267 research outputs found
Boundary States for Supertubes in Flat Spacetime and Godel Universe
We construct boundary states for supertubes in the flat spacetime. The T-dual
objects of supertubes are moving spiral D1-branes (D-helices). Since we can
obtain these D-helices from the usual D1-branes via null deformation, we can
construct the boundary states for these moving D-helices in the covariant
formalism. Using these boundary states, we calculate the vacuum amplitude
between two supertubes in the closed string channel and read the open string
spectrum via the open closed duality. We find there are critical values of the
energy for on-shell open strings on the supertubes due to the non-trivial
stringy correction. We also consider supertubes in the type IIA Godel universe
in order to use them as probes of closed timelike curves. This universe is the
T-dual of the maximally supersymmetric type IIB PP-wave background. Since the
null deformations of D-branes are also allowed in this PP-wave, we can
construct the boundary states for supertubes in the type IIA Godel universe in
the same way. We obtain the open string spectrum on the supertube from the
vacuum amplitude between supertubes. As a consequence, we find that the
tachyonic instability of open strings on the supertube, which is the signal of
closed time like curves, disappears due to the stringy correction.Comment: 26 pages, 3 figures, v2: explanations added, references added, v3:
explanations adde
Supertube domain-walls and elimination of closed time-like curves in string theory
We show that some novel physics of supertubes removes closed time-like curves
from many supersymmetric spaces which naively suffer from this problem. The
main claim is that supertubes naturally form domain-walls, so while analytical
continuation of the metric would lead to closed time-like curves, across the
domain-wall the metric is non-differentiable, and the closed time-like curves
are eliminated. In the examples we study the metric inside the domain-wall is
always of the G\"odel type, while outside the shell it looks like a localized
rotating object, often a rotating black hole. Thus this mechanism prevents the
appearance of closed time-like curves behind the horizons of certain rotating
black holes.Comment: 22 pages, JHEP3 class. V2: Some corrections and clariffications,
references added. V3: more corrections to formulas, results unchanged. V4:
minor typos, as published in PR
Killing spectroscopy of closed timelike curves
We analyse the existence of closed timelike curves in spacetimes which
possess an isometry. In particular we check which discrete quotients of such
spaces lead to closed timelike curves. As a by-product of our analysis, we
prove that the notion of existence or non-existence of closed timelike curves
is a T-duality invariant notion, whenever the direction along which we apply
such transformations is everywhere spacelike. Our formalism is
straightforwardly applied to supersymmetric theories. We provide some new
examples in the context of D-branes and generalized pp-waves.Comment: 1+35 pages, no figures; v2, new references added. Final version to
appear in JHE
Goedel-type Universes and the Landau Problem
We point out a close relation between a family of Goedel-type solutions of
3+1 General Relativity and the Landau problem in S^2, R^2 and H_2; in
particular, the classical geodesics correspond to Larmor orbits in the Landau
problem. We discuss the extent of this relation, by analyzing the solutions of
the Klein-Gordon equation in these backgrounds. For the R^2 case, this relation
was independently noticed in hep-th/0306148. Guided by the analogy with the
Landau problem, we speculate on the possible holographic description of a
single chronologically safe region.Comment: Latex, 21 pages, 1 figure. v2 missing references to previous work on
the subject adde
Seiberg-Witten Transforms of Noncommutative Solitons
We evaluate the Seiberg-Witten map for solitons and instantons in
noncommutative gauge theories in various dimensions. We show that solitons
constructed using the projection operators have delta-function supports when
expressed in the commutative variables. This gives a precise identification of
the moduli of these solutions as locations of branes. On the other hand, an
instanton solution in four dimensions allows deformation away from the
projection operator construction. We evaluate the Seiberg-Witten transform of
the U(2) instanton and show that it has a finite size determined by the
noncommutative scale and by the deformation parameter \rho. For large \rho, the
profile of the D0-brane density of the instanton agrees surprisingly well with
that of the BPST instanton on commutative space.Comment: 29 pages, LaTeX; comments added, typos corrected, and a reference
added; comments added, typos correcte
Classical and Quantum Strings in compactified pp-waves and Godel type Universes
We consider Neveu-Schwarz pp-waves with spacetime supersymmetry. Upon
compactification of a spacelike direction, these backgrounds develop Closed
Null Curves (CNCs) and Closed Timelike Curves (CTCs), and are U-dual to
supersymmetric Godel type universes. We study classical and quantum strings in
this background, with emphasis on the strings winding around the compact
direction. We consider two types of strings: long strings stabilized by NS flux
and rotating strings which are stabilized against collapse by angular momentum.
Some of the latter strings wrap around CNCs and CTCs, and are thus a potential
source of pathology. We analyze the partition function, and in particular
discuss the effects of these string states. Although our results are not
conclusive, the partition function seems to be dramatically altered due to the
presence of CNCs and CTCs. We discuss some interpretations of our results,
including a possible sign of unitary violation.Comment: 42 pages, LaTeX, 2 figure
Dynamics and stability of the Godel universe
We use covariant techniques to describe the properties of the Godel universe
and then consider its linear response to a variety of perturbations. Against
matter aggregations, we find that the stability of the Godel model depends
primarily upon the presence of gradients in the centrifugal energy, and
secondarily on the equation of state of the fluid. The latter dictates the
behaviour of the model when dealing with homogeneous perturbations. The
vorticity of the perturbed Godel model is found to evolve as in almost-FRW
spacetimes, with some additional directional effects due to shape distortions.
We also consider gravitational-wave perturbations by investigating the
evolution of the magnetic Weyl component. This tensor obeys a simple plane-wave
equation, which argues for the neutral stability of the Godel model against
linear gravity-wave distortions. The implications of the background rotation
for scalar-field Godel cosmologies are also discussed.Comment: Revised version, to match paper published in Class. Quantum Gra
Consumer Adoption of Self-Service Technologies in the Context of the Jordanian Banking Industry: Examining the Moderating Role of Channel Types
YesThis study aimed to examine the key factors predicting Jordanian consumers’ intentions and
usage of three types of self-service banking technologies. This study also sought to test if the
impacts of these main predictors could be moderated by channel type. This study proposed a
conceptual model by integrating factors from the unified theory of acceptance and use of
technology (UTAUT), along with perceived risk. The required data were collected from a
convenience sample of Jordanian banking customers using a survey questionnaire. The
statistical results strongly support the significant influence of performance expectancy, social
influence, and perceived risk on customer intentions for the three types of SSTs examined. The
results of the X2 differences test also indicate that there are significant differences in the
influence of the main predictors due to the moderating effect of channel type. One of the key
contributions of this study is that three types of SSTs were tested in a single study, which had
not been done before, leading to the identification of the factors common to all three types, as
well as the salient factors unique to each type
A proteomic approach to investigating gene cluster expression and secondary metabolite functionality in Aspergillus fumigatus.
A combined proteomics and metabolomics approach was utilised to advance the identification and characterisation of secondary metabolites in Aspergillus fumigatus. Here, implementation of a shotgun proteomic strategy led to the identification of non-redundant mycelial proteins (n = 414) from A. fumigatus including proteins typically under-represented in 2-D proteome maps: proteins with multiple transmembrane regions, hydrophobic proteins and proteins with extremes of molecular mass and pI. Indirect identification of secondary metabolite cluster expression was also achieved, with proteins (n = 18) from LaeA-regulated clusters detected, including GliT encoded within the gliotoxin biosynthetic cluster. Biochemical analysis then revealed that gliotoxin significantly attenuates H2O2-induced oxidative stress in A. fumigatus (p>0.0001), confirming observations from proteomics data. A complementary 2-D/LC-MS/MS approach further elucidated significantly increased abundance (p<0.05) of proliferating cell nuclear antigen (PCNA), NADH-quinone oxidoreductase and the gliotoxin oxidoreductase GliT, along with significantly attenuated abundance (p<0.05) of a heat shock protein, an oxidative stress protein and an autolysis-associated chitinase, when gliotoxin and H2O2 were present, compared to H2O2 alone. Moreover, gliotoxin exposure significantly reduced the abundance of selected proteins (p<0.05) involved in de novo purine biosynthesis. Significantly elevated abundance (p<0.05) of a key enzyme, xanthine-guanine phosphoribosyl transferase Xpt1, utilised in purine salvage, was observed in the presence of H2O2 and gliotoxin. This work provides new insights into the A. fumigatus proteome and experimental strategies, plus mechanistic data pertaining to gliotoxin functionality in the organism
- …