333 research outputs found
Brane Worlds, the Cosmological Constant and String Theory
We argue that traditional methods of compactification of string theory make
it very difficult to understand how the cosmological constant becomes zero.
String inspired models can give zero cosmological constant after fine tuning
but since string theory has no free parameters it is not clear that this is
allowed. Brane world scenarios on the other hand while they do not answer the
question as to why the cosmological constant is zero do actually allow a choice
of integration constants that permit flat four space solutions. In this paper
we discuss gauged supergravity realizations of such a world. To the extent that
this starting point can be considered a low energy effective action of string
theory (and there is some recent evidence supporting this) our model may be
considered a string theory realization of this scenario.Comment: 18 pages, 5 figures. Shorter version and a few new comments adde
What Do We Mean by “Transformation”?
What is “defense transformation” (by whatever name), and how might it affect strategy? What might it cost, and how could its cost affect military forces? What systemic impediments to implementation does it face, and what are its competitors in the budgetary realm? A distinguished panel of the Secretary of the Navy’s Current Strategy Forum held at the Naval War College on 12–13 June 2001 addressed these issues from various perspectives
Designed for Disease: The Link Between Local Food Environments and Obesity and Diabetes
Examines the link between a community's retail food environment -- the ratio of fast-food outlets and convenience stores to grocery stores and produce vendors, with income level as a factor -- and the prevalence of adult obesity and diabetes
Duality Twists on a Group Manifold
We study duality-twisted dimensional reductions on a group manifold G, where
the twist is in a group \tilde{G} and examine the conditions for consistency.
We find that if the duality twist is introduced through a group element
\tilde{g} in \tilde{G}, then the flat \tilde{G}-connection A =\tilde{g}^{-1}
d\tilde{g} must have constant components M_n with respect to the basis 1-forms
on G, so that the dependence on the internal coordinates cancels out in the
lower dimensional theory. This condition can be satisfied if and only if M_n
forms a representation of the Lie algebra of G, which then ensures that the
lower dimensional gauge algebra closes. We find the form of this gauge algebra
and compare it to that arising from flux compactifications on twisted tori. As
an example of our construction, we find a new five dimensional gauged, massive
supergravity theory by dimensionally reducing the eight dimensional Type II
supergravity on a three dimensional unimodular, non-semi-simple, non-abelian
group manifold with an SL(3,R) twist.Comment: 22 page
Live-cell imaging approaches for the investigation of xenobiotic-induced oxidant stress
Oxidant stress is arguably a universal feature in toxicology. Research studies on the role of oxidant stress induced by xenobiotic exposures have typically relied on the identification of damaged biomolecules using a variety of conventional biochemical and molecular techniques. However, there is increasing evidence that low-level exposure to a variety of toxicants dysregulates cellular physiology by interfering with redox-dependent processes
Quantum Mechanics of the Doubled Torus
We investigate the quantum mechanics of the doubled torus system, introduced
by Hull [1] to describe T-folds in a more geometric way. Classically, this
system consists of a world-sheet Lagrangian together with some constraints,
which reduce the number of degrees of freedom to the correct physical number.
We consider this system from the point of view of constrained Hamiltonian
dynamics. In this case the constraints are second class, and we can quantize on
the constrained surface using Dirac brackets. We perform the quantization for a
simple T-fold background and compare to results for the conventional
non-doubled torus system. Finally, we formulate a consistent supersymmetric
version of the doubled torus system, including supersymmetric constraints.Comment: 31 pages, 1 figure; v2: references added, minor corrections to final
sectio
Darkfield-Confocal Microscopy detection of nanoscale particle internalization by human lung cells
<p>Abstract</p> <p>Background</p> <p>Concerns over the health effects of nanomaterials in the environment have created a need for microscopy methods capable of examining the biological interactions of nanoparticles (NP). Unfortunately, NP are beyond the diffraction limit of resolution for conventional light microscopy (~200 nm). Fluorescence and electron microscopy techniques commonly used to examine NP interactions with biological substrates have drawbacks that limit their usefulness in toxicological investigation of NP. EM is labor intensive and slow, while fluorescence carries the risk of photobleaching the sample and has size resolution limits. In addition, many relevant particles lack intrinsic fluorescence and therefore can not be detected in this manner. To surmount these limitations, we evaluated the potential of a novel combination of darkfield and confocal laser scanning microscopy (DF-CLSM) for the efficient 3D detection of NP in human lung cells. The DF-CLSM approach utilizes the contrast enhancements of darkfield microscopy to detect objects below the diffraction limit of 200 nm based on their light scattering properties and interfaces it with the power of confocal microscopy to resolve objects in the z-plane.</p> <p>Results</p> <p>Validation of the DF-CLSM method using fluorescent polystyrene beads demonstrated spatial colocalization of particle fluorescence (Confocal) and scattered transmitted light (Darkfield) along the X, Y, and Z axes. DF-CLSM imaging was able to detect and provide reasonable spatial locations of 27 nm TiO<sub>2 </sub>particles in relation to the stained nuclei of exposed BEAS 2B cells. Statistical analysis of particle proximity to cellular nuclei determined a significant difference between 5 min and 2 hr particle exposures suggesting a time-dependant internalization process.</p> <p>Conclusions</p> <p>DF-CLSM microscopy is an alternative to current conventional light and electron microscopy methods that does not rely on particle fluorescence or contrast in electron density. DF-CLSM is especially well suited to the task of establishing the spatial localization of nanoparticles within cells, a critical topic in nanotoxicology. This technique has advantages to 2D darkfield microscopy as it visualizes nanoparticles in 3D using confocal microscopy. Use of this technique should aid toxicological studies related to observation of NP interactions with biological endpoints at cellular and subcellular levels.</p
Global Aspects of T-Duality, Gauged Sigma Models and T-Folds
The gauged sigma-model argument that string backgrounds related by T-dual
give equivalent quantum theories is revisited, taking careful account of global
considerations. The topological obstructions to gauging sigma-models give rise
to obstructions to T-duality, but these are milder than those for gauging: it
is possible to T-dualise a large class of sigma-models that cannot be gauged.
For backgrounds that are torus fibrations, it is expected that T-duality can be
applied fibrewise in the general case in which there are no globally-defined
Killing vector fields, so that there is no isometry symmetry that can be
gauged; the derivation of T-duality is extended to this case. The T-duality
transformations are presented in terms of globally-defined quantities. The
generalisation to non-geometric string backgrounds is discussed, the conditions
for the T-dual background to be geometric found and the topology of T-folds
analysed.Comment: Minor corrections and addition
D-branes in T-fold conformal field theory
We investigate boundary dynamics of orbifold conformal field theory involving
T-duality twists. Such models typically appear in contexts of non-geometric
string compactifications that are called monodrofolds or T-folds in recent
literature. We use the framework of boundary conformal field theory to analyse
the models from a microscopic world-sheet perspective. In these backgrounds
there are two kinds of D-branes that are analogous to bulk and fractional
branes in standard orbifold models. The bulk D-branes in T-folds allow
intuitive geometrical interpretations and are consistent with the classical
analysis based on the doubled torus formalism. The fractional branes, on the
other hand, are `non-geometric' at any point in the moduli space and their
geometric counterparts seem to be missing in the doubled torus analysis. We
compute cylinder amplitudes between the bulk and fractional branes, and find
that the lightest modes of the open string spectra show intriguing non-linear
dependence on the moduli (location of the brane or value of the Wilson line),
suggesting that the physics of T-folds, when D-branes are involved, could
deviate from geometric backgrounds even at low energies. We also extend our
analysis to the models with SU(2) WZW fibre at arbitrary levels.Comment: 38 pages, no figure, ams packages. Essentially the published versio
Superstring partition functions in the doubled formalism
Computation of superstring partition function for the non-linear sigma model
on the product of a two-torus and its dual within the scope of the doubled
formalism is presented. We verify that it reproduces the partition functions of
the toroidally compactified type--IIA and type--IIB theories for appropriate
choices of the GSO projection.Comment: 15 page
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