85 research outputs found
Transients from initial conditions based on Lagrangian perturbation theory in N-body simulations
We explore the initial conditions for cosmological N-body simulations
suitable for calculating the skewness and kurtosis of the density field. In
general, the initial conditions based on the perturbation theory (PT) provide
incorrect second-order and higher-order growth. These errors implied by the use
of the perturbation theory to set up the initial conditions in N-body
simulations are called transients. Unless these transients are completely
suppressed compared with the dominant growing mode, we can not reproduce the
correct evolution of cumulants with orders higher than two, even though there
is no problem with the numerical scheme. We investigate the impact of
transients on the observable statistical quantities by performing -body
simulations with initial conditions based on Lagrangian perturbation theory
(LPT). We show that the effects of transients on the kurtosis from the initial
conditions, based on second-order Lagrangian perturbation theory (2LPT) have
almost disappeared by , as long as the initial conditions are set at . This means that for practical purposes, the initial conditions based on
2LPT are accurate enough for numerical calculations of skewness and kurtosis.Comment: 21 pages, 5 figures; accepted for publication in JCA
Universal Non-Gaussian Velocity Distribution in Violent Gravitational Processes
We study the velocity distribution in spherical collapses and cluster-pair
collisions by use of N-body simulations. Reflecting the violent gravitational
processes, the velocity distribution of the resultant quasi-stationary state
generally becomes non-Gaussian. Through the strong mixing of the violent
process, there appears a universal non-Gaussian velocity distribution, which is
a democratic (equal-weighted) superposition of many Gaussian distributions (DT
distribution). This is deeply related with the local virial equilibrium and the
linear mass-temperature relation which characterize the system. We show the
robustness of this distribution function against various initial conditions
which leads to the violent gravitational process. The DT distribution has a
positive correlation with the energy fluctuation of the system. On the other
hand, the coherent motion such as the radial motion in the spherical collapse
and the rotation with the angular momentum suppress the appearance of the DT
distribution.Comment: 11 pages, 19 eps figures, RevTex, submitted to PRE, Revised version,
minor change
Gravitational instability on the brane: the role of boundary conditions
An outstanding issue in braneworld theory concerns the setting up of proper
boundary conditions for the brane-bulk system. Boundary conditions (BC's)
employing regulatory branes or demanding that the bulk metric be nonsingular
have yet to be implemented in full generality. In this paper, we take a
different route and specify boundary conditions directly on the brane thereby
arriving at a local and closed system of equations (on the brane). We consider
a one-parameter family of boundary conditions involving the anisotropic stress
of the projection of the bulk Weyl tensor on the brane and derive an exact
system of equations describing scalar cosmological perturbations on a generic
braneworld with induced gravity. Depending upon our choice of boundary
conditions, perturbations on the brane either grow moderately (region of
stability) or rapidly (instability). In the instability region, the evolution
of perturbations usually depends upon the scale: small scale perturbations grow
much more rapidly than those on larger scales. This instability is caused by a
peculiar gravitational interaction between dark radiation and matter on the
brane. Generalizing the boundary conditions obtained by Koyama and Maartens, we
find for the Dvali-Gabadadze-Porrati model an instability, which leads to a
dramatic scale-dependence of the evolution of density perturbations in matter
and dark radiation. A different set of BC's, however, leads to a more moderate
and scale-independent growth of perturbations. For the mimicry braneworld,
which expands like LCDM, this class of BC's can lead to an earlier epoch of
structure formation.Comment: 35 pages, 9 figures, an appendix and references added, version to be
published in Classical and Quantum Gravit
Cosmic structures via Bose Einstein condensation and its collapse
We develop our novel model of cosmology based on the Bose-Einstein
condensation. This model unifies the Dark Energy and the Dark Matter, and
predicts multiple collapse of condensation, followed by the final acceleration
regime of cosmic expansion. We first explore the generality of this model,
especially the constraints on the boson mass and condensation conditions. We
further argue the robustness of this model over the wide range of parameters of
mass, self coupling constant and the condensation rate. Then the dynamics of
BEC collapse and the preferred scale of the collapse are studied. Finally, we
describe possible observational tests of our model, especially, the periodicity
of the collapses and the gravitational wave associated with them.Comment: 21 pages, 5 figure
Non-Gaussianity of the density distribution in accelerating universes II:N-body simulations
We explore the possibility of putting constraints on dark energy models with
statistical property of large scale structure in the non-linear region. In
particular, we investigate the dependence of non-Gaussianity of the
smoothed density distribution generated by the nonlinear dynamics. In order to
follow the non-linear evolution of the density fluctuations, we apply N-body
simulations based on scheme. We show that the relative difference
between non-Gaussianity of model and that of model is (skewness) and (kurtosis) for Mpc. We also calculate the
correspondent quantities for Mpc, (skewness) and
(kurtosis), and the difference turn out to be greater, even though
non-linearity in this scale is so strong that the complex physical processes
about galaxy formation affect the galaxy distribution. From this, we can expect
that the difference can be tested by all sky galaxy surveys with the help of
mock catalogs created by selection functions, which suggests that
non-Gaussianity of the density distribution potentially plays an important role
for extracting information on dark energy.Comment: 21 pages, 14 figure
Solving the Vlasov equation for one-dimensional models with long range interactions on a GPU
We present a GPU parallel implementation of the numeric integration of the
Vlasov equation in one spatial dimension based on a second order time-split
algorithm with a local modified cubic-spline interpolation. We apply our
approach to three different systems with long-range interactions: the
Hamiltonian Mean Field, Ring and the self-gravitating sheet models. Speedups
and accuracy for each model and different grid resolutions are presented
Large bowel obstruction due to sesame seed bezoar: a case report
peer-reviewedWe report a case of a 79 year old man with a known benign anastomotic stricture presenting with
large bowel obstruction. At laparotomy the obstruction was found to be caused by a large sesame
seed bezoar. Seed bezoars are well known to cause impaction in the rectum but have never been
previously reported to cause large bowel obstruction. We recommend that patients with known
large bowel strictures should be advised not to eat seeds as this could ultimately lead to
obstruction, ischaemia or perforationPUBLISHEDpeer-reviewe
Third-order perturbative solutions in the Lagrangian perturbation theory with pressure
Lagrangian perturbation theory for cosmological fluid describes structure
formation in the quasi-nonlinear stage well. We present a third-order
perturbative equation for Lagrangian perturbation with pressure in both the
longitudinal and transverse modes. Then we derive the perturbative solution for
simplest case.Comment: 11 pages, 1 figure; accepted for publication in Physical Review
Relativistic cosmological perturbation scheme on a general background: scalar perturbations for irrotational dust
In standard perturbation approaches and N-body simulations, inhomogeneities
are described to evolve on a predefined background cosmology, commonly taken as
the homogeneous-isotropic solutions of Einstein's field equations
(Friedmann-Lema\^itre-Robertson-Walker (FLRW) cosmologies). In order to make
physical sense, this background cosmology must provide a reasonable description
of the effective, i.e. spatially averaged, evolution of structure
inhomogeneities also in the nonlinear regime. Guided by the insights that (i)
the average over an inhomogeneous distribution of matter and geometry is in
general not given by a homogeneous solution of general relativity, and that
(ii) the class of FLRW cosmologies is not only locally but also globally
gravitationally unstable in relevant cases, we here develop a perturbation
approach that describes the evolution of inhomogeneities on a general
background being defined by the spatially averaged evolution equations. This
physical background interacts with the formation of structures. We derive and
discuss the resulting perturbation scheme for the matter model `irrotational
dust' in the Lagrangian picture, restricting our attention to scalar
perturbations.Comment: 18 pages. Matches published version in CQ
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