514 research outputs found
Spacetime structure of the global vortex
We analyse the spacetime structure of the global vortex and its maximal
analytic extension in an arbitrary number of spacetime dimensions. We find that
the vortex compactifies space on the scale of the Hubble expansion of its
worldvolume, in a manner reminiscent of that of the domain wall. We calculate
the effective volume of this compactification and remark on its relevance to
hierarchy resolution with extra dimensions. We also consider strongly
gravitating vortices and derive bounds on the existence of a global vortex
solution.Comment: 19 pages revtex, 2 figures, minor changes, references adde
NASA/LaRC jet plume research
The following provides a summary for research being conducted by NASA/LaRC and its contractors and grantees to develop jet engine noise suppression technology under the NASA High Speed Research (HSR) program for the High Speed Civil Transport (HSCT). The objective of this effort is to explore new innovative concepts for reducing noise to Federally mandated guidelines with minimum compromise on engine performance both in take-off and cruise. The research program is divided into four major technical areas: (1) jet noise research on advanced nozzles; (2) plume prediction and validation; (3) passive and active control; and (4) methodology for noise prediction
Selenium interactions with algae: Chemical processes at biological uptake sites, bioaccumulation, and intracellular metabolism
Selenium (Se) uptake by primary producers is the most variable and important step in determining Se concentrations at higher trophic levels in aquatic food webs. We gathered data available about the Se bioaccumulation at the base of aquatic food webs and analyzed its relationship with Se concentrations in water. This important dataset was separated into lotic and lentic systems to provide a reliable model to estimate Se in primary producers from aqueous exposure. We observed that lentic systems had higher organic selenium and selenite concentrations than in lotic systems and selenate concentrations were higher in lotic environments. Selenium uptake by algae is mostly driven by Se concentrations, speciation and competition with other anions, and is as well influenced by pH. Based on Se species uptake by algae in the laboratory, we proposed an accurate mechanistic model of competition between sulfate and inorganic Se species at algal uptake sites. Intracellular Se transformations and incorporation into selenoproteins as well as the mechanisms through which Se can induce toxicity in algae has also been reviewed. We provided a new tool for risk assessment strategies to better predict accumulation in primary consumers and consequently to higher trophic levels, and we identified some research needs that could fill knowledge gaps
Scalar Hair of Global Defect and Black Brane World
We consider a complex scalar field in (p+3)-dimensional bulk with a negative
cosmological constant and study global vortices in two extra-dimensions. We
reexamine carefully the coupled scalar and Einstein equations, and show that
the boundary value of scalar amplitude at infinity of the extra-dimensions
should be smaller than vacuum expectation value. The brane world has a
cigar-like geometry with an exponentially decaying warp factor and a flat thick
p-brane is embedded. Since a coordinate transformation identifies the obtained
brane world as a black p-brane world bounded by a horizon, this strange
boundary condition of the scalar amplitude is understood as existence of a
short scalar hair.Comment: 26 pages, 2 figure
Charting the Landscape of Modified Gravity
We explore brane induced gravity on a 3-brane in six locally flat dimensions.
To regulate the short distance singularities in the brane core, we resolve the
thin brane by a cylindrical 4-brane, with the geometry of 4D Minkowski
a circle, which has an axion flux to cancel the vacuum pressure in the compact
direction. We discover a large diversity of possible solutions controlled by
the axion flux, as governed by its boundary conditions. Hence brane induced
gravity models really give rise to a {\it landscape} of vacua, at least
semiclassically. For sub-critical tensions, the crossover scale, below which
gravity may look 4D, and the effective 4D gravitational coupling are sensitive
to vacuum energy. This shows how the vacuum energy problem manifests in brane
induced gravity: instead of tuning the 4D curvature, generically one must tune
the crossover scale. On the other hand, in the near-critical limit, branes live
inside very deep throats which efficiently compactify the angular dimension. In
there, 4D gravity first changes to , and only later to . The crossover
scale saturates at the gravitational see-saw scale, independent of the tension.
Using the fields of static loops on a wrapped brane, we check the perturbative
description of long range gravity below the crossover scale. In sub-critical
cases the scalars are strongly coupled already at the crossover scale even in
the vacuum, because the brane bending is turned on by the axion flux. Near the
critical limit, linearized perturbation theory remains under control below the
crossover scale, and we find that linearized gravity around the vacuum looks
like a scalar-tensor theory.Comment: 47 LaTeX pages, 3 .eps figures, typos fixed to match the published
versio
Modulus stabilization of generalized Randall Sundrum model with bulk scalar field
We study the stabilization of inter-brane spacing modulus of generalized
warped brane models with a nonzero brane cosmological constant. Employing
Goldberger-Wise stabilization prescription of brane world models with a bulk
scalar field, we show that the stabilized value of the modulus generally
depends on the value of the brane cosmological constant. Our result further
reveals that the stabilized modulus value corresponding to a vanishingly small
cosmological constant can only resolve the gauge hierarchy problem
simultaneously. This in turn vindicates the original Randall-Sundrum model
where the 3-brane cosmological constant was chosen to be zero.Comment: 12 Pages, 1 figure, Revtex, Version to appear in Euro. Phys. Let
Finite temperature Casimir effect for massive scalar field in spacetime with extra dimensions
We compute the finite temperature Casimir energy for massive scalar field
with general curvature coupling subject to Dirichlet or Neumann boundary
conditions on the walls of a closed cylinder with arbitrary cross section,
located in a background spacetime of the form ,
where is the -dimensional Minkowski spacetime and
is an -dimensional internal manifold. The Casimir energy is
regularized using the criteria that it should vanish in the infinite mass
limit. The Casimir force acting on a piston moving freely inside the closed
cylinder is derived and it is shown that it is independent of the
regularization procedure. By letting one of the chambers of the cylinder
divided by the piston to be infinitely long, we obtain the Casimir force acting
on two parallel plates embedded in the cylinder. It is shown that if both the
plates assume Dirichlet or Neumann boundary conditions, the strength of the
Casimir force is reduced by the increase in mass. Under certain conditions, the
passage from massless to massive will change the nature of the force from long
range to short range. Other properties of the Casimir force such as its sign,
its behavior at low and high temperature, and its behavior at small and large
plate separations, are found to be similar to the massless case. Explicit exact
formulas and asymptotic behaviors of the Casimir force at different limits are
derived. The Casimir force when one plate assumes Dirichlet boundary condition
and one plate assumes Neumann boundary condition is also derived and shown to
be repulsive.Comment: 28 pages, 4 figure
Gravity induced over a smooth soliton
I consider gravity induced over a smooth (finite thickness) soliton. Graviton
kinetic term is coupled to bulk scalar that develops solitonic vacuum
expectation value. Couplings of Kaluza-Klein modes to soliton-localized matter
are suppressed, giving rise to crossover distance between
4D and 5D behavior. This system can be viewed as a finite thickness brane
regularization of the model of Dvali, Gabadadze and Porrati.Comment: 12 pages, 2 figure
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