulerian space, simple and computaionally
efficient. This technique uses the velocities of particles and thus takes into
account the dynamical information. This is in contrast with the majority of
standard methods that use the coordinates of particles only. Two quantities are
computed in every mesh cell: the mean and variance of the velocity field. In
the cells where the velocity is single-valued the variance must be equal to
zero exactly, therefore the cells with non-zero variance are identified as
multi-stream flows. The technique has been tested in a N-body simulation of the
\L CDM model. The preliminary analysis has shown that numerical noise does not
pose a significant problem. The web identified by the new method has been
compared with the web identified by the standard technique using only the
particle coordinates. The comparison has shown overall similarity of two webs
as expected, however they by no means are identical. For example, the
isocontours of the corresponding fields have significantly different shapes and
some density peaks of similar heights exhibit significant differences in the
velocity variance and vice versa. This suggest that the density and velocity
variance have a significant degree of independence. The shape of the
two-dimensional pdf of density and velocity variance confirms this proposition.
Thus, we conclude that the dynamical information probed by this technique
introduces an additional dimension into analysis of the web.Comment: 19 pages, 10 figure
Reheating and turbulence
We show that the ''turbulent'' particle spectra found in numerical
simulations of the behavior of matter fields during reheating admit a simple
interpretation in terms of hydrodynamic models of the reheating period. We
predict a particle number spectrum with for Comment: 10 pages, one figure included in tex
Statistical nature of non-Gaussianity from cubic order primordial perturbations: CMB map simulations and genus statistic
We simulate CMB maps including non-Gaussianity arising from cubic order
perturbations of the primordial gravitational potential, characterized by the
non-linearity parameter . The maps are used to study the characteristic
nature of the resulting non-Gaussian temperature fluctuations. We measure the
genus and investigate how it deviates from Gaussian shape as a function of
and smoothing scale. We find that the deviation of the non-Gaussian
genus curve from the Gaussian one has an antisymmetric, sine function like
shape, implying more hot and more cold spots for and less of both
for . The deviation increases linearly with and also
exhibits mild increase as the smoothing scale increases. We further study other
statistics derived from the genus, namely, the number of hot spots, the number
of cold spots, combined number of hot and cold spots and the slope of the genus
curve at mean temperature fluctuation. We find that these observables carry
signatures of that are clearly distinct from the quadratic order
perturbations, encoded in the parameter . Hence they can be very useful
tools for distinguishing not only between non-Gaussian temperature fluctuations
and Gaussian ones but also between and type
non-Gaussianities.Comment: 18+1 page
The clustering instability of inertial particles spatial distribution in turbulent flows
A theory of clustering of inertial particles advected by a turbulent velocity
field caused by an instability of their spatial distribution is suggested. The
reason for the clustering instability is a combined effect of the particles
inertia and a finite correlation time of the velocity field. The crucial
parameter for the clustering instability is a size of the particles. The
critical size is estimated for a strong clustering (with a finite fraction of
particles in clusters) associated with the growth of the mean absolute value of
the particles number density and for a weak clustering associated with the
growth of the second and higher moments. A new concept of compressibility of
the turbulent diffusion tensor caused by a finite correlation time of an
incompressible velocity field is introduced. In this model of the velocity
field, the field of Lagrangian trajectories is not divergence-free. A mechanism
of saturation of the clustering instability associated with the particles
collisions in the clusters is suggested. Applications of the analyzed effects
to the dynamics of droplets in the turbulent atmosphere are discussed. An
estimated nonlinear level of the saturation of the droplets number density in
clouds exceeds by the orders of magnitude their mean number density. The
critical size of cloud droplets required for clusters formation is more than
m.Comment: REVTeX 4, 15 pages, 2 figures(included), PRE submitte
Renormalization group and nonequilibrium action in stochastic field theory
We investigate the renormalization group approach to nonequilibrium field
theory. We show that it is possible to derive nontrivial renormalization group
flow from iterative coarse graining of a closed-time-path action. This
renormalization group is different from the usual in quantum field theory
textbooks, in that it describes nontrivial noise and dissipation. We work out a
specific example where the variation of the closed-time-path action leads to
the so-called Kardar-Parisi-Zhang equation, and show that the renormalization
group obtained by coarse graining this action, agrees with the dynamical
renormalization group derived by directly coarse graining the equations of
motion.Comment: 33 pages, 3 figures included in the text. Revised; one reference
adde
On Dark Matter Annihilation in the Local Group
Under the hypothesis of a Dark Matter composed by supersymmetric particles
like neutralinos, we investigate the possibility that their annihilation in the
haloes of nearby galaxies could produce detectable fluxes of -photons.
Expected fluxes depend on several, poorly known quantities such as the density
profiles of Dark Matter haloes, the existence and prominence of central density
cusps and the presence of a population of sub-haloes. We find that, for all
reasonable choices of Dark Matter halo models, the intensity of the
-ray flux from some of the nearest extragalactic objects, like M31, is
comparable or higher than the diffuse Galactic foreground. We show that next
generation ground-based experiments could have the sensitivity to reveal such
fluxes which could help us unveiling the nature of Dark Matter particles.Comment: 11 pages, 10 figures. Accepted for publication in Phys. Rev. D.;
added a new paragraph on the detectability of Galactic sub-halos in our
Galaxy; added a discussion on their model dependence. The relation of our
results with the "CDM crisis" issue has also been adde
Some remarks on the angular momenta of galaxies, their clusters and superclusters
We discuss the relation between angular momenta and masses of galaxy
structures base on the Li model of the universe with global rotation. In our
previous paper (God{\l}owski et al 2002) it was shown that the model predicts
the presence of a minimum in this relation. In the present paper we discuss
observational evidence allowing us to verify this relation. We find null
angular momentum J=0 for the masses corresponding to mass of galaxy grups and
non-vanishing angular momenta for other galactic structures. We check these
theoretical predictions analysing Tully's galaxy grups. The existing data
comparing alignment in different galactic structure are consistent with
obtained theoretical relation if we interpret the groving alignment as
the galactic increasing angular momenta in the galactic structure.Comment: 20 pages 1 figure. GRG accepte