8,130 research outputs found

    Semi-transparent brane-worlds

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    We study the evolution of a closed Friedmann brane perturbed by the Hawking radiation escaping a bulk black hole. The semi-transparent brane absorbes some of the infalling radiation, the rest being transmitted across the brane to the other bulk region. We characterize the cosmological evolution in terms of the transmission rate ϵ\epsilon. For small values of ϵ\epsilon a critical-like behaviour could be observed, when the acceleration due to radiation pressure and the deceleration induced by the increasing self-gravity of the brane roughly compensate each other, and cosmological evolution is approximately the same as without radiation. Lighter (heavier) branes than those with the critical energy density will recollapse slower (faster). This feature is obstructed at high values of ϵ\epsilon , where the overall effect of the radiation is to speed-up the recollapse. We determine the maximal value of the transmission rate for which the critical-like behaviour is observed. We also study the effect of transmission on the evolution of different source terms of the Friedmann equation. We conclude that among all semi-transparent branes the slowest recollapse occurs for light branes with total absorption.Comment: 15 pages, 8 figure

    A MOOC for Adult Learners of Mathematics and Statistics: Tensions and Compromises in Design

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    There are many adults with low mathematical/statistical knowledge who would like to enhance that understanding. There are insufficient teachers to respond to the level of need and so innovative solutions must be found. In the UK, the Ufi Charitable Trust has funded a project to develop a free open online course to offer motivated adults access to powerful ideas. We reflect on the tensions and compromises that emerged during its design. More specifically, referring to data collected from users, we consider the challenge of developing resources that will support heterogeneous students from unknown backgrounds, who may have already been failed by the conventional educational system and who will have no interactive tutor support within this course

    Low Temperature Physics

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    Contains research objectives and reports on one research project

    Low Temperature Physics

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    Contains reports on two research projects

    The ART of IAM: The Winning Strategy for the 2006 Competition

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    In many dynamic open systems, agents have to interact with one another to achieve their goals. Here, agents may be self-interested, and when trusted to perform an action for others, may betray that trust by not performing the actions as required. In addition, due to the size of such systems, agents will often interact with other agents with which they have little or no past experience. This situation has led to the development of a number of trust and reputation models, which aim to facilitate an agent's decision making in the face of uncertainty regarding the behaviour of its peers. However, these multifarious models employ a variety of different representations of trust between agents, and measure performance in many different ways. This has made it hard to adequately evaluate the relative properties of different models, raising the need for a common platform on which to compare competing mechanisms. To this end, the ART Testbed Competition has been proposed, in which agents using different trust models compete against each other to provide services in an open marketplace. In this paper, we present the winning strategy for this competition in 2006, provide an analysis of the factors that led to this success, and discuss lessons learnt from the competition about issues of trust in multiagent systems in general. Our strategy, IAM, is Intelligent (using statistical models for opponent modelling), Abstemious (spending its money parsimoniously based on its trust model) and Moral (providing fair and honest feedback to those that request it)

    PREDICTION OF AN OPTIMUM TECHNIQUE FOR THE WOMEN’S YURCHENKO LAYOUT VAULT

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    The purpose of this study was to identify an optimum technique for the women’s Yurchenko layout vault through an application of optimal control theory on a five-segment model consisting of the hand, whole arm, upper trunk, lower trunk and whole leg. An optimum technique for the vault was determined which, compared to the data, had greater post-flight amplitude and a better layout posture throughout post-flight. However, it involved a larger angular velocity of the segments and greater shoulder extension by about 9º, than the data. The impact phase of the optimum technique was shorter than the data by 0.003 s, and served to increase both the angular momentum of the model as well as the vertical horse takeoff velocity. There is thus, evidence of a ‘blocking technique’ during impact

    Friedmann branes with variable tension

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    We introduce brane-worlds with non-constant tension, strenghtening the analogy with fluid membranes, which exhibit a temperature-dependence according to the empirical law established by E\"otv\"os. This new degree of freedom allows for evolving gravitational and cosmological constants, the latter being a natural candidate for dark energy. We establish the covariant dynamics on a brane with variable tension in full generality, by considering asymmetrically embedded branes and allowing for non-standard model fields in the 5-dimensional space-time. Then we apply the formalism for a perfect fluid on a Friedmann brane, which is embedded in a 5-dimensional charged Vaidya-Anti de Sitter space-time.Comment: 12 pages, to appear in Phys. Rev.
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