9,794 research outputs found
Linearized Gravity in Brane Backgrounds
A treatment of linearized gravity is given in the Randall-Sundrum background.
The graviton propagator is found in terms of the scalar propagator, for which
an explicit integral expression is provided. This reduces to the
four-dimensional propagator at long distances along the brane, and provides
estimates of subleading corrections. Asymptotics of the propagator off the
brane yields exponential falloff of gravitational fields due to matter on the
brane. This implies that black holes bound to the brane have a "pancake"-like
shape in the extra dimension, and indicates validity of a perturbative
treatment off the brane. Some connections with the AdS/CFT correspondence are
described.Comment: 31 pages, harvmac. v2: minor typo and reference corrections. v3:
minor corrections to eqs and discussio
The embedding of the spacetime in five dimensions: an extension of Campbell-Magaard theorem
We extend Campbell-Magaard embedding theorem by proving that any
n-dimensional semi-Riemannian manifold can be locally embedded in an
(n+1)-dimensional Einstein space. We work out some examples of application of
the theorem and discuss its relevance in the context of modern
higher-dimensional spacetime theories.Comment: 22pages, Revte
Why supersymmetry should be restored at the TeV scale
It is explained why the curvature associated to the vacuum energy density
arising from SUSY breaking cannot be completely transferred to the extra
spatial dimensions of a bulk space-time manifold, and it is shown -- without
using hierarchy arguments but only the results of current large-scale
observations -- why the Tev scale should correspond to the maximal allowed
SUSY-breaking scale.Comment: 6 pages. Essay written for the 2008 Awards for Essays on Gravitation
(Gravity Research Foundation, USA), and awarded with "Honorable Mention
Fabrication and Characterization of Controllable Grain Boundary Arrays in Solution Processed Small Molecule Organic Semiconductor Films
We have produced solution-processed thin films of
6,13-bis(triisopropyl-silylethynyl) pentacene with grain sizes from a few
micrometers up to millimeter scale by lateral crystallization from a
rectangular stylus. Grains are oriented along the crystallization direction,
and the grain size transverse to the crystallization direction depends
inversely on the writing speed, hence forming a regular array of oriented grain
boundaries with controllable spacing. We utilize these controllable arrays to
systematically study the role of large-angle grain boundaries in carrier
transport and charge trapping in thin film transistors. The effective mobility
scales with the grain size, leading to an estimate of the potential drop at
individual large-angle grain boundaries of more than one volt. This result
indicates that the structure of grain boundaries is not molecularly abrupt,
which may be a general feature of solution processed small molecule organic
semiconductor thin films where relatively high energy grain boundaries are
typically formed. This may be due to the crystal Transient measurements after
switching from positive to negative gate bias or between large and small
negative gate bias reveal reversible charge trapping with time constants on the
order of 10 s, and trap densities that are correlated with grain boundary
density. We suggest that charge diffusion along grain boundaries and other
defects is the rate determining mechanism of the reversible trapping.Comment: 12 pages, 11 figure
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
Iodine sorption study on the proposed use of Viton A in a shuttle galley water accumulator
The installation of a Viton A accumulator in the Shuttle galley has been proposed to prevent overpressurization of the hot water supply system. A laboratory study has been conducted to determine if there would be any interaction between the Viton A material and the iodine used to disinfect the water. Coupons of Viton A were exposed for 24 hours to aqueous iodine solutions similar in quality to the Shuttle's potable water. Changes in the iodine residual were monitored to determine the rate of iodine sorption by the coupon. Total organic carbon (TOC) was monitored to determine the rate of desorption of organic materials from the Viton A. The same coupons were then soaked in reagent-grade water for 24 hours, and iodine was monitored to determine the rate of iodine desorption. The coupons were again exposed to iodine solutions for 24 hours and iodine and TOC were monitored. No significant change in the iodine sorption rate was detected between the first and second exposures. A triangle taste test indicated at a 1 percent confidence level that the water exposed to Viton A had a different taste which was less acceptable to the panelists
- …