3,379 research outputs found
Bulk gravitons from a cosmological brane
We investigate the emission of gravitons by a cosmological brane into an Anti
de Sitter five-dimensional bulk spacetime. We focus on the distribution of
gravitons in the bulk and the associated production of `dark radiation' in this
process. In order to evaluate precisely the amount of dark radiation in the
late low-energy regime, corresponding to standard cosmology, we study
numerically the emission, propagation and bouncing off the brane of bulk
gravitons.Comment: 27 pages, 5 figures, minor corrections. Final versio
Cosmic Microwave Background Dipole induced by double inflation
The observed CMBR dipole is generally interpreted as the consequence of the
peculiar motion of the Sun with respect to the reference frame of the CMBR.
This article proposes an alternative interpretation in which the observed
dipole is the result of isocurvature perturbations on scales larger than the
present Hubble radius. These perturbations are produced in the simplest model
of double inflation, depending on three parameters. The observed dipole and
quadrupole can be explained in this model, while severely constraining its
parameters.Comment: Latex, 9 pages, no figure, to appear in Phys. Rev.
Correlated adiabatic and isocurvature perturbations from double inflation
It is shown that double inflation (two minimally coupled massive scalar
fields) can produce correlated adiabatic and isocurvature primordial
perturbations. Depending on the two relevant parameters of the model, the
contributions to the primordial perturbations are computed, with special
emphasis on the correlation, which can be quantitatively represented by a
correlation spectrum. Finally the primordial spectra are evolved numerically to
obtain the CMBR anisotropy multipole expectation values. It turns out that the
existence of mixing and correlation can alter very significantly the
temperature fluctuation predictions.Comment: 23 pages, 7 figures, RevTex. To appear in Phys. Rev.
(1+3) Covariant Dynamics of Scalar Perturbations in Braneworlds
We discuss the dynamics of linear, scalar perturbations in an almost
Friedmann-Robertson-Walker braneworld cosmology of Randall-Sundrum type II
using the 1+3 covariant approach. We derive a complete set of frame-independent
equations for the total matter variables, and a partial set of equations for
the non-local variables which arise from the projection of the Weyl tensor in
the bulk. The latter equations are incomplete since there is no propagation
equation for the non-local anisotropic stress. We supplement the equations for
the total matter variables with equations for the independent constituents in a
cold dark matter cosmology, and provide solutions in the high and low-energy
radiation-dominated phase under the assumption that the non-local anisotropic
stress vanishes. These solutions reveal the existence of new modes arising from
the two additional non-local degrees of freedom. Our solutions should prove
useful in setting up initial conditions for numerical codes aimed at exploring
the effect of braneworld corrections on the cosmic microwave background (CMB)
power spectrum. As a first step in this direction, we derive the covariant form
of the line of sight solution for the CMB temperature anisotropies in
braneworld cosmologies, and discuss possible mechanisms by which braneworld
effects may remain in the low-energy universe.Comment: 22 pages replaced with additional references and minor corrections in
Revtex4, and accepted for publication in Phys. Rev.
Influence of heavy modes on perturbations in multiple field inflation
We investigate linear cosmological perturbations in multiple field
inflationary models where some of the directions are light while others are
heavy (with respect to the Hubble parameter). By integrating out the massive
degrees of freedom, we determine the multi-dimensional effective theory for the
light degrees of freedom and give explicitly the propagation matrix that
replaces the effective sound speed of the one-dimensional case. We then examine
in detail the consequences of a sudden turn along the inflationary trajectory,
in particular the possible breakdown of the low energy effective theory in case
the heavy modes are excited. Resorting to a new basis in field space, instead
of the usual adiabatic/entropic basis, we study the evolution of the
perturbations during the turn. In particular, we compute the power spectrum and
compare with the result obtained from the low energy effective theory.Comment: 24 pages, 13 figures; v2 substantial changes in sec.V; v3 matching
the published version on JCA
Bulk inflaton shadows of vacuum gravity
We introduce a -dimensional vacuum description of five-dimensional
bulk inflaton models with exponential potentials that makes analysis of
cosmological perturbations simple and transparent. We show that various
solutions, including the power-law inflation model recently discovered by
Koyama and Takahashi, are generated from known -dimensional vacuum
solutions of pure gravity. We derive master equations for all types of
perturbations, and each of them becomes a second order differential equation
for one master variable supplemented by simple boundary conditions on the
brane. One exception is the case for massive modes of scalar perturbations. In
this case, there are two independent degrees of freedom, and in general it is
difficult to disentangle them into two separate sectors.Comment: 22 pages, 4 figures, revtex; v2: references adde
Controlling Silver Nanoparticle Size and Morphology with Photostimulated Synthesis
Photo-induced synthesis and control over the size and shape of colloidal
silver nanoparticles is investigated in contrast to photo-stimulated
aggregation of small nanoparticles into large fractal-type structures. The
feasibility of light-driven nanoengineering which enables manipulation of the
sizes and shapes of the isolated nanoparticles is studied by varying the amount
and type of the stabilizing agent and the type of optical irradiation.Comment: 10 pages, 7 figures, 11 image
Bulk Gravitational Field and Cosmological Perturbations on the Brane
We investigate the effect of the bulk gravitational field on the cosmological
perturbations on a brane embedded in the 5D Anti-de Sitter (AdS) spacetime. The
effective 4D Einstein equations for the scalar cosmological perturbations on
the brane are obtained by solving the perturbations in the bulk. Then the
behaviour of the corrections induced by the bulk gravitational field to the
conventional 4D Einstein equation are determined. Two types of the corrections
are found. First we investigate the corrections which become significant at
scales below the AdS curvature scales and in the high energy universe with the
energy density larger than the tension of the brane. The evolution equation for
the perturbations on the brane is found and solved. Another type of the
corrections is induced on the brane if we consider the bulk perturbations which
do not contribute to the metric perturbations but do contribute to the matter
perturbations. At low energies, they have imaginary mass m^2=-(2/3) \k^2 in
the bulk where \k is the 3D comoving wave number of the perturbations. They
diverge at the horizon of the AdS spacetime. The induced density perturbations
behave as sound waves with sound velocity in the low energy
universe. At large scales, they are homogeneous perturbations that depend only
on time and decay like radiation. They can be identified as the perturbations
of the dark radiation. They produce isocurvature perturbations in the matter
dominated era. Their effects can be observed as the shifts of the location and
the height of the acoustic peak in the CMB spectrum.Comment: 35 pages, 1 figur
Non-Gaussian isocurvature perturbations in dark radiation
We study non-Gaussian properties of the isocurvature perturbations in the
dark radiation, which consists of the active neutrinos and extra light species,
if exist. We first derive expressions for the bispectra of primordial
perturbations which are mixtures of curvature and dark radiation isocurvature
perturbations. We also discuss CMB bispectra produced in our model and forecast
CMB constraints on the nonlinearity parameters based on the Fisher matrix
analysis. Some concrete particle physics motivated models are presented in
which large isocurvature perturbations in extra light species and/or the
neutrino density isocurvature perturbations as well as their non-Gaussianities
may be generated. Thus detections of non-Gaussianity in the dark radiation
isocurvature perturbation will give us an opportunity to identify the origin of
extra light species and lepton asymmetry.Comment: 32 pages, 7 figure
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