234,288 research outputs found

    Limits on Cosmic Chiral Vortons

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    We study chiral vorton production for Witten-type superconducting string models in the context of a recently developed analytic formalism. We delineate three distinct scenarios: First, a low energy regime (including the electroweak scale) where vortons can be a source of dark matter. Secondly, an intermediate energy regime where the vorton density is too high to be compatible with the standard cosmology (thereby excluding these models). Finally, a high energy regime (including the GUT scale) in which no vortons are expected to form. The vorton density is most sensitive to the order of the string-forming phase transition and relatively insensitive to the current-forming transition. For a second-order string transition, vorton production is cosmologically disastrous for the range 10^{-28}\lsim G\mu \lsim 10^{-10} (10^{5} GeV \lsim T_{c} \lsim 10^{14} GeV), while for the first-order case we can only exclude 10^{-20}\lsim G\mu \lsim 10^{-14} (10^{9} GeV \lsim T_{c} \lsim 10^{12} GeV). We provide a fitting formula which summarises our results.Comment: 9 LaTeX pages, 5 .eps files; submitted to Phys.Lett.

    Accurate Calibration of the Velocity-dependent One-scale Model for Domain Walls

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    We study the asymptotic scaling properties of standard domain wall networks in several cosmological epochs. We carry out the largest field theory simulations achieved to date, with simulation boxes of size 20483, and confirm that a scale-invariant evolution of the network is indeed the attractor solution. The simulations are also used to obtain an accurate calibration for the velocity-dependent one-scale model for domain walls: we numerically determine the two free model parameters to have the values cw=0.34±0.16c_w = 0.34\pm0.16 and kw=0.98±0.07k_w = 0.98\pm0.07, which are higher precision than (but in agreement with) earlier estimates.Comment: 8 pages, version to appear in Phys. Lett. B. arXiv admin note: substantial text overlap with arXiv:1110.348

    Simulated emergence of cyclic sexual-asexual reproduction

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    Motivated by the cyclic pattern of reproductive regimes observed in some species of green flies (``{\it aphids}''), we simulate the evolution of a population enduring harsh seasonal conditions for survival. The reproductive regime of each female is also seasonal in principle and genetically acquired, and can mutate for each newborn with some small probability. The results show a sharp transition at a critical value of the survival probability in the winter, between a reproductive regime in the fall that is predominantly sexual, for low values of this probability, or asexual, for high values.Comment: 9 pages, 4 figures, requires RevTe

    UV, optical and near-IR diagnostics of massive stars

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    We present an overview of a few spectroscopic diagnostics of massive stars. We explore the following wavelength ranges: UV (1000 to 2000 A), optical (4000--7000 A) and near-infrared (mainly H and K bands). The diagnostics we highlight are available in O and Wolf-Rayet stars as well as in B supergiants. We focus on the following parameters: effective temperature, gravity, surface abundances, luminosity, mass loss rate, terminal velocity, wind clumping, rotation/macroturbulence and surface magnetic field.Comment: 12 pages, 8 figures. Proceedings of the 39th Li\`ege Astrophysical Colloquium "The multi-wavelength view of hot, massive stars". Referee's comments include

    Joint Dispersion Model with a Flexible Link

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    The objective is to model longitudinal and survival data jointly taking into account the dependence between the two responses in a real HIV/AIDS dataset using a shared parameter approach inside a Bayesian framework. We propose a linear mixed effects dispersion model to adjust the CD4 longitudinal biomarker data with a between-individual heterogeneity in the mean and variance. In doing so we are relaxing the usual assumption of a common variance for the longitudinal residuals. A hazard regression model is considered in addition to model the time since HIV/AIDS diagnostic until failure, being the coefficients, accounting for the linking between the longitudinal and survival processes, time-varying. This flexibility is specified using Penalized Splines and allows the relationship to vary in time. Because heteroscedasticity may be related with the survival, the standard deviation is considered as a covariate in the hazard model, thus enabling to study the effect of the CD4 counts' stability on the survival. The proposed framework outperforms the most used joint models, highlighting the importance in correctly taking account the individual heterogeneity for the measurement errors variance and the evolution of the disease over time in bringing new insights to better understand this biomarker-survival relation.Comment: 27 pages, 3 figures, 2 table

    I knew I Shouldn’t Do It; But I Did It: Davidson on Causal Strength and Weakness of Will

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    Reasons for action is a widely employed methodology in practical philosophy, and especially in moral philosophy. Reasons are facts that explain and justify actions. But, conceptually, if reasons were causes, incontinent actions would be impossible. When an agent ranks an evaluation about what to do as his best judgement, it entails that he has a reason for acting as that judgement prescribes. But when an agent acts incontinently, he acts in accordance to an intention that is not aligned with his best evaluative judgement. Yet, if the agent’s best evaluative judgement provides him a reason for action, this reason should also be his strongest reason, and therefore, the strongest cause. How then can it be possible that an agent incontinently acts according to a reason of inferior causal strength? In this paper, I analyze how Davidson’s argument for the possibility of incontinent actions interacts with his causal theory of actions. I argue that Davidson’s proposal does not fully respect the two principles of intentional rationality, that he himself claims to be compelling. Lastly, I sketch some initial steps that might be helpful to drawing more precise conceptual distinctions in terms of the rationality of incontinent actions

    The art of tropical travel, 1768-1830

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    Book synopsis: Georgian Geographies provides an innovative interdisciplinary examination of the geographical nature of culture and society in eighteenth-century Britain and the British world. The book's introduction identifies the key areas of study as the geographical constitution of empire, the Enlightenment and the public sphere. These themes are explored by examining the connections between space, place and landscape in the eighteenth century in relation to the emergent empire in the Caribbean and north-west America, and in Britain itself. The topics considered include landscape painting, London's art world, geography's book, mapping, the geography of erotic fiction, provincial science, and the production of domestic space in the early English novel. It will be an essential contribution to eighteenth-century studies for research and teaching staff, postgraduates and advanced undergraduate students in geography, history, literary studies, the history of art, postcolonial studies and the history of science
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