179,769 research outputs found

    The excitation spectrum of rotating strings with masses at the ends

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    We compute the spectrum of excitations of the rotating Nambu-Goto string with masses at the ends. We find interesting quasi-massless modes in the limit of slow rotation and comment on the nontrivial relation between world-sheet and target space energy.Comment: 13 pages, 1 figure, v2: Typos corrected, v3: fina

    An evolutionary model for simple ecosystems

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    In this review some simple models of asexual populations evolving on smooth landscapes are studied. The basic model is based on a cellular automaton, which is analyzed here in the spatial mean-field limit. Firstly, the evolution on a fixed fitness landscape is considered. The correspondence between the time evolution of the population and equilibrium properties of a statistical mechanics system is investigated, finding the limits for which this mapping holds. The mutational meltdown, Eigen's error threshold and Muller's ratchet phenomena are studied in the framework of a simplified model. Finally, the shape of a quasi-species and the condition of coexistence of multiple species in a static fitness landscape are analyzed. In the second part, these results are applied to the study of the coexistence of quasi-species in the presence of competition, obtaining the conditions for a robust speciation effect in asexual populations.Comment: 36 pages, including 16 figures, to appear in Annual Review of Computational Physics, D. Stauffer (ed.), World Scientific, Singapor

    Higher harmonics and ac transport from time dependent density functional theory

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    We report on dynamical quantum transport simulations for realistic molecular devices based on an approximate formulation of time-dependent Density Functional Theory with open boundary conditions. The method allows for the computation of various properties of junctions that are driven by alternating bias voltages. Besides the ac conductance for hexene connected to gold leads via thiol anchoring groups, we also investigate higher harmonics in the current for a benzenedithiol device. Comparison to a classical quasi-static model reveals that quantum effects may become important already for small ac bias and that the full dynamical simulations exhibit a much lower number of higher harmonics. Current rectification is also briefly discussed.Comment: submitted to J. Comp. Elec. (special issue

    Quasi-Metric Relativity

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    This is a survey of a new type of relativistic space-time framework; the so-called quasi-metric framework. The basic geometric structure underlying quasi-metric relativity is quasi-metric space-time; this is defined as a 4-dimensional differentiable manifold N{\cal N} equipped with two one-parameter families gˉt{\bf {\bar g}}_t and gt{\bf g}_t of Lorentzian 4-metrics parametrized by a global time function tt. The metric family gˉt{\bf {\bar g}}_t is found from field equations, whereas the metric family gt{\bf g}_t is used to propagate sources and to compare predictions to experiments. A linear and symmetric affine connection compatible with the family gt{\bf g}_t is defined, giving rise to equations of motion. Furthermore a quasi-metric theory of gravity, including field equations and local conservation laws, is presented. Just as for General Relativity, the field equations accommodate two independent propagating dynamical degrees of freedom. On the other hand, the particular structure of quasi-metric geometry allows only a partial coupling of space-time geometry to the active stress-energy tensor. Besides, the field equations are defined from projections of physical and geometrical tensors with respect to a ``preferred'' foliation of quasi-metric space-time into spatial hypersurfaces. The dynamical nature of this foliation makes the field equations unsuitable for a standard PPN-analysis. This implies that the experimental status of the theory is not completely clear at this point in time. The theory seems to be consistent with a number of cosmological observations and it satisfies all the classical solar system tests, though. Moreover, in its non-metric sector the new theory has experimental support where General Relativity fails or is irrelevant.Comment: 39 pages, no figures, LaTeX; v2: some points clarified; v3: connection changed; v4: extended and local conservation laws changed; v5: major revision; v6: accepted for publication in G&C; v7: must have non-universal gravitational coupling; v8: rewritten with fully coupled theory; v9: major revision (fully coupled theory abandoned

    Vacuum solutions of the gravitational field equations in the brane world model

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    We consider some classes of solutions of the static, spherically symmetric gravitational field equations in the vacuum in the brane world scenario, in which our Universe is a three-brane embedded in a higher dimensional space-time. The vacuum field equations on the brane are reduced to a system of two ordinary differential equations, which describe all the geometric properties of the vacuum as functions of the dark pressure and dark radiation terms (the projections of the Weyl curvature of the bulk, generating non-local brane stresses). Several classes of exact solutions of the vacuum gravitational field equations on the brane are derived. In the particular case of a vanishing dark pressure the integration of the field equations can be reduced to the integration of an Abel type equation. A perturbative procedure, based on the iterative solution of an integral equation, is also developed for this case. Brane vacuums with particular symmetries are investigated by using Lie group techniques. In the case of a static vacuum brane admitting a one-parameter group of conformal motions the exact solution of the field equations can be found, with the functional form of the dark radiation and pressure terms uniquely fixed by the symmetry. The requirement of the invariance of the field equations with respect to the quasi-homologous group of transformations also imposes a unique, linear proportionality relation between the dark energy and dark pressure. A homology theorem for the static, spherically symmetric gravitational field equations in the vacuum on the brane is also proven.Comment: 13 pages, no figures, to appear in PR

    Large Scale Structure Formation of Normal Branch in DGP Brane World Model

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    In this paper, we study the large scale structure formation of the normal branch in DGP model (Dvail, Gabadadze and Porrati brane world model) by applying the scaling method developed by Sawicki, Song and Hu for solving the coupled perturbed equations of motion of on-brane and off-brane. There is detectable departure of perturbed gravitational potential from LCDM even at the minimal deviation of the effective equation of state w_eff below -1. The modified perturbed gravitational potential weakens the integrated Sachs-Wolfe effect which is strengthened in the self-accelerating branch DGP model. Additionally, we discuss the validity of the scaling solution in the de Sitter limit at late times.Comment: 6 pages, 2 figure

    Extra dimensions and the strong CP problem

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    In higher-dimensional theories such as Brane World models with quasi-localized non-Abelian gauge fields the vacuum structure turns out to be trivial. Since the gauge theory behaves at large distances as a 4+δ4+\delta-dimensional and thus the topology of the infinity is that of of S3+δS^{3+\delta} rather than S3S^{3}, the set of gauge mappings are homotopically trivial and the CP-violating θ\theta-term vanishes on the brane world-volume. As well there are no contributions to the θ\theta-term from the higher-dimensional solitonic configurations. In this way, the strong CP problem is absent in the models with quasi-localized gluons.Comment: 7 pp., latex, no figure
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