1,885 research outputs found
Particle linear theory on a self-gravitating perturbed cubic Bravais lattice
Discreteness effects are a source of uncontrolled systematic errors of N-body
simulations, which are used to compute the evolution of a self-gravitating
fluid. We have already developed the so-called "Particle Linear Theory" (PLT),
which describes the evolution of the position of self-gravitating particles
located on a perturbed simple cubic lattice. It is the discrete analogue of the
well-known (Lagrangian) linear theory of a self-gravitating fluid. Comparing
both theories permits to quantify precisely discreteness effects in the linear
regime. It is useful to develop the PLT also for other perturbed lattices
because they represent different discretizations of the same continuous system.
In this paper we detail how to implement the PLT for perturbed cubic Bravais
lattices (simple, body and face-centered) in a cubic simulation box. As an
application, we will study the discreteness effects -- in the linear regime --
of N-body simulations for which initial conditions have been set-up using these
different lattices.Comment: 9 pages, 4 figures and 4 tables. Minor corrections to match published
versio
The Measure of Cosmological Parameters
New, large, ground and space telescopes are contributing to an exciting and
rapid period of growth in observational cosmology. The subject is now far from
its earlier days of being data-starved and unconstrained, and new data are
fueling a healthy interplay between observations and experiment and theory. I
briefly review here the status of measurements of a number of quantities of
interest in cosmology: the Hubble constant, the total mass-energy density, the
matter density, the cosmological constant or dark energy component, and the
total optical background light.Comment: 12 pages, 4 figures, to be published in "2001: A Spacetime Odyssey:
Proceedings of the Inaugural Conference of the Michigan Center for
Theoretical Physics", Michael J. Duff & James T. Liu, eds., (World
Scientific, Singapore), in pres
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
New Constraints on the Variable Equation of State Parameter from X-Ray Gas Mass Fractions and SNe Ia
Recent measurements are suggesting that we live in a flat Universe and that
its present accelerating stage is driven by a dark energy component whose
equation of state may evolve in time. Assuming two different parameterizations
for the function , we constrain their free parameters from a joint
analysis involving measurements from X-Ray luminosity of galaxy clusters and
SNe type Ia data.Comment: paper, 6 pages, 1 figure Accepted by Int. Journal of Modern Physics D
(IJPMD
Primordial fractal density perturbations and structure formation in the Universe: 1-Dimensional collisionless sheet model
Two-point correlation function of galaxy distribution shows that the
structure in the present Universe is scale-free up to a certain scale (at least
several tens Mpc), which suggests that a fractal structure may exist. If small
primordial density fluctuations have a fractal structure, the present
fractal-like nonlinear structure below the horizon scale could be naturally
explained. We analyze the time evolution of fractal density perturbations in
Einstein-de Sitter universe, and study how the perturbation evolves and what
kind of nonlinear structure will come out. We assume a one-dimensional
collisionless sheet model with initial Cantor-type fractal perturbations. The
nonlinear structure seems to approach some attractor with a unique fractal
dimension, which is independent of the fractal dimensions of initial
perturbations. A discrete self-similarity in the phase space is also found when
the universal nonlinear fractal structure is reached.Comment: 17 pages, 19 jpeg figures. Accepted for publication in ApJ. Figures
are also available from
http://www.phys.waseda.ac.jp/gravity/~tatekawa/0003124/figs.tar.g
Dark Matter and Dark Radiation
We explore the feasibility and astrophysical consequences of a new long-range
U(1) gauge field ("dark electromagnetism") that couples only to dark matter,
not to the Standard Model. The dark matter consists of an equal number of
positive and negative charges under the new force, but annihilations are
suppressed if the dark matter mass is sufficiently high and the dark
fine-structure constant is sufficiently small. The correct relic
abundance can be obtained if the dark matter also couples to the conventional
weak interactions, and we verify that this is consistent with particle-physics
constraints. The primary limit on comes from the demand that the
dark matter be effectively collisionless in galactic dynamics, which implies
for TeV-scale dark matter. These values are
easily compatible with constraints from structure formation and primordial
nucleosynthesis. We raise the prospect of interesting new plasma effects in
dark matter dynamics, which remain to be explored.Comment: 14 pages, 6 figures Updated equations and figure
Nonlinear Velocity-Density Coupling: Analysis by Second-Order Perturbation Theory
Cosmological linear perturbation theory predicts that the peculiar velocity
and the matter overdensity at a same point are
statistically independent quantities, as log as the initial density
fluctuations are random Gaussian distributed. However nonlinear gravitational
effects might change the situation. Using framework of second-order
perturbation theory and the Edgeworth expansion method, we study local density
dependence of bulk velocity dispersion that is coarse-grained at a weakly
nonlinear scale. For a typical CDM model, the first nonlinear correction of
this constrained bulk velocity dispersion amounts to (Gaussian
smoothing) at a weakly nonlinear scale with a very weak dependence on
cosmological parameters. We also compare our analytical prediction with
published numerical results given at nonlinear regimes.Comment: 16 pages including 2 figures, ApJ 537 in press (July 1
Cavity evolution in relativistic self-gravitating fluids
We consider the evolution of cavities within spherically symmetric
relativistic fluids, under the assumption that proper radial distance between
neighboring fluid elements remains constant during their evolution (purely
areal evolution condition). The general formalism is deployed and solutions are
presented. Some of them satisfy Darmois conditions whereas others present
shells and must satisfy Israel conditions, on either one or both boundary
surfaces. Prospective applications of these results to some astrophysical
scenarios is suggested.Comment: 10 pages Revtex. To appear in Class. Quantum Grav
Effect of Peculiar Motion in Weak Lensing
We study the effect of peculiar motion in weak gravitational lensing. We
derive a fully relativistic formula for the cosmic shear and the convergence in
a perturbed Friedmann Universe. We find a new contribution related to galaxies
peculiar velocity. This contribution does not affect cosmic shear in a
measurable way, since it is of second order in the velocity. However, its
effect on the convergence (and consequently on the magnification, which is a
measurable quantity) is important, especially for redshifts z < 1. As a
consequence, peculiar motion modifies also the relation between the shear and
the convergence.Comment: 11 pages, 7 figures; v2: discussion on the reduced shear added (5.C),
additional references, version accepted in PRD; v3: mistakes corrected in
eqs. (26), (31), (33) and (44); results unchange
Neutrino Mass and Dark Energy from Weak Lensing
Weak gravitational lensing of background galaxies by intervening matter
directly probes the mass distribution in the universe. This distribution, and
its evolution at late times, is sensitive to both the dark energy, a negative
pressure energy density component, and neutrino mass. We examine the potential
of lensing experiments to measure features of both simultaneously. Focusing on
the radial information contained in a future deep 4000 square degree survey, we
find that the expected (1-sigma) error on a neutrino mass is 0.1 eV, if the
dark energy parameters are allowed to vary. The constraints on dark energy
parameters are similarly restrictive, with errors on w of 0.09. Much of the
restrictive power on the dark energy comes not from the evolution of the
gravitational potential but rather from how distances vary as a function of
redshift in different cosmologies
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