44 research outputs found

    Possible black universes in a brane world

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    A black universe is a nonsingular black hole where, beyond the horizon, there is an expanding, asymptotically isotropic universe. Such spherically symmetric configurations have been recently found as solutions to the Einstein equations with phantom scalar fields (with negative kinetic energy) as sources of gravity. They have a Schwarzschild-like causal structure but a de Sitter infinity instead of a singularity. It is attempted to obtain similar configurations without phantoms, in the framework of an RS2 type brane world scenario, considering the modified Einstein equations that describe gravity on the brane. By building an explicit example, it is shown that black-universe solutions can be obtained there in the presence of a scalar field with positive kinetic energy and a nonzero potential.Comment: 8 pages, 5 figures, gc styl

    Wave interaction with defects in pressurised composite structures

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    There exists a great variety of structural failure modes which must be frequently inspected to ensure continuous structural integrity of composite structures. This work presents a Finite Element (FE) based method for calculating wave interaction with damage within structures of arbitrary layering and geometric complexity. The principal novelty is the investigation of pre-stress effect on wave propagation and scattering in layered structures. A Wave Finite Element (WFE) method, which combines FE analysis with periodic structure theory (PST), is used to predict the wave propagation properties along periodic waveguides of the structural system. This is then coupled to the full FE model of a coupling joint within which structural damage is modelled, in order to quantify wave interaction coeffcients through the joint. Pre-stress impact is quantified by comparison of results under pressurised and non-pressurised scenarios. The results show that including these pressurisation effects in calculations is essential. This is of specific relevance to aircraft structures being intensely pressurised while on air. Numerical case studies are exhibited for different forms of damage type. The exhibited results are validated against available analytical and experimental results

    The impact of negative selection on thymocyte migration in the medulla

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    Developing thymocytes are screened for self-reactivity before they exit the thymus, but how thymocytes scan the medulla for self antigens is unclear. Using two-photon microscopy, we observed that medullary thymocytes migrated rapidly and made frequent, transient contacts with dendritic cells. In the presence of a negative selecting ligand, thymocytes slowed, became confined to areas of approximately 30 mum in diameter and had increased contact with dendritic cells surrounding confinement zones. One third of polyclonal medullary thymocytes also showed confined, slower migration and may correspond to autoreactive thymocytes. Our data suggest that many autoreactive thymocytes do not undergo immediate arrest and death after encountering a negative selecting ligand but instead adopt an altered migration program while remaining in the medullary microenvironment

    The earliest thymic T cell progenitors sustain B cell and myeloid lineage potential

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    The stepwise commitment from hematopoietic stem cells in the bone marrow to T lymphocyte-restricted progenitors in the thymus represents a paradigm for understanding the requirement for distinct extrinsic cues during different stages of lineage restriction from multipotent to lineage-restricted progenitors. However, the commitment stage at which progenitors migrate from the bone marrow to the thymus remains unclear. Here we provide functional and molecular evidence at the single-cell level that the earliest progenitors in the neonatal thymus had combined granulocyte-monocyte, T lymphocyte and B lymphocyte lineage potential but not megakaryocyte-erythroid lineage potential. These potentials were identical to those of candidate thymus-seeding progenitors in the bone marrow, which were closely related at the molecular level. Our findings establish the distinct lineage-restriction stage at which the T cell lineage-commitment process transits from the bone marrow to the remote thymus. © 2012 Nature America, Inc. All rights reserved

    Study of the influence of the basic spectra of discrete light sources on the seeds of greenhouse cultures

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    © Published under licence by IOP Publishing Ltd. This article refers to studies conducted on the seeds of vegetable and green crops with low useful mass, which require light stimulation at all periods of development. Research was conducted using RGBW LEDs. There is a large number of scientific studies proving the effectiveness of the technology of photostimulation of seeds and seedlings of various crops. However, there is no consensus on what the positive effect of the variable light field on a biological object is based. This is due to the complexity of the process of photosynthesis, where the productivity of photosynthesis is considered as an integral response to the influence of external conditions. The study of the influence of various light modes on plants is necessary to search for performance criteria. The found efficiency criteria will provide an opportunity to systematize the many well-known results of the impact of variable light conditions on plants in order to identify plants as a biological object by optical properties, as well as to use to standardize technological lighting in greenhouse production

    Acoustic metamaterials with piezoelectric resonant structures

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    International audienceA resonant structure of a hard-core coated by piezoelectric composite materials is proposed as an acoustic metamaterial (AM), in which a negative effective mass density and elastic modulus are simultaneously achieved. The double negativity, appearing within a certain range of the filling ratio, is numerically demonstrated by the switch of the electrical boundary from open to closed. The bandwidth of the negative effective elastic modulus is sensitive to the piezoelectric constant e(33). The multi-unit AM offers the advantages of broadening the double-negativity domain and of reducing the primary frequency, while the cut-up frequency remains the same as that of the single-unit cell AM
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