78 research outputs found
Non-Singularity of the Exact Two-Dimensional String Black Hole
We study the global structure of the exact two-dimensional space-time which
emerges from string theory. Previous work has shown that in the semi-classical
limit, this is a black hole similar to the Schwarzschild solution. However, we
find that in the exact case, a new Euclidean region appears "between" the
singularity and black hole interior. However the boundary between the
Lorentzian and Euclidean regions is a coordinate singularity, which turns out
to be a surface of time reflection symmetry in an extended space-time. Thus
strings having fallen through the black hole horizon would eventually emerge
through another one into a new asymptotically flat region. The maximally
extended space-time consists of an infinite number of universes connected by
wormholes. There are no singularities present in this geometry. We also
calculate the mass and temperature associated with the space-time.Comment: 9 pages, latex, DAMTP R93/
Numerical simulations of a kilometre-thick Arctic ice shelf consistent with ice grounding observations
Recently obtained geophysical data show sets of parallel erosional features on the Lomonosov
Ridge in the central Arctic Basin, indicative of ice grounding in water depths up to 1280
m. These features have been interpreted as being formed by an ice shelfâeither restricted to
the Amerasian Basin (the âminimum modelâ) or extending across the entire Arctic Basin.
Here, we use a numerical ice sheet-shelf model to explore how such an ice shelf could form.
We rule out the âminimum modelâ and suggest that grounding on the Lomonosov Ridge
requires complete Arctic ice shelf cover; this places a minimum estimate on its volume, which
would have exceeded that of the modern Greenland Ice Sheet. Buttressing provided by an
Arctic ice shelf would have increased volumes of the peripheral terrestrial ice sheets. An
Arctic ice shelf could have formed even in the absence of a hypothesised East Siberian Ice
Sheet
Axions In String Theory
In the context of string theory, axions appear to provide the most plausible
solution of the strong CP problem. However, as has been known for a long time,
in many string-based models, the axion coupling parameter F_a is several orders
of magnitude higher than the standard cosmological bounds. We re-examine this
problem in a variety of models, showing that F_a is close to the GUT scale or
above in many models that have GUT-like phenomenology, as well as some that do
not. On the other hand, in some models with Standard Model gauge fields
supported on vanishing cycles, it is possible for F_a to be well below the GUT
scale.Comment: 62 pages, v2; references, acknowledgements and minor corrections
adde
Branes, Instantons, And Taub-NUT Spaces
ALE and Taub-NUT (or ALF) hyper-Kahler four-manifolds can be naturally
constructed as hyper-Kahler quotients. In the ALE case, this construction has
long been understood in terms of D-branes; here we give a D-brane derivation in
the Taub-NUT case. Likewise, instantons on ALE spaces and on Taub-NUT spaces
have ADHM-like constructions related to hyper-Kahler quotients. Here we refine
the analysis in the Taub-NUT case by making use of a D-brane probe, and give an
application to M-theory.Comment: 63 p
Reflectance spectra of "featureless" materials and the surface mineralogies of M- and E-class asteroids
The 0.3- to 2.6-ÎŒm reflectance spectra of meteoritic enstatite (nearly pure MgSiO3), iron meteorite metal, magnetite and amorphous carbon, and various mixtures of these materials with mafic silicates have been measured in order to search for diagnostic spectral parameters which can be used to differentiate among the different materials and to provide information on the detection limits for mafic silicates. Enstatite and metal can be discriminated on the basis of albedo and spectral slopes. Metal,
magnetite, and amorphous carbon are variably effective at suppressing mafic silicate absorption bands. Metal-silicate spectra suggest that >25 wt% olivine must be present to be spectrally resolvable while the lower limit for orthopyroxene is ~10 wt %. Magnetite-silicate spectra indicate that magnetite is not an effective suppressor of mafic silicate absorption bands and that fine-grained magnetite imparts a less red overall slope to mafic silicate spectra than coarse-grained magnetite. Carbon-silicate spectra
indicate that suppression of olivme absorption bands is enhanced when the olivine is finely comminuted and is less effective at larger grain sizes. Carbon and magnetite both seem to be required to impart a blue overall slope to mafic silicate spectra and to suppress Fe2+ absorption bands. The small amount of carbon present in ordinary chondrites is probably not the cause of the flat reflectance spectra of metal-rich ordinary chondrite separates. The observational data for a representative M-class asteroid, (16) Psyche, are largely consistent with a fine-grained metal-rich surface assemblage, although an enstatite chondrite-like assemblage cannot be ruled out. The observational data for the E-class asteroid (44) Nysa indicate that its surface is composed of fine-grained material similar to enstatite achondrites (aubrites), with a small amount of material comparable to the chondritic inclusions found in the Cumberland Falls aubrite.This study was supported by grants-in-aid of research from the Central Research Fund of the University of Alberta (to D.G.W.S.), from Sigma Xi, The Scientific Research Society and the Geological Society of America 3741-87 (to E.A.C.) and NASA Planetary Geology and Geophysics grant NAGW 642 (to M.J.G.).https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JB095iB01p0028
Quantum Monte Carlo Calculations of Pion Scattering from Li
We show that the neutron and proton transition densities predicted by recent
quantum Monte Carlo calculations for A=6,7 nuclei are consistent with pion
scattering from 6Li and 7Li at energies near the Delta resonance. This has
provided a microscopic understanding of the enhancement factors for quadrople
excitations, which were needed to describe pion inelastic scattering within the
nuclear shell model of Cohen and Kurath.Comment: 10 pages, REVTeX, 3 postscript figures; added calculation of elastic
and inelastic pion scattering from 6Li at multiple energie
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Mutations in the Satellite Cell Gene MEGF10 Cause a Recessive Congenital Myopathy with Minicores
We ascertained a nuclear family in which three of four siblings were affected with an unclassified autosomal recessive myopathy characterized by severe weakness, respiratory impairment, scoliosis, joint contractures, and an unusual combination of dystrophic and myopathic features on muscle biopsy. Whole genome sequence from one affected subject was filtered using linkage data and variant databases. A single gene, MEGF10, contained nonsynonymous mutations that co-segregated with the phenotype. Affected subjects were compound heterozygous for missense mutations c.976T > C (p.C326R) and c.2320T > C (p.C774R). Screening the MEGF10 open reading frame in 190 patients with genetically unexplained myopathies revealed a heterozygous mutation, c.211C > T (p.R71W), in one additional subject with a similar clinical and histological presentation as the discovery family. All three mutations were absent from at least 645 genotyped unaffected control subjects. MEGF10 contains 17 atypical epidermal growth factor-like domains, each of which contains eight cysteine residues that likely form disulfide bonds. Both the p.C326R and p.C774R mutations alter one of these residues, which are completely conserved in vertebrates. Previous work showed that murine Megf10 is required for preserving the undifferentiated, proliferative potential of satellite cells, myogenic precursors that regenerate skeletal muscle in response to injury or disease. Here, knockdown of megf10 in zebrafish by four different morpholinos resulted in abnormal phenotypes including unhatched eggs, curved tails, impaired motility, and disorganized muscle tissue, corroborating the pathogenicity of the human mutations. Our data establish the importance of MEGF10 in human skeletal muscle and suggest satellite cell dysfunction as a novel myopathic mechanism
Past continental shelf evolution increased Antarctic ice sheet sensitivity to climatic conditions
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Exome sequencing identifies BRAF mutations in papillary craniopharyngiomas
The Eocene-Oligocene boundary climate transition: An Antarctic perspective
Antarctica underwent a complex evolution over the course of the Cenozoic, which influenced the history of the Earthâs climate system. The Eocene-Oligocene boundary is a divide of this history when the ice-free âgreenhouse worldâ transitioned to the âicehouseâ with the glaciation of Antarctica. Prior to this, Antarctica experienced warm climates, peaking during Early Eocene when tropical-like conditions existed at the margins of the continent where geological evidence is present. Climate signals in the geological record show that the climate then cooled, but not enough to allow the existence of significant ice until the latest Eocene. Glacial deposits from several areas around the continental margin indicate that ice was present by the earliest Oligocene. This matches the major oxygen isotope positive shift captured by marine records. On land, vegetation was able to persist, but the thermophylic plants of the Eocene were replaced by shrubby vegetation with the southern beech Nothofagus, mosses and ferns, which survived in tundra-like conditions. Coupled climateâice sheet modelling indicates that changing levels of atmospheric CO2 controlled Antarcticaâs climate and the onset of glaciation. Factors such as mountain uplift, vegetation changes, ocean gateway opening and orbital forcing all played a part in cooling the polar climate, but only when CO2 levels reached critical thresholds was Antarctica tipped into an icy glacial world
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