189 research outputs found
Liquid-liquid coexistence in the phase diagram of a fluid confined in fractal porous materials
Multicanonical ensemble sampling simulations have been performed to calculate
the phase diagram of a Lennard-Jones fluid embedded in a fractal random matrix
generated through diffusion limited cluster aggregation. The study of the
system at increasing size and constant porosity shows that the results are
independent from the matrix realization but not from the size effects. A
gas-liquid transition shifted with respect to bulk is found. On growing the
size of the system on the high density side of the gas-liquid coexistence curve
it appears a second coexistence region between two liquid phases. These two
phases are characterized by a different behaviour of the local density inside
the interconnected porous structure at the same temperature and chemical
potential.Comment: 5 pages, 4 figures. To be published in Europhys. Letter
Lattice-gas Monte Carlo study of adsorption in pores
A lattice gas model of adsorption inside cylindrical pores is evaluated with
Monte Carlo simulations. The model incorporates two kinds of site: (a line of)
``axial'' sites and surrounding ``cylindrical shell'' sites, in ratio 1:7. The
adsorption isotherms are calculated in either the grand canonical or canonical
ensembles. At low temperature, there occur quasi-transitions that would be
genuine thermodynamic transitions in mean-field theory. Comparison between the
exact and mean-field theory results for the heat capacity and adsorption
isotherms are provided
A Mass Matrix for Atmospheric, Solar, and LSND Neutrino Oscillations
We construct a mass matrix for the four neutrino flavors, three active and
one sterile, needed to fit oscillations in all three neutrino experiments:
atmospheric, solar, and LSND, simultaneously. It organizes the neutrinos into
two doublets whose central values are about 1 eV apart, and whose splittings
are of the order of 10^(-3) eV. Atmospheric neutrino oscillations are described
as maximal mixing within the upper doublet, and solar as the same within the
lower doublet. Then LSND is a weak transition from one doublet to the other. We
comment on the Majorana versus Dirac nature of the active neutrinos and show
that our mass matrix can be derived from an S_2 x S_2 permutation symmetry plus
an equal splitting rule.Comment: 4 pages, 0 figures, minor text change
The Local Group as a test of cosmological models
The dynamics of the Local Group and its environment provide a unique
challenge to cosmological models. The velocity field within 5h-1 Mpc of the
Local Group (LG) is extremely ``cold''. The deviation from a pure Hubble flow,
characterized by the observed radial peculiar velocity dispersion, is measured
to be about 60km/s. We compare the local velocity field with similarly defined
regions extracted from N-body simulations of Universes dominated by cold dark
matter (CDM). This test is able to strongly discriminate between models that
have different mean mass densities. We find that neither the Omega=1 (SCDM) nor
Omega=0.3 (OCDM) cold dark matter models can produce a single candidate Local
Group that is embedded in a region with such small peculiar velocities. For
these models, we measure velocity dispersions between 500-700km/s and
150-300km/s respectively, more than twice the observed value. Although both CDM
models fail to produce environments similar to those of our Local Group on a
scale of a few Mpc, they can give rise to many binary systems that have similar
orbital properties as the Milky Way--Andromeda system. The local,
gravitationally induced bias of halos in the CDM ``Local Group'' environment,
if defined within a sphere of 10 Mpc around each Local Group is about 1.5,
independent of Omega. No biasing scheme could reconcile the measured velocity
dispersions around Local Groups with the observed one. Identification of binary
systems using a halo finder (named Skid
(http://www-hpcc.astro.washington.edu/tools/DENMAX for a public version)) based
on local density maxima instead of a simple linking algorithm, gives a much
more complete sample. We show that a standard ``friend of friends'' algorithm
would miss 40% of the LG candidates present in the simulations.Comment: Latex file (19 pages) + 13 figures. Submitted to New Astronomy. Two
MPEG movies were not included. Also available (this time with the movies) at
http://www-hpcc.astro.washington.edu/faculty/fabio/index.htm
A New Statistic for Redshift Surveys: the Redshift Dispersion of Galaxies
We present a new statistic-the redshift dispersion-- which may prove useful
for comparing next generation redshift surveys (e.g., the Sloan Digital Sky
Survey) and cosmological simulations. Our statistic is specifically designed
for the projection of phase space which is directly measured by redshift
surveys. We find that the redshift dispersion of galaxies as a function of the
projected overdensity has a strong dependence on the cosmological density
parameter Omega. The redshift dispersion statistic is easy to compute and can
be motivated by applying the Cosmic Virial Theorem to subsets of galaxies with
the same local density. We show that the velocity dispersion of particles in
these subsets is proportional to the product of Omega and the local density.
Low resolution N-body simulations of several cosmological models (open/closed
CDM, CDM+Lambda, HDM) indicate that the proportionality between velocity
dispersion, local density and Omega holds over redshift scales in the range 50
km/s to 500 km/s. The redshift dispersion may provide an interesting means for
comparing volume-limited subsamples of the Sloan Digital Sky Survey to
equivalent N-body/hydrodynamics simulations.Comment: LaTex 27 pages, 8 Postscript figures, added one figure and some
changes to the tex
Wetting of a symmetrical binary fluid mixture on a wall
We study the wetting behaviour of a symmetrical binary fluid below the
demixing temperature at a non-selective attractive wall. Although it demixes in
the bulk, a sufficiently thin liquid film remains mixed. On approaching
liquid/vapour coexistence, however, the thickness of the liquid film increases
and it may demix and then wet the substrate. We show that the wetting
properties are determined by an interplay of the two length scales related to
the density and the composition fluctuations. The problem is analysed within
the framework of a generic two component Ginzburg-Landau functional
(appropriate for systems with short-ranged interactions). This functional is
minimized both numerically and analytically within a piecewise parabolic
potential approximation. A number of novel surface transitions are found,
including first order demixing and prewetting, continuous demixing, a
tricritical point connecting the two regimes, or a critical end point beyond
which the prewetting line separates a strongly and a weakly demixed film. Our
results are supported by detailed Monte Carlo simulations of a symmetrical
binary Lennard-Jones fluid at an attractive wall.Comment: submitted to Phys. Rev.
Adsorption hysteresis and capillary condensation in disordered porous solids: a density functional study
We present a theoretical study of capillary condensation of fluids adsorbed
in mesoporous disordered media. Combining mean-field density functional theory
with a coarse-grained description in terms of a lattice-gas model allows us to
investigate both the out-of-equilibrium (hysteresis) and the equilibrium
behavior. We show that the main features of capillary condensation in
disordered solids result from the appearance of a complex free-energy landscape
with a large number of metastable states. We detail the numerical procedures
for finding these states, and the presence or absence of transitions in the
thermodynamic limit is determined by careful finite-size studies.Comment: 30 pages, 18 figures. To appear in J. Phys.: Condens. Matte
Lattice model of gas condensation within nanopores
We explore the thermodynamic behavior of gases adsorbed within a nanopore.
The theoretical description employs a simple lattice gas model, with two
species of site, expected to describe various regimes of adsorption and
condensation behavior. The model includes four hypothetical phases: a
cylindrical shell phase (S), in which the sites close to the cylindrical wall
are occupied, an axial phase (A), in which sites along the cylinder's axis are
occupied, a full phase (F), in which all sites are occupied, and an empty phase
(E). We obtain exact results at T=0 for the phase behavior, which is a function
of the interactions present in any specific problem. We obtain the
corresponding results at finite T from mean field theory. Finally, we examine
the model's predicted phase behavior of some real gases adsorbed in nanopores
The density and peculiar velocity fields of nearby galaxies
We review the quantitative science that can be and has been done with
redshift and peculiar velocity surveys of galaxies in the nearby universe.
After a brief background setting the cosmological context for this work, the
first part of this review focuses on redshift surveys. The practical issues of
how redshift surveys are carried out, and how one turns a distribution of
galaxies into a smoothed density field, are discussed. Then follows a
description of major redshift surveys that have been done, and the local
cosmography out to 8,000 km/s that they have mapped. We then discuss in some
detail the various quantitative cosmological tests that can be carried out with
redshift data. The second half of this review concentrates on peculiar velocity
studies, beginning with a thorough review of existing techniques. After
discussing the various biases which plague peculiar velocity work, we survey
quantitative analyses done with peculiar velocity surveys alone, and finally
with the combination of data from both redshift and peculiar velocity surveys.
The data presented rule out the standard Cold Dark Matter model, although
several variants of Cold Dark Matter with more power on large scales fare
better. All the data are consistent with the hypothesis that the initial
density field had a Gaussian distribution, although one cannot rule out broad
classes of non-Gaussian models. Comparison of the peculiar velocity and density
fields constrains the Cosmological Density Parameter. The results here are
consistent with a flat universe with mild biasing of the galaxies relative to
dark matter, although open universe models are by no means ruled out.Comment: In press, Physics Reports. 153 pages. gzip'ed postscript of text plus
20 embedded figures. Also available via anonymous ftp at
ftp://eku.ias.edu/pub/strauss/review/physrep.p
Reconstructing the Inflaton Potential---in Principle and in Practice
Generalizing the original work by Hodges and Blumenthal, we outline a
formalism which allows one, in principle, to reconstruct the potential of the
inflaton field from knowledge of the tensor gravitational wave spectrum or the
scalar density fluctuation spectrum, with special emphasis on the importance of
the tensor spectrum. We provide some illustrative examples of such
reconstruction. We then discuss in some detail the question of whether one can
use real observations to carry out this procedure. We conclude that in
practice, a full reconstruction of the functional form of the potential will
not be possible within the foreseeable future. However, with a knowledge of the
dark matter components, it should soon be possible to combine
intermediate-scale data with measurements of large-scale cosmic microwave
background anisotropies to yield useful information regarding the potential.Comment: 39 pages plus 2 figures (upon request:[email protected]), LaTeX,
FNAL--PUB--93/029-A; SUSSEX-AST 93/3-
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