1,357 research outputs found
Thermal distortion analysis of a deployable parabolic reflector
A thermal distortion analysis of the ATS-6 Satellite parabolic reflector was performed using NASTRAN level 15.1. The same NASTRAN finite element method was used to conduct a one g static load analysis and a dynamic analysis of the reflector. In addition, a parametric study was made to determine which parameters had the greatest effect on the thermal distortions. The method used to model the construction of the reflector is described and the results of the analyses are presented
Density perturbations in the brane-world
In Randall-Sundrum-type brane-world cosmologies, density perturbations
generate Weyl curvature in the bulk, which in turn backreacts on the brane via
stress-energy perturbations. On large scales, the perturbation equations
contain a closed system on the brane, which may be solved without solving for
the bulk perturbations. Bulk effects produce a non-adiabatic mode, even when
the matter perturbations are adiabatic, and alter the background dynamics. As a
consequence, the standard evolution of large-scale fluctuations in general
relativity is modified. The metric perturbation on large-scales is not constant
during high-energy inflation. It is constant during the radiation era, except
at most during the very beginning, if the energy is high enough.Comment: Additional arguments and minor corrections; version accepted by 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.
Large-scale cosmological perturbations on the brane
In brane-world cosmologies of Randall-Sundrum type, we show that evolution of
large-scale curvature perturbations may be determined on the brane, without
solving the bulk perturbation equations. The influence of the bulk
gravitational field on the brane is felt through a projected Weyl tensor which
behaves effectively like an imperfect radiation fluid with anisotropic stress.
We define curvature perturbations on uniform density surfaces for both the
matter and Weyl fluids, and show that their evolution on large scales follows
directly from the energy conservation equations for each fluid. The total
curvature perturbation is not necessarily constant for adiabatic matter
perturbations, but can change due to the Weyl entropy perturbation. To relate
this curvature perturbation to the longitudinal gauge metric potentials
requires knowledge of the Weyl anisotropic stress which is not determined by
the equations on the brane. We discuss the implications for large-angle
anisotropies on the cosmic microwave background sky.Comment: 13 pages, latex with revtex, no figure
Boundary Inflation and the WMAP Data
Inflation in a five-dimensional brane world model with two boundary branes is
studied. We make use of the moduli space approximation whereby the low energy
theory reduces to a four-dimensional biscalar-tensor gravity plus a minimally
coupled scalar field. After a detailed analysis of the inflationary solutions,
we derive the evolution equations of the linear perturbations separating the
adiabatic mode from two entropy modes. We then examine the primordial scalar
and tensor power spectra and show that their tilt depends on the scalar-tensor
coupling constant. Finally, the induced CMB anisotropies are computed and we
present a Monte Carlo Markov Chains exploration of the parameter space using
the first year WMAP data. We find a marginalized probability bound for the
associated Eddington parameter at the end of inflation 1 - gamma < 0.002, at
95% confidence level. This suggests that future CMB data could provide crucial
information helping to distinguish scalar-tensor and standard inflationary
scenarios.Comment: 24 pages, 19 figures, uses RevTex. Qualitative discussions added,
matches published versio
The Initial Mass Function of the Stellar Association NGC 602 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations
We present our photometric study of the stellar association NGC 602 in the
wing of the Small Magellanic Cloud (SMC). The data were taken in the filters
F555W and F814W using the Advanced Camera for Surveys (ACS) on-board the Hubble
Space Telescope (HST). Photometry was performed using the ACS module of the
stellar photometry package DOLPHOT. We detected more than 5,500 stars with a
magnitude range of 14 \lsim m_{555} \lsim 28 mag. Three prominent stellar
concentrations are identified with star counts in the observed field, the
association NGC 602 itself, and two clusters, one of them not being currently
in any known catalog. The Color-Magnitude Diagrams (CMDs) of both clusters show
features typical for young open clusters, while that of the association reveals
bright main sequence (MS) and faint pre-main sequence (PMS) stars as the
members of the system. We construct the initial mass spectrum (IMS) of the
association by applying an age-independent method of counting the PMS stars
within evolutionary tracks, while for the bright MS stars we transform their
magnitudes to masses with the use of mass-luminosity relations. The IMS of NGC
602 is found to be well represented by a single-power law, corresponding to an
Initial Mass Function (IMF) of slope \Gamma\approx -1.2 for 1 \lsim M/M{\solar}
\lsim 45. This indicates that the shape of the IMF of a star forming system in
the SMC for stars with masses higher than 1 M{\solar} seems to be quite similar
to the field IMF in the solar neighborhood.Comment: Accepted for publication in ApJ, 13 pages, 14 figures, emulateapj.cls
LaTeX style, full resolution version available on
http://www.astro.uni-bonn.de/~dgoulier/Science/NGC602/ms.pd
Scalar perturbations from brane-world inflation
We investigate the scalar metric perturbations about a de Sitter brane
universe in a 5-dimensional anti de Sitter bulk. We compare the master-variable
formalism, describing metric perturbations in a 5-dimensional longitudinal
gauge, with results in a Gaussian normal gauge. For a vacuum brane (with
constant brane tension) there is a continuum of normalizable Kaluza-Klein
modes, with m>3H/2, which remain in the vacuum state. A light radion mode, with
m=\sqrt{2}H, satisfies the boundary conditions for two branes but is not
normalizable in the single-brane case. When matter is introduced (as a test
field) on the brane, this mode, together with the zero-mode and an infinite
ladder of discrete tachyonic modes, become normalizable. However, the boundary
condition requires the self-consistent 4-dimensional evolution of scalar field
perturbations on the brane and the dangerous growing modes are not excited.
These normalizable discrete modes introduce corrections at first-order to the
scalar field perturbations computed in a slow-roll expansion. On super-Hubble
scales, the correction is smaller than slow-roll corrections to the de Sitter
background. However on small scales the corrections can become significant.Comment: 15 page
Magnetic-Anisotropy in UTX Compounds
Havela, L.; Sechovsky, V.; Nakotte, H.; Bruck, E.H. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible
Axisymmetric metrics in arbitrary dimensions
We consider axially symmetric static metrics in arbitrary dimension, both
with and without a cosmological constant. The most obvious such solutions have
an SO(n) group of Killing vectors representing the axial symmetry, although one
can also consider abelian groups which represent a flat `internal space'. We
relate such metrics to lower dimensional dilatonic cosmological metrics with a
Liouville potential. We also develop a duality relation between vacuum
solutions with internal curvature and those with zero internal curvature but a
cosmological constant. This duality relation gives a solution generating
technique permitting the mapping of different spacetimes. We give a large class
of solutions to the vacuum or cosmological constant spacetimes. We comment on
the extension of the C-metric to higher dimensions and provide a novel solution
for a braneworld black hole.Comment: 36 pages, LaTeX (JHEP), 4 figures, section added (published version
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