223 research outputs found
Velocity dispersion in N-body simulations of CDM models
This work reports on a study of the spatially coarse-grained velocity
dispersion in cosmological N-body simulations (OCDM and Lambda-CDM models) as a
function of time (redshifts z=0-4) and of the coarsening length (0.6-20 Mpc/h).
The main result is the discovery of a polytropic relationship I_1 ~ rho^{2-eta}
between the velocity-dispersion kinetic energy density of the coarsening cells,
I_1, and their mass density, rho. The exponent eta, dependent on time and
coarsening scale, is a compact measure of the deviations from the naive virial
prediction eta_virial=0. This relationship supports the ``polytropic
assumption'' which has been employed in theoretical models for the growth of
cosmological structure by gravitational instability.Comment: Minor, unimportant changes. Matches published versio
Comment on ''On the problem of initial conditions in cosmological N-body simulations'' (Europhys. Lett. 57, 322)
In astro-ph/0109199, the initial conditions (IC's) of cosmological N-body
simulations by the Virgo Consortium are analyzed and it is concluded that the
density fluctuations are rather different from the desired ones. We have
repeated the analysis of the IC's using our own code and the code provided by
the authors of astro-ph/0109199, obtaining results that disprove the
criticisms.Comment: 2 pages, 4 eps figures, epl.cls style fil
Influence of the cosmological expansion on small systems
The effect of the large-scale cosmological expansion on small systems is
studied in the light of modern cosmological models of large-scale structure. We
identify certain assumptions of earlier works which render them unrealistic
regarding these cosmological models. The question is reanalyzed by dropping
these assumptions to conclude that a given small system can experience either
an expansion or a contraction of cosmological origin.Comment: 7 pages. Accepted for publication in Europhysics Letter
Dynamics of colloidal particles with capillary interactions
We investigate the dynamics of colloids at a fluid interface driven by
attractive capillary interactions. At submillimeter length scales, the
capillary attraction is formally analogous to two-dimensional gravity. In
particular it is a non-integrable interaction and it can be actually relevant
for collective phenomena in spite of its weakness at the level of the pair
potential. We introduce a mean-field model for the dynamical evolution of the
particle number density at the interface. For generic values of the physical
parameters the homogeneous distribution is found to be unstable against
large-scale clustering driven by the capillary attraction. We also show that
for the instability to be observable, the appropriate values for the relevant
parameters (colloid radius, surface charge, external electric field, etc.) are
experimentally well accessible. Our analysis contributes to current studies of
the structure and dynamics of systems governed by long-ranged interactions and
points towards their experimental realizations via colloidal suspensions.Comment: Matches version accepted for publication. New refs. added, misprints
corrected in figs.6,8,9,1
Study of corrections to the dust model via perturbation theory
This work reports on the application of the Eulerian perturbation theory to a
recently proposed model of cosmological structure formation by gravitational
instability (astro-ph/0009414). Its physical meaning is discussed in detail and
put in perspective of previous works. The model incorporates in a systematic
fashion corrections to the popular dust model due to multistreaming and, more
generally, the small-scale, virialized degrees of freedom. It features a
time-dependent length scale L(t) estimated to be L/r0 ~ 0.1 (r0(t) is the
nonlinear scale, at which the density variance =1). The model provides a new
angle on the dust model and allows to overcome some of its limitations. Thus,
the scale L(t) works as a physically meaningful short-distance cutoff for the
divergences appearing in the perturbation expansion of the dust model when
there is too much initial power on small scales. The model also incorporates
the generation of vorticity by tidal forces; according to the perturbational
result, the filtered vorticity for standard CDM initial conditions should be
significant today only at scales below ~ 1 Mpc/h.Comment: 9 pages, coincides with the published versio
Extending the scope of models for large-scale structure formation in the Universe
We propose a phenomenological generalization of the models of large-scale
structure formation in the Universe by gravitational instability in two ways:
we include pressure forces to model multi-streaming, and noise to model
fluctuations due to neglected short-scale physical processes. We show that
pressure gives rise to a viscous-like force of the same character as that one
introduced in the ``adhesion model'', while noise leads to a roughening of the
density field yielding a scaling behavior of its correlations.Comment: matches published version in A&A, incl. 3 figure
- …