974 research outputs found
Black hole pair creation and the stability of flat space
We extend the Gross-Perry-Yaffe approach of hot flat space instability to
Minkowski space. This is done by a saddle point approximation of the partition
function in a Schwarzschild wormhole background which is coincident with an
eternal black hole. The appearance of an instability in the whole manifold is
here interpreted as a black hole pair creation.Comment: 11 pages,RevTeX4, 2 figures. Accepted for publication in Int. J. Mod.
Phys.
The cosmological constant as an eigenvalue of the Hamiltonian constraint in Horava-Lifshits theory
In the framework of Horava-Lifshitz theory, we study the eigenvalues
associated with the Wheeler-DeWitt equation representing the vacuum expectation
values associated with the cosmological constant. The explicit calculation is
performed with the help of a variational procedure with trial wave functionals
of the Gaussian type. We analyze both the case with the detailed balanced
condition and the case without it. In the case without the detailed balance, we
find the existence of an eigenvalue depending on the set of coupling constants
(g2,g3) and (g4,g5,g6), respectively, and on the physical scale.Comment: RevTeX,11 Pages, Substantial Improvements. References added. To
appear in Phys.Rev.
How realistic are solar model atmospheres?
Recently, new solar model atmospheres have been developed to replace
classical 1D LTE hydrostatic models and used to for example derive the solar
chemical composition. We aim to test various models against key observational
constraints. In particular, a 3D model used to derive the solar abundances, a
3D MHD model (with an imposed 10 mT vertical magnetic field), 1D models from
the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical model of
Holweger & M\"uller. We confront the models with observational diagnostics of
the temperature profile: continuum centre-to-limb variations (CLV), absolute
continuum fluxes, and the wings of hydrogen lines. We also test the 3D models
for the intensity distribution of the granulation and spectral line shapes. The
predictions from the 3D model are in excellent agreement with the continuum CLV
observations, performing even better than the Holweger & M\"uller model
(constructed largely to fulfil such observations). The predictions of the 1D
theoretical models are worse, given their steeper temperature gradients. For
the continuum fluxes, predictions for most models agree well with the
observations. No model fits all hydrogen lines perfectly, but again the 3D
model comes ahead. The 3D model also reproduces the observed continuum
intensity fluctuations and spectral line shapes very well. The excellent
agreement of the 3D model with the observables reinforces the view that its
temperature structure is realistic. It outperforms the MHD simulation in all
diagnostics, implying that recent claims for revised abundances based on MHD
modelling are premature. Several weaknesses in the 1D models are exposed. The
differences between the PHOENIX LTE and NLTE models are small. We conclude that
the 3D hydrodynamical model is superior to any of the tested 1D models, which
gives further confidence in the solar abundance analyses based on it.Comment: 17 pages, 15 figures. Accepted for publication in A&
Staging Glaucoma Patient: Why and How?
Staging glaucomatous damage into appropriatecategories enhances management of the disease. Automated static perimetry is the benchmark for testing visual function in glaucoma. Numerous examples of standard automated perimetry staging systems have been proposed but difficulties such as lack of accuracy, absence of information related to location and depth of the defect(s) and need of time-consuming analysis of every visual field test result may reduce their day-to-day clinical usefulness.A new visual field staging system is proposed: the University of SĂŁo Paulo Glaucoma Visual Field Staging System (USP-GVFSS). In this system, qualitative and quantitative characteristics of the visual field defect are described. The method is intuitive, comprehensible and describes severity, extension and hemi field involvement
Experimental maps of DNA structure at nucleotide resolution distinguish intrinsic from protein-induced DNA deformations
Recognition of DNA by proteins depends on DNA sequence and structure. Often unanswered is whether the structure of naked DNA persists in a proteinâDNA complex, or whether protein binding changes DNA shape. While X-ray structures of proteinâDNA complexes are numerous, the structure of naked cognate DNA is seldom available experimentally. We present here an experimental and computational analysis pipeline that uses hydroxyl radical cleavage to map, at single-nucleotide resolution, DNA minor groove width, a recognition feature widely exploited by proteins. For 11 proteinâDNA complexes, we compared experimental maps of naked DNA minor groove width with minor groove width measured from X-ray co-crystal structures. Seven sites had similar minor groove widths as naked DNA and when bound to protein. For four sites, part of the DNA in the complex had the same structure as naked DNA, and part changed structure upon protein binding. We compared the experimental map with minor groove patterns of DNA predicted by two computational approaches, DNAshape and ORChID2, and found good but not perfect concordance with both. This experimental approach will be useful in mapping structures of DNA sequences for which high-resolution structural data are unavailable. This approach allows probing of protein family-dependent readout mechanisms.National Institutes of Health [R01GM106056 to R.R., T.D.T.; U54CA121852 in part to T.D.T.]; Boston University Undergraduate Research Opportunities Program [Faculty Matching Grants to D.O. and Y.J.]; USC Graduate School [Research Enhancement Fellowship and Manning Endowed Fellowship to T.P.C.]. R.R. is an Alfred P. Sloan Research Fellow. Funding for open access charge: Boston University. (R01GM106056 - National Institutes of Health; U54CA121852 - National Institutes of Health; Boston University Undergraduate Research Opportunities Program; USC Graduate School; Boston University)https://academic.oup.com/nar/article/46/5/2636/4829691?searchresult=1https://academic.oup.com/nar/article/46/5/2636/4829691?searchresult=1Published versio
Surface induced magnetization reversal of MnP nanoclusters embedded in GaP
We investigate the quasi-static magnetic behavior of ensembles of
non-interacting ferromagnetic nanoparticles consisting of MnP nanoclusters
embedded in GaP(001) epilayers grown at 600, 650 and 700{\deg}C. We use a
phenomenological model, in which surface effects are included, to reproduce the
experimental hysteresis curves measured as a function of temperature (120-260
K) and direction of the applied field. The slope of the hysteresis curve during
magnetization reversal is determined by the MnP nanoclusters size distribution,
which is a function of the growth temperature. Our results show that the
coercive field is very sensitive to the strength of the surface anisotropy,
which reduces the energy barrier between the two states of opposite
magnetization. Notably, this reduction in the energy barrier increases by a
factor of 3 as the sample temperature is lowered from 260 to 120 K.Comment: 7 pages, 5 figure
Multiâmodel assessment of subâseasonal predictive skill for yearâround AtlanticâEuropean weather regimes
Constraint-based protocols for distributed problem solving
AbstractDistributed Problem Solving (DPS) approaches decompose problems into subproblems to be solved by interacting, cooperative software agents. Thus, DPS is suitable for solving problems characterized by many interdependencies among subproblems in the context of parallel and distributed architectures. Concurrent Constraint Programming (CCP) provides a powerful execution framework for DPS where constraints define local problem solving and the exchange of information among agents declaratively. To optimize DPS, the protocol for constraint communication must be tuned to the specific kind of DPS problem and the characteristics of the underlying system architecture. In this paper, we provide a formal framework for modeling different problems and we show how the framework applies to simple yet generalizable examples
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