9,477 research outputs found

    A point mass in an isotropic universe: III. The region R≤2mR\leq 2m

    Full text link
    McVittie's solution of Einstein's field equations, representing a point mass embedded into an isotropic universe, possesses a scalar curvature singularity at proper radius R=2mR=2m. The singularity is space-like and precedes, in the expanding case, all other events in the space-time. It is shown here that this singularity is gravitationally weak, and the possible structure of the region R≤2mR\leq 2m is investigated. A characterization of this solution which does not involve asymptotics is given.Comment: Revtex, 11pp. To appear in Class.Quant.Grav. Paper II appeared as Class. Quant. Grav. 16 (1999) 122

    Causalities of war: The connection between type VI secretion system and microbiota

    Full text link
    Cellular Microbiology published by John Wiley & Sons Ltd Microbiota niches have space and/or nutrient restrictions, which has led to the coevolution of cooperation, specialisation, and competition within the population. Different animal and environmental niches contain defined resident microbiota that tend to be stable over time and offer protection against undesired intruders. Yet fluxes can occur, which alter the composition of a bacterial population. In humans, the microbiota are now considered a key contributor to maintenance of health and homeostasis, and its alteration leads to dysbiosis. The bacterial type VI secretion system (T6SS) transports proteins into the environment, directly into host cells or can function as an antibacterial weapon by killing surrounding competitors. Upon contact with neighbouring cells, the T6SS fires, delivering a payload of effector proteins. In the absence of an immunity protein, this results in growth inhibition or death of prey leading to a competitive advantage for the attacker. It is becoming apparent that the T6SS has a role in modulating and shaping the microbiota at multiple levels, which is the focus of this review. Discussed here is the T6SS, its role in competition, key examples of its effect upon the microbiota, and future avenues of researc

    Odd-parity perturbations of self-similar Vaidya spacetime

    Get PDF
    We carry out an analytic study of odd-parity perturbations of the self-similar Vaidya space-times that admit a naked singularity. It is found that an initially finite perturbation remains finite at the Cauchy horizon. This holds not only for the gauge invariant metric and matter perturbation, but also for all the gauge invariant perturbed Weyl curvature scalars, including the gravitational radiation scalars. In each case, `finiteness' refers to Sobolev norms of scalar quantities on naturally occurring spacelike hypersurfaces, as well as pointwise values of these quantities.Comment: 28 page

    A note on behaviour at an isotropic singularity

    Get PDF
    The behaviour of Jacobi fields along a time-like geodesic running into an isotropic singularity is studied. It is shown that the Jacobi fields are crushed to zero length at a rate which is the same in every direction orthogonal to the geodesic. We show by means of a counter-example that this crushing effect depends crucially on a technicality of the definition of isotropic singularities, and not just on the uniform degeneracy of the metric at the singularity.Comment: 13 pp. plain latex. To appear in Classical and Quantum Gravit

    Expansion-induced contribution to the precession of binary orbits

    Get PDF
    We point out the existence of new effects of global spacetime expansion on local binary systems. In addition to a possible change of orbital size, there is a contribution to the precession of elliptic orbits, to be added to the well-known general relativistic effect in static spacetimes, and the eccentricity can change. Our model calculations are done using geodesics in a McVittie metric, representing a localized system in an asymptotically Robertson-Walker spacetime; we give a few numerical estimates for that case, and indicate ways in which the model should be improved.Comment: revtex, 7 pages, no figures; revised for publication in Classical and Quantum Gravity, with minor changes in response to referees' comment

    Chrysotile biopersistence in the lungs of persons in the general population and exposed workers.

    Get PDF
    Lung burden analysis was performed on 126 autopsy cases of persons who died in New York City from 1966 through 1968. Of the 126 cases, 107 were probably non-occupationally exposed, judging by occupational history and asbestos body content of lung. Fifty-three of the 107 cases contained short chrysotile fibers/fibrils, < 5 microns in length, present in 3-fold greater amounts than were found in laboratory background controls. The fiber concentrations ranged from 1.8 to 15.7 x 10(6) f/gm/dry lung tissue, and the proportion of fibers > or = 5 microns in length was only 0.34% of the total chrysotile population found. Other inorganic particles present included fragments of amphiboles. In contrast to these data, the lung parenchyma of persons occupationally exposed to asbestos commonly showed the presence of other fiber types, especially amosite and crocidolite, at very much higher concentrations and greater fiber length. Any chrysotile present would usually be in fiber bundle form, with both fibers and fibrils > 5 microns in length. Comparison of the lung fiber content of occupationally exposed persons with that of the general population showed marked qualitative and quantitative differences. Fibers are durable, and are retained in a range of concentrations. Their length and dose, among other factors, which control their biological potential are different in the two populations; the risk factors for chrysotile-induced disease are not the same

    A process-based approach to health-related quality of life as a “way of living”

    Get PDF
    Introduction There is an historical initiative to establish common theoretical ground to support a framework for assessing health-related quality of life (HRQL). Our aim was to add to this effort with an analysis of theoretical/philosophical themes embedded in HRQL questionnaires and patient reports. Methods and Results We reviewed recent developments in HRQL assessment. This included analyzing a representative sample of psychometric measures of HRQL to schematically summarize core theoretical/philosophical themes that are embedded in questionnaire items. This analysis indicated a state-based framework for HRQL that was characterized by themes of hedonic and eudaimonic well-being, and desire-satisfaction. In contrast, a review of patient reports of HRQL indicated a process-based framework where goal-directed activities aimed to secure aspirational life goals while striving to accept the reality of declining health. Given this difference in HRQL themes we used a meta-philosophical approach, based on Hadot’s idea of philosophy as a way of living, to identify a process-based theoretical framework for HRQL assessment that addressed patient-reported themes. The Stoic modification of eudaimonic well-being was examined where HRQL and well-being are viewed as a process (vs. state) aimed at transforming the experience of loss or grief in response to adversity through goal-directed activities/exercises (euroia biou, good flow in life). We then introduced a complementary research agenda for HRQL assessment that incorporates self-reported, goal-directed activities that are initiated or maintained to promote HRQL. Conclusion A process-based approach to HRQL assessment may increase the spectrum of clinically relevant features that currently comprise operational measures of this patient-reported appraisal

    The Gamma Ray Pulsar Population

    Get PDF
    We apply a likelihood analysis to pulsar detections, pulsar upper limits, and diffuse background measurements from the OSSE and EGRET instruments on the Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray pulsars and some properties of the gamma-ray pulsar population. We find that the dependence of luminosity on spin period and dipole magnetic field is much steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively), suggesting that different emission mechanisms are responsible for low- and high-energy gamma-ray emission. Incorporating a spin-down model and assuming a pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray background due to unidentified pulsars and find that pulsars may be an important component of the OSSE diffuse flux, but are most likely not important at EGRET energies. Using measurements of the diffuse background flux from these instruments, we are able to place constraints on the braking index, initial spin period, and magnetic field of the Galactic pulsar population. We are also able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr). Our results are based on a large gamma-ray beam, but they do not scale in a simple way with beam size. With our assumed beam size, the implied gamma-ray efficiency for the EGRET detections is no more than 20%. We estimate that about 20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use our model to predict the pulsar population that will be seen by future gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray pulsars as steady sources, only 120 of which are currently known radio pulsars.Comment: 32 pages, including figures. submitted to Ap
    • …
    corecore