72 research outputs found
Non-Gaussianity of the density distribution in accelerating universes
According to recent observations, the existence of the dark energy has been
considered. Even though we have obtained the constraint of the equation of the
state for dark energy () as by combining WMAP
data with other astronomical data, in order to pin down , it is necessary to
use other independent observational tools. For this purpose, we consider the
dependence of the non-Gaussianity of the density distribution generated by
nonlinear dynamics. To extract the non-Gaussianity, we follow a semi-analytic
approach based on Lagrangian linear perturbation theory, which provides an
accurate value for the quasi-nonlinear region. From our results, the difference
of the non-Gaussianity between and is about 4% while that
between and is about . For the highly non-linear
region, we estimate the difference by combining this perturbative approach with
N-body simulation executed for our previous paper. From this, we can expect the
difference to be more enhanced in the low- region, which suggests that the
non-Gaussianity of the density distribution potentially plays an important role
for extracting the information of dark energy.Comment: 15 pages, 4 figures, accepted for publication in JCAP; v2: smoothing
scale has been 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
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
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
A new approach to cosmological perturbations in f(R) models
We propose an analytic procedure that allows to determine quantitatively the
deviation in the behavior of cosmological perturbations between a given f(R)
modified gravity model and a LCDM reference model. Our method allows to study
structure formation in these models from the largest scales, of the order of
the Hubble horizon, down to scales deeply inside the Hubble radius, without
employing the so-called "quasi-static" approximation. Although we restrict our
analysis here to linear perturbations, our technique is completely general and
can be extended to any perturbative order.Comment: 21 pages, 2 figures; Revised version according to reviewer's
suggestions; Typos corrected; Added Reference
Third-order perturbative solutions in the Lagrangian perturbation theory with pressure II: Effect of the transverse modes
Lagrangian perturbation theory for cosmological fluid describes structure
formation in the quasi-nonlinear stage well. In a previous paper, we presented
a third-order perturbative equation for Lagrangian perturbation with pressure.
There we considered only the longitudinal modes for the first-order
perturbation. In this paper, we generalize the perturbation, i.e., we consider
both the longitudinal and the transverse modes for the first-order
perturbation. Then we derive third-order perturbative equations and solutions.Comment: 10 pages, no figure; accepted for publication in Phys.Rev.
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
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
De Sitter ground state of scalar-tensor gravity and its primordial perturbation
Scalar-tensor gravity is one of the most competitive gravity theory to
Einstein's relativity. We reconstruct the exact de Sitter solution in
scalar-tensor gravity, in which the non-minimal coupling scalar is rolling
along the potential. This solution may have some relation to the early
inflation and present acceleration of the universe. We investigated its
primordial quantum perturbation around the adiabatic vacuum. We put forward for
the first time that exact de Sitter generates non-exactly scale invariant
perturbations. In the conformal coupling case, this model predicts that the
tensor mode of the perturbation (gravity wave) is strongly depressed.Comment: 9 page
Second-order matter density perturbations and skewness in scalar-tensor modified gravity models
We study second-order cosmological perturbations in scalar-tensor models of
dark energy that satisfy local gravity constraints, including f(R) gravity. We
derive equations for matter fluctuations under a sub-horizon approximation and
clarify conditions under which first-order perturbations in the scalar field
can be neglected relative to second-order matter and velocity perturbations. We
also compute the skewness of the matter density distribution and find that the
difference from the LCDM model is only less than a few percent even if the
growth rate of first-order perturbations is significantly different from that
in the LCDM model. This shows that the skewness provides a model-independent
test for the picture of gravitational instability from Gaussian initial
perturbations including scalar-tensor modified gravity models.Comment: 15 pages, 1 figure, version to appear in JCA
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