87 research outputs found
Aspects of Puff Field Theory
We describe some features of the recently constructed "Puff Field Theory,"
and present arguments in favor of it being a field theory decoupled from
gravity. We construct its supergravity dual and calculate the entropy of this
theory in the limit of large 't Hooft coupling. We also determine the leading
irrelevant operator that governs its deviation from N=4 super Yang-Mills
theory.Comment: 31 pages, 1 figur
Dualities, Twists, and Gauge Theories with Non-Constant Non-Commutativity
We study the world volume theory of D3-branes wrapping the Melvin universe
supported by background NSNS B-field. In the appropriate decoupling limit, the
open string dynamics is that of non-commutative guage field theory with
non-constant non-commutativity. We identify this model as a simple Melvin twist
of flat D3 branes. Along similar lines, one recognizes the model of Hashimoto
and Sethi as being the Melvin null twist, and the model of Dolan and Nappi as
being the null Melvin twist, of the flat D3-brane. This construction therefore
offers a unified perspective on most of the known explicit constructions of
non-commutative gauge theories as a decoupled theory of D-branes in a B-field
background. We also describe the world volume theory on the D3-brane in Melvin
universe which is decaying via the nucleation of monopole anti-monopole pair.Comment: 18 pages, 1 figure, References added, typo correcte
Studies Of The Over-Rotating BMPV Solution
We study unphysical features of the BMPV black hole and how each can be
resolved using the enhancon mechanism. We begin by reviewing how the enhancon
mechanism resolves a class of repulson singularities which arise in the BMPV
geometry when D--branes are wrapped on K3. In the process, we show that the
interior of an enhancon shell can be a time machine due to non-vanishing
rotation. We link the resolution of the time machine to the recently proposed
resolution of the BMPV naked singularity / "over-rotating" geometry through the
expansion of strings in the presence of RR flux. We extend the analysis to
include a general class of BMPV black hole configurations, showing that any
attempt to "over-rotate" a causally sound BMPV black hole will be thwarted by
the resolution mechanism. We study how it may be possible to lower the entropy
of a black hole due to the non-zero rotation. This process is prevented from
occurring through the creation of a family of resolving shells. The second law
of thermodynamics is thereby enforced in the rotating geometry - even when
there is no risk of creating a naked singularity or closed time-like curves
Penrose Limit and String Theories on Various Brane Backgrounds
We investigate the Penrose limit of various brane solutions including
Dp-branes, NS5-branes, fundamental strings, (p,q) fivebranes and (p,q) strings.
We obtain special null geodesics with the fixed radial coordinate (critical
radius), along which the Penrose limit gives string theories with constant
mass. We also study string theories with time-dependent mass, which arise from
the Penrose limit of the brane backgrounds. We examine equations of motion of
the strings in the asymptotic flat region and around the critical radius. In
particular, for (p,q) fivebranes, we find that the string equations of motion
in the directions with the B field are explicitly solved by the spheroidal wave
functions.Comment: 41 pages, Latex, minor correction
From D-Dbar Pairs to Branes in Motion
We investigate various supersymmetric brane intersections. Motivated by the
recent results on supertubes, we investigate general constraints in which
parallel brane-antibrane configurations are supersymmetric. Dual descriptions
of these configurations involve systems of branes in relative motion. In
particular, we find new supersymmetric configurations which are not related to
a static brane intersection by a boost. In these new configurations, the
intersection point moves at the speed of light. These systems provide
interesting time dependent backgrounds for open strings.Comment: 28+1 pages, 8 figures, uses JHEP3.cl
What is the Oxygen Isotope Composition of Venus? The Scientific Case for Sample Return from Earth’s “Sister” Planet
Venus is Earth’s closest planetary neighbour and both bodies are of similar size and mass. As a consequence, Venus is often described as Earth’s sister planet. But the two worlds have followed very different evolutionary paths, with Earth having benign surface conditions, whereas Venus has a surface temperature of 464 °C and a surface pressure of 92 bar. These inhospitable surface conditions may partially explain why there has been such a dearth of space missions to Venus in recent years.The oxygen isotope composition of Venus is currently unknown. However, this single measurement (Δ17O) would have first order implications for our understanding of how large terrestrial planets are built. Recent isotopic studies indicate that the Solar System is bimodal in composition, divided into a carbonaceous chondrite (CC) group and a non-carbonaceous (NC) group. The CC group probably originated in the outer Solar System and the NC group in the inner Solar System. Venus comprises 41% by mass of the inner Solar System compared to 50% for Earth and only 5% for Mars. Models for building large terrestrial planets, such as Earth and Venus, would be significantly improved by a determination of the Δ17O composition of a returned sample from Venus. This measurement would help constrain the extent of early inner Solar System isotopic homogenisation and help to identify whether the feeding zones of the terrestrial planets were narrow or wide.Determining the Δ17O composition of Venus would also have significant implications for our understanding of how the Moon formed. Recent lunar formation models invoke a high energy impact between the proto-Earth and an inner Solar System-derived impactor body, Theia. The close isotopic similarity between the Earth and Moon is explained by these models as being a consequence of high-temperature, post-impact mixing. However, if Earth and Venus proved to be isotopic clones with respect to Δ17O, this would favour the classic, lower energy, giant impact scenario.We review the surface geology of Venus with the aim of identifying potential terrains that could be targeted by a robotic sample return mission. While the potentially ancient tessera terrains would be of great scientific interest, the need to minimise the influence of venusian weathering favours the sampling of young basaltic plains. In terms of a nominal sample mass, 10 g would be sufficient to undertake a full range of geochemical, isotopic and dating studies. However, it is important that additional material is collected as a legacy sample. As a consequence, a returned sample mass of at least 100 g should be recovered.Two scenarios for robotic sample return missions from Venus are presented, based on previous mission proposals. The most cost effective approach involves a “Grab and Go” strategy, either using a lander and separate orbiter, or possibly just a stand-alone lander. Sample return could also be achieved as part of a more ambitious, extended mission to study the venusian atmosphere. In both scenarios it is critical to obtain a surface atmospheric sample to define the extent of atmosphere-lithosphere oxygen isotopic disequilibrium. Surface sampling would be carried out by multiple techniques (drill, scoop, “vacuum-cleaner” device) to ensure success. Surface operations would take no longer than one hour.Analysis of returned samples would provide a firm basis for assessing similarities and differences between the evolution of Venus, Earth, Mars and smaller bodies such as Vesta. The Solar System provides an important case study in how two almost identical bodies, Earth and Venus, could have had such a divergent evolution. Finally, Venus, with its runaway greenhouse atmosphere, may provide data relevant to the understanding of similar less extreme processes on Earth. Venus is Earth’s planetary twin and deserves to be better studied and understood. In a wider context, analysis of returned samples from Venus would provide data relevant to the study of exoplanetary systems
Novel genetic loci associated with hippocampal volume
The hippocampal formation is a brain structure integrally involved in episodic memory, spatial navigation, cognition and stress responsiveness. Structural abnormalities in hippocampal volume and shape are found in several common neuropsychiatric disorders. To identify the genetic underpinnings of hippocampal structure here we perform a genome-wide association study (GWAS) of 33,536 individuals and discover six independent loci significantly associated with hippocampal volume, four of them novel. Of the novel loci, three lie within genes (ASTN2, DPP4 and MAST4) and one is found 200 kb upstream of SHH. A hippocampal subfield analysis shows that a locus within the MSRB3 gene shows evidence of a localized effect along the dentate gyrus, subiculum, CA1 and fissure. Further, we show that genetic variants associated with decreased hippocampal volume are also associated with increased risk for Alzheimer's disease (rg =-0.155). Our findings suggest novel biological pathways through which human genetic variation influences hippocampal volume and risk for neuropsychiatric illness
The genetic architecture of the human cerebral cortex
INTRODUCTION
The cerebral cortex underlies our complex cognitive capabilities. Variations in human cortical surface area and thickness are associated with neurological, psychological, and behavioral traits and can be measured in vivo by magnetic resonance imaging (MRI). Studies in model organisms have identified genes that influence cortical structure, but little is known about common genetic variants that affect human cortical structure.
RATIONALE
To identify genetic variants associated with human cortical structure at both global and regional levels, we conducted a genome-wide association meta-analysis of brain MRI data from 51,665 individuals across 60 cohorts. We analyzed the surface area and average thickness of the whole cortex and 34 cortical regions with known functional specializations.
RESULTS
We identified 306 nominally genome-wide significant loci (P < 5 × 10−8) associated with cortical structure in a discovery sample of 33,992 participants of European ancestry. Of the 299 loci for which replication data were available, 241 loci influencing surface area and 14 influencing thickness remained significant after replication, with 199 loci passing multiple testing correction (P < 8.3 × 10−10; 187 influencing surface area and 12 influencing thickness).
Common genetic variants explained 34% (SE = 3%) of the variation in total surface area and 26% (SE = 2%) in average thickness; surface area and thickness showed a negative genetic correlation (rG = −0.32, SE = 0.05, P = 6.5 × 10−12), which suggests that genetic influences have opposing effects on surface area and thickness. Bioinformatic analyses showed that total surface area is influenced by genetic variants that alter gene regulatory activity in neural progenitor cells during fetal development. By contrast, average thickness is influenced by active regulatory elements in adult brain samples, which may reflect processes that occur after mid-fetal development, such as myelination, branching, or pruning. When considered together, these results support the radial unit hypothesis that different developmental mechanisms promote surface area expansion and increases in thickness.
To identify specific genetic influences on individual cortical regions, we controlled for global measures (total surface area or average thickness) in the regional analyses. After multiple testing correction, we identified 175 loci that influence regional surface area and 10 that influence regional thickness. Loci that affect regional surface area cluster near genes involved in the Wnt signaling pathway, which is known to influence areal identity.
We observed significant positive genetic correlations and evidence of bidirectional causation of total surface area with both general cognitive functioning and educational attainment. We found additional positive genetic correlations between total surface area and Parkinson’s disease but did not find evidence of causation. Negative genetic correlations were evident between total surface area and insomnia, attention deficit hyperactivity disorder, depressive symptoms, major depressive disorder, and neuroticism.
CONCLUSION
This large-scale collaborative work enhances our understanding of the genetic architecture of the human cerebral cortex and its regional patterning. The highly polygenic architecture of the cortex suggests that distinct genes are involved in the development of specific cortical areas. Moreover, we find evidence that brain structure is a key phenotype along the causal pathway that leads from genetic variation to differences in general cognitive function
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