2,800 research outputs found
Cosmological CMBR dipole in open universes ?
The observed CMBR dipole is generally interpreted as a Doppler effect arising
from the motion of the Earth relative to the CMBR frame. An alternative
interpretation, proposed in the last years, is that the dipole results from
ultra-large scale isocurvature perturbations. We examine this idea in the
context of open cosmologies and show that the isocurvature interpretation is
not valid in an open universe, unless it is extremely close to a flat universe,
.Comment: 26 pages, Latex, 6 figures, to appear in Phys. Rev.
Vortexing Off A Common Suction Header
This project is the first iteration of a testing rig to determine the critical submergence of a vortex off a main suction header. The rig was designed with supplies purchased and provided by PG&E, including 4-6â schedule 40 PVC pipe and fittings, a large water tank, a pump, and a flowmeter. PG&E at Diablo Canyon Power Plant presented their problem to the Cal Poly Mechanical Engineering Senior Project class, and three mechanical engineers took up the project. The following report details the ideation, design, build, and test processes used during the 2016-17 academic year to create the vortex testing rig. We determined through testing that the location of hydraulic jump can be influenced by how open or closed the branch line valve was, which in turn influenced when gas ingestion to the pump occurred
Solutions of gauge invariant cosmological perturbations in long-wavelength limit
We investigate gauge invariant cosmological perturbations in a spatially flat
Friedman-Robertson-Walker universe with scalar fields. It is well known that
the evolution equation for the gauge invariant quantities has exact solutions
in the long-wavelength limit. We find that these gauge invariant solutions can
be obtained by differentiating the background solution with respect to
parameters contained in the background system. This method is very useful when
we analyze the long-wavelength behavior of cosmological perturbation with
multiple scalar fields.Comment: 17 pages, will appear in Classical and Quantum Gravit
Primordial gravitational waves in inflationary braneworld
We study primordial gravitational waves from inflation in Randall-Sundrum
braneworld model. The effect of small change of the Hubble parameter during
inflation is investigated using a toy model given by connecting two de Sitter
branes. We analyze the power spectrum of final zero-mode gravitons, which is
generated from the vacuum fluctuations of both initial Kaluza-Klein modes and
zero-mode. The amplitude of fluctuations is confirmed to agree with the
four-dimensional one at low energies, whereas it is enhanced due to the
normalization factor of zero-mode at high energies. We show that the
five-dimensional spectrum can be well approximated by applying a simple mapping
to the four-dimensional fluctuation amplitude.Comment: 16 pages, 4 figures, typos correcte
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.
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
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
Exactly solvable model for cosmological perturbations in dilatonic brane worlds
We construct a model where cosmological perturbations are analytically solved
based on dilatonic brane worlds. A bulk scalar field has an exponential
potential in the bulk and an exponential coupling to the brane tension. The
bulk scalar field yields a power-law inflation on the brane. The exact
background metric can be found including the back-reaction of the scalar field.
Then exact solutions for cosmological perturbations which properly satisfy the
junction conditions on the brane are derived. These solutions provide us an
interesting model to understand the connection between the behavior of
cosmological perturbations on the brane and the geometry of the bulk. Using
these solutions, the behavior of an anisotropic stress induced on the
inflationary brane by bulk gravitational fields is investigated.Comment: 30 pages, typos corrected, reference adde
Non-linear isocurvature perturbations and non-Gaussianities
We study non-linear primordial adiabatic and isocurvature perturbations and
their non-Gaussianity. After giving a general formulation in the context of an
extended delta N-formalism, we analyse in detail two illustrative examples. The
first is a mixed curvaton-inflaton scenario in which fluctuations of both the
inflaton and a curvaton (a light isocurvature field during inflation)
contribute to the primordial density perturbation. The second example is that
of double inflation involving two decoupled massive scalar fields during
inflation. In the mixed curvaton-inflaton scenario we find that the bispectrum
of primordial isocurvature perturbations may be large and comparable to the
bispectrum of adiabatic curvature perturbations.Comment: 24 pages, typos corrected, references adde
Models for the Brane-Bulk Interaction: Toward Understanding Braneworld Cosmological Perturbation
Using some simple toy models, we explore the nature of the brane-bulk
interaction for cosmological models with a large extra dimension. We are in
particular interested in understanding the role of the bulk gravitons, which
from the point of view of an observer on the brane will appear to generate
dissipation and nonlocality, effects which cannot be incorporated into an
effective (3+1)-dimensional Lagrangian field theoretic description. We
explicitly work out the dynamics of several discrete systems consisting of a
finite number of degrees of freedom on the boundary coupled to a
(1+1)-dimensional field theory subject to a variety of wave equations. Systems
both with and without time translation invariance are considered and moving
boundaries are discussed as well. The models considered contain all the
qualitative feature of quantized linearized cosmological perturbations for a
Randall-Sundrum universe having an arbitrary expansion history, with the sole
exception of gravitational gauge invariance, which will be treated in a later
paper.Comment: 47 pages, RevTeX (or Latex, etc) with 5 eps figure
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