24,111 research outputs found

    Regulating the infrared by mode matching: A massless scalar in expanding spaces with constant deceleration

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    In this paper we consider a massless scalar field, with a possible coupling ξ\xi to the Ricci scalar in a DD dimensional FLRW spacetime with a constant deceleration parameter q=ϵ−1q=\epsilon-1, ϵ=−H˙/H2\epsilon=-{\dot{H}}/{H^2}. Correlation functions for the Bunch-Davies vacuum of such a theory have long been known to be infrared divergent for a wide range of values of ϵ\epsilon. We resolve these divergences by explicitly matching the spacetime under consideration to a spacetime without infrared divergencies. Such a procedure ensures that all correlation functions with respect to the vacuum in the spacetime of interest are infrared finite. In this newly defined vacuum we construct the coincidence limit of the propagator and as an example calculate the expectation value of the stress energy tensor. We find that this approach gives both in the ultraviolet and in the infrared satisfactory results. Moreover, we find that, unless the effective mass due to the coupling to the Ricci scalar ξR\xi R is negative, quantum contributions to the energy density always dilute away faster, or just as fast, as the background energy density. Therefore, quantum backreaction is insignificant at the one loop order, unless ξR\xi R is negative. Finally we compare this approach with known results where the infrared is regulated by placing the Universe in a finite box. In an accelerating universe, the results are qualitatively the same, provided one identifies the size of the Universe with the physical Hubble radius at the time of the matching. In a decelerating universe however, the two schemes give different late time behavior for the quantum stress energy tensor. This happens because in this case the length scale at which one regulates the infrared becomes sub-Hubble at late times.Comment: 55 pages, 6 figure

    Observations of the binary pulsar system PSR B1718-19 -- The Role of Tidal Circularisation

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    We present optical and infrared observations taken with the Very Large Telescope of the eclipsing binary pulsar system PSR B1718-19. The candidate companion of the pulsar, identified earlier in Hubble Space Telescope observations, has been detected in all three bands, R, I, and J. These detections allowed us to derive constraints on temperature, radius, and mass, pointing to a companion that has expanded to a radius between one of a main sequence star and one at the Roche-limit. We focus on the role of tidal circularisation in the system, which will have transformed the initially eccentric orbit expected from formation scenarios into the nearly circular orbit presently observed. Based on simple energy balance arguments, we are able to draw a picture of the companion's evolution resulting from the energy deposition in the star due to circularisation. In this picture, our measurement of the companion's parameters is consistent with the expected initial eccentricity. However, with the present understanding of tidal dissipation it remains difficult to account for the short time in which the system was circularised.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

    Dynamics of modulated and composite aperiodic crystals: the signature of the inner polarization in the neutron coherent inelastic scattering

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    We compare within an unifying formalism the dynamical properties of modulated and composite aperiodic (incommensurate) crystals. We discuss the concept of inner polarization and we define an inner polarization parameter beta that distinguishes between different acoustic modes of aperiodic crystals. Although this concept has its limitations, we show that it can be used to extract valuable information from neutron coherent inelastic scattering experiments. Within certain conditions, the ratio between the dynamic and the static structure factors at various Bragg peaks depends on beta. We show how the knowledge of beta for modes of an unknown structure can be used to decide whether the structure is composite or modulated. Furthermore, the same information can be used to predict scattered intensity within unexplored regions of the reciprocal space, being thus a guide for experiment

    Entropic Stabilization of Tunable Planar Modulated Superstructures

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    Self-assembling novel ordered structures with nanoparticles has recently received much attention. Here we use computer simulations to study a two-dimensional model system characterized by a simple isotropic interaction that could be realized with building blocks on the nanoscale. We find that the particles arrange themselves into hexagonal superstructures of twin boundaries whose superlattice vector can be tuned reversibly by changing the temperature. Thermodynamic stability is confirmed by calculating the free energy with a combination of thermodynamic integration and the Frenkel-Ladd method. Different contributions to the free energy difference are discussed.Comment: 4 pages, 5 figures plus 7 pages of supplementary figure

    Monte Carlo simulations of the classical two-dimensional discrete frustrated Ï•4\phi ^4 model

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    The classical two-dimensional discrete frustrated ϕ4\phi ^4 model is studied by Monte Carlo simulations. The correlation function is obtained for two values of a parameter dd that determines the frustration in the model. The ground state is a ferro-phase for d=−0.35d=-0.35 and a commensurate phase with period N=6 for d=−0.45d=-0.45. Mean field predicts that at higher temperature the system enters a para-phase via an incommensurate state, in both cases. Monte Carlo data for d=−0.45d=-0.45 show two phase transitions with a floating-incommensurate phase between them. The phase transition at higher temperature is of the Kosterlitz-Thouless type. Analysis of the data for d=−0.35d=-0.35 shows only a single phase transition between the floating-fluid phase and the ferro-phase within the numerical error.Comment: 5 figures, submitted to the European Physical Journal

    Problems and hopes in nonsymmetric gravity

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    We consider the linearized nonsymmetric theory of gravitation (NGT) within the background of an expanding universe and near a Schwarzschild mass. We show that the theory always develops instabilities unless the linearized nonsymmetric lagrangian reduces to a particular simple form. This form contains a gauge invariant kinetic term, a mass term for the antisymmetric metric-field and a coupling with the Ricci curvature scalar. This form cannot be obtained within NGT. Based on the linearized lagrangian we know to be stable, we consider the generation and evolution of quantum fluctuations of the antisymmetric gravitational field (B-field) from inflation up to the present day. We find that a B-field with a mass m ~ 0.03(H_I/10^(13)GeV)^4 eV is an excellent dark matter candidate.Comment: 9 pages, 1 figure. Based on two talks by the authors at the 2nd International Conference on Quantum Theories and Renormalization Group in Gravity and Cosmology (IRGAC) 2006, Barcelon

    High voltage solid-state relay

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    Hybrid microelectronics relay has characteristics significantly superior to conventional solid state relays. Relay provides 2500 Vdc input to output isolation and operates from high threshold logic signal to switch load of 400 Vdc at 2 mA. Technology should be of interest to manufacturers of discrete components

    Bridging Two Ways of Describing Final-State Interactions in A(e,e'p) Reactions

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    We outline a relativistic and unfactorized framework to treat the final-state interactions in quasi-elastic A(e,e'p) reactions for four-momentum transfers Q2≳0.3^{2} \gtrsim 0.3 (GeV/c)2^{2}. The model, which relies on the eikonal approximation, can be used in combination with optical potentials, as well as with the Glauber multiple-scattering method. We argue that such a model can bridge the gap between a typical ``low'' and ``high-energy'' description of final-state interactions, in a reasonably smooth fashion. This argument is made on the basis of calculated structure functions, polarization observables and nuclear transparencies for the target nuclei 12^{12}C and 16^{16}O.Comment: revised versio

    Perceptions of physiotherapists towards research: a mixed methods study

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    OBJECTIVES: To explore the perceptions of physiotherapists towards the use of and participation in research. DESIGN: Concurrent mixed methods research, combining in-depth interviews with three questionnaires (demographics, Edmonton Research Orientation Survey, visual analogue scales for confidence and motivation to participate in research). SETTING: One physiotherapy department in a rehabilitation hospital, consisting of seven specialised areas. PARTICIPANTS: Twenty-five subjects {four men and 21 women, mean age 38 [standard deviation (SD) 11] years} who had been registered as a physiotherapist for a mean period of 15 (SD 10) years participated in this study. They were registered with the New Zealand Board of Physiotherapy, held a current practising certificate, and were working as a physiotherapist or physiotherapy/allied health manager at the hospital. MAIN OUTCOME MEASURE: The primary outcome measure was in-depth interviews and the secondary outcome measures were the three questionnaires. RESULTS: Physiotherapists were generally positive towards research, but struggled with the concept of research, the available literature and the time to commit to research. Individual confidence and orientation towards research seemed to influence how these barriers were perceived. CONCLUSION: This study showed that physiotherapists struggle to implement research in their daily practice and become involved in research. Changing physiotherapists' conceptions of research, making it more accessible and providing dedicated research time could facilitate increased involvement in the physiotherapy profession
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