27,452 research outputs found
Spacetime Emergence in the Robertson-Walker Universe from a Matrix model
Using a novel, string theory-inspired formalism based on a Hamiltonian
constraint, we obtain a conformal mechanical system for the spatially flat
four-dimensional Robertson-Walker Universe. Depending on parameter choices,
this system describes either a relativistic particle in the Robertson-Walker
background, or metric fluctuations of the Robertson-Walker geometry. Moreover
we derive a tree-level M-theory matrix model in this time-dependent background.
Imposing the Hamiltonian constraint forces the spacetime geometry to be fuzzy
near the big bang, while the classical Robertson-Walker geometry emerges as the
Universe expands. From our approach we also derive the temperature of the
Universe interpolating between the radiation and matter dominated eras.Comment: 4 pages - accepted for publication in Physical Review Letter
The polymeric stability of the Escherichia coli F4 (K88) fimbriae enhances its mucosal immunogenicity following oral immunization
<p>Only a few vaccines are commercially available against intestinal infections since the induction of a protective intestinal immune response is difficult to achieve. For instance, oral administration of most proteins results in oral tolerance instead of an antigen-specific immune response. We have shown before that as a result of oral immunization of piglets with F4 fimbriae purified from pathogenic enterotoxigenic Escherichia coli (ETEC), the fimbriae bind to the F4 receptor (F4R) in the intestine and induce a protective F4-specific immune response. F4 fimbriae are very stable polymeric structures composed of some minor subunits and a major subunit FaeG that is also the fimbrial adhesin. In the present study, the mutagenesis experiments identified FaeG amino acids 97 (N to K) and 201 (I to V) as determinants for F4 polymeric stability. The interaction between the FaeG subunits in mutant F4 fimbriae is reduced but both mutant and wild type fimbriae behaved identically in F4R binding and showed equal stability in the gastro-intestinal lumen. Oral immunization experiments indicated that a higher degree of polymerisation of the fimbriae in the intestine was correlated with a better F4-specific mucosal immunogenicity. These data suggest that the mucosal immunogenicity of soluble virulence factors can be increased by the construction of stable polymeric structures and therefore help in the development of effective mucosal vaccines.</p></p
Metodologia para determinação da zona de abscisão foliar em pomáceas.
Estudos anatômicos e de ontogênese da abscisão foliar em pomáceas são escassos. A queda foliar promovida após formada a zona de abscisão é um processo natural nas pomáceas, e prolonga-se durante o outono, até que ocorram as primeiras geadas no Sul do Brasil. Ainda não se conhece até quando as folhas de macieiras permanecem funcionais. O objetivo deste trabalho foi desenvolver uma metodologia que permita estudar o inÃcio e o fi m das zonas de abscisão foliar de macieiras. O trabalho foi realizado nas cultivares Galaxy e Fuji em pomares comerciais de macieira, de Vacaria, Rio Grande do Sul. O método proposto consistiu em coletar,ramos do ano, com 10cm a 20cm de comprimento, e utilizar um segmento contendo uma parte do caule, uma gema axilar e a base do pecÃolo, descartando a parte das quatro folhas do ápice. Os segmentos foram fixados, seccionados e corados para a observação da zona de abscisão em microscopia óptica. Os resultados mostraram que a metodologia utilizada permitiu descrever a zona de abscisão em macieiras e que ela é formada por células alongadas com parede celular delgada, apresentando lignifi cação junto à parede celular. O processo manteve a gema intacta e ocorreu no sentido adaxial para o abaxial. A abscisão foliar natural, em plantas de Galaxy e Fuji em sintomas de doença e em condições ambientais favoráveis, pode ser acompanhada 7 e 14 dias antes e depois da colheita, e após, com intervalos de 14 e 21 dias até maio. A partir de maio, as coletas devem ser feitas semanalmente. Essas informações serão úteis na tomada de decisão para realizar práticas culturais e/ou de controle quÃmico, para a manutenção das folhas sadias, resultando no maior acúmulo de reservas de carboidratos e/ou redução do número de aplicações de fungicidas
Chaos Synchronization and Spontaneous Symmetry Breaking in Symmetrically Delay Coupled Semiconductor Lasers
PACS: 05.45.Xt, 42.55.Px, 42.65.SfWe present experimental and numerical investigations of the dynamics of two device-identical, optically coupled semiconductor lasers exhibiting a delay in the coupling. Our results give evidence for subnanosecond coupling-induced synchronized chaotic dynamics in conjunction with a spontaneous symmetry-breaking: we find a well-defined time lag between the dynamics of the two lasers, and an asymmetric physical role of the subsystems. We demonstrate that the leading laser synchronizes its lagging counterpart, whereas the synchronized lagging laser drives the coupling-induced instabilities.Peer reviewe
Order and Frustration in Chiral Liquid Crystals
This paper reviews the complex ordered structures induced by chirality in
liquid crystals. In general, chirality favors a twist in the orientation of
liquid-crystal molecules. In some cases, as in the cholesteric phase, this
favored twist can be achieved without any defects. More often, the favored
twist competes with applied electric or magnetic fields or with geometric
constraints, leading to frustration. In response to this frustration, the
system develops ordered structures with periodic arrays of defects. The
simplest example of such a structure is the lattice of domains and domain walls
in a cholesteric phase under a magnetic field. More complex examples include
defect structures formed in two-dimensional films of chiral liquid crystals.
The same considerations of chirality and defects apply to three-dimensional
structures, such as the twist-grain-boundary and moire phases.Comment: 39 pages, RevTeX, 14 included eps figure
Vertical Structure of the Outer Accretion Disk in Persistent Low-Mass X-Ray Binaries
We have investigated the influence of X-ray irradiation on the vertical
structure of the outer accretion disk in low-mass X-ray binaries by performing
a self-consistent calculation of the vertical structure and X-ray radiation
transfer in the disk. Penetrating deep into the disk, the field of scattered
X-ray photons with energy \,keV exerts a significant influence on
the vertical structure of the accretion disk at a distance
\,cm from the neutron star. At a distance \,cm,
where the total surface density in the disk reaches
\,g\,cm, X-ray heating affects all layers of an
optically thick disk. The X-ray heating effect is enhanced significantly in the
presence of an extended atmospheric layer with a temperature
\,K above the accretion disk. We have derived
simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This
approximation has a \,% accuracy in the range of X-ray photon
energies \,keV.Comment: 19 pages, 8 figures, published in Astronomy Letter
Wavelets: mathematics and applications
The notion of wavelets is defined. It is briefly described {\it what} are
wavelets, {\it how} to use them, {\it when} we do need them, {\it why} they are
preferred and {\it where} they have been applied. Then one proceeds to the
multiresolution analysis and fast wavelet transform as a standard procedure for
dealing with discrete wavelets. It is shown which specific features of signals
(functions) can be revealed by this analysis, but can not be found by other
methods (e.g., by the Fourier expansion). Finally, some examples of practical
application are given (in particular, to analysis of multiparticle production}.
Rigorous proofs of mathematical statements are omitted, and the reader is
referred to the corresponding literature.Comment: 16 pages, 5 figures, Latex, Phys. Atom. Nuc
Local Magnetic Anomalies Explain Bias in Paleomagnetic Data: Consequences for Sampling
Volcanic rocks are considered reliable recorders of past changes in the Earth's magnetic field. Recent flows, however, sometimes fail to produce the known magnetic field at the time of cooling. Previous research on Mt. Etna suggests paleomagnetic data might not be accurately recorded. Here we test the accuracy of paleomagnetic data obtained from Mt. Etna lavas by comparing paleomagnetic data from historical flows to direct measurements of the magnetic field above the current topography. The inclinations and intensities in both data sets are biased toward lower values, while there is no such trend for the declination. Inclinations are on average 2.9° lower than expected; intensities are on average 8.8 µT lower. The deviations from the expected values depend on the height above the flow. Moreover, the inclinations and intensities vary as a function of topography. Both are higher above ridges and lower in gullies; the variations within a site are up to 14.1° in inclination and 12.9 µT for intensity. To suppress this paleomagnetic data bias it is important to take samples several meters apart and from different parts of the flow whenever possible. While this leads to a higher degree of scatter in paleodirections, the results better represent the Earth's magnetic field at the time of cooling. This emphasizes the importance of reporting paleomagnetic sampling strategies in detail
Limit-(quasi)periodic point sets as quasicrystals with p-adic internal spaces
Model sets (or cut and project sets) provide a familiar and commonly used
method of constructing and studying nonperiodic point sets. Here we extend this
method to situations where the internal spaces are no longer Euclidean, but
instead spaces with p-adic topologies or even with mixed Euclidean/p-adic
topologies.
We show that a number of well known tilings precisely fit this form,
including the chair tiling and the Robinson square tilings. Thus the scope of
the cut and project formalism is considerably larger than is usually supposed.
Applying the powerful consequences of model sets we derive the diffractive
nature of these tilings.Comment: 11 pages, 2 figures; dedicated to Peter Kramer on the occasion of his
65th birthda
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