1,742 research outputs found
The changing tide: Federal support of civilian-sector R and D
The involvement of the Federal government in civilian sector research and development is discussed. Relevant policies are put in an historical perspective. The roles played by industrial research and public funding are reveiwed. Government support of basic an generic research, clientele-oriented applied research, and research with commercial ends is studied. Procurement, anti-trust, and patent policies, all of which affect the climate for private research and development, are examined
Viscous Brane Cosmology with a Brane-Bulk Energy Interchange Term
We assume a flat brane located at y=0, surrounded by an AdS space, and
consider the 5D Einstein equations when the energy flux component of the
energy-momentum tensor is related to the Hubble parameter through a constant Q.
We calculate the metric tensor, as well as the Hubble parameter on the brane,
when Q is small. As a special case, if the brane is tensionless, the influence
from Q on the Hubble parameter is absent. We also consider the emission of
gravitons from the brane, by means of the Boltzmann equation. Comparing the
energy conservation equation derived herefrom with the energy conservation
equation for a viscous fluid on the brane, we find that the entropy change for
the fluid in the emission process has to be negative. This peculiar effect is
related to the fluid on the brane being a non-closed thermodynamic system. The
negative entropy property for non-closed systems is encountered in other areas
in physics also, in particular, in connection with the Casimir effect at finite
temperature.Comment: 12 pages, latex, no figure
A general proof of the equivalence between the \delta N and covariant formalisms
Recently, the equivalence between the \delta N and covariant formalisms has
been shown (Suyama et al. 2012), but they essentially assumed Einstein gravity
in their proof. They showed that the evolution equation of the curvature
covector in the covariant formalism on uniform energy density slicings
coincides with that of the curvature perturbation in the \delta N formalism
assuming the coincidence of uniform energy and uniform expansion (Hubble)
slicings, which is the case on superhorizon scales in Einstein gravity. In this
short note, we explicitly show the equivalence between the \delta N and
covariant formalisms without specifying the slicing condition and the
associated slicing coincidence, in other words, regardless of the gravity
theory.Comment: 7 pages,a reference added, to be published in EP
An Off-Axis Telescope Concept for Antarctic Astronomy
International audienc
Strong gravitational lensing by braneworld black holes
In this paper, we use the strong field limit approach to investigate the
gravitational lensing properties of braneworld black holes. Applying this
method to the supermassive black hole at the centre of our galaxy, the lensing
observables for some candidate braneworld black hole metrics are compared with
those for the standard Schwarzschild case. It is found that braneworld black
holes could have significantly different observational signatures to the
Schwarzschild black hole.Comment: 8 pages, 4 figures, RevTeX4; v2 reference added; v3 minor technical
correctio
Designing Volumetric Truss Structures
We present the first algorithm for designing volumetric Michell Trusses. Our
method uses a parametrization approach to generate trusses made of structural
elements aligned with the primary direction of an object's stress field. Such
trusses exhibit high strength-to-weight ratios. We demonstrate the structural
robustness of our designs via a posteriori physical simulation. We believe our
algorithm serves as an important complement to existing structural optimization
tools and as a novel standalone design tool itself
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
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
Randall-Sundrum Model in the Presence of a Brane Bulk Viscosity
The presence of a bulk viscosity for the cosmic fluid on a single
Randall-Sundrum brane is considered. The spatial curvature is assumed to be
zero. The five-dimensional Friedmann equation is derived, together with the
energy conservation equation for the viscous fluid. These governing equations
are solved for some special cases: (i) in the low-energy limit when the matter
energy density is small compared with brane tension; (ii) for a
matter-dominated universe, and (iii) for a radiation-dominated universe. Rough
numerical estimates, for the extreme case when the universe is at its Planck
time, indicate that the viscous effect can be significant.Comment: 18 pages, RevTeX4, no figures. Discussion in Sec. III expanded; new
references. To appear in Phys. Rev.
Godel brane
We consider the brane-world generalisation of the Godel universe and analyse
its dynamical interaction with the bulk. The exact homogeneity of the standard
Godel spacetime no longer holds, unless the bulk is also static. We show how
the anisotropy of the Godel-type brane is dictated by that of the bulk and find
that the converse is also true. This determines the precise evolution of the
nonlocal anisotropic stresses, without any phenomenological assumptions, and
leads to a self-consistent closed set of equations for the evolution of the
Godel brane. We also examine the causality of the Godel brane and show that the
presence of the bulk cannot prevent the appearance of closed timelike curves.Comment: Revised version, to match paper published in Phys. Rev.
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