5,956 research outputs found

    Cosmic scalar fields with flat potential

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    The dynamics of cosmic scalar fields with flat potential is studied. Their contribution to the expansion rate of the universe is analyzed, and their behaviour in a simple model of phase transitions is discussed.Comment: 9 page

    Ensemble Kalman filter for neural network based one-shot inversion

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    We study the use of novel techniques arising in machine learning for inverse problems. Our approach replaces the complex forward model by a neural network, which is trained simultaneously in a one-shot sense when estimating the unknown parameters from data, i.e. the neural network is trained only for the unknown parameter. By establishing a link to the Bayesian approach to inverse problems, an algorithmic framework is developed which ensures the feasibility of the parameter estimate w.r. to the forward model. We propose an efficient, derivative-free optimization method based on variants of the ensemble Kalman inversion. Numerical experiments show that the ensemble Kalman filter for neural network based one-shot inversion is a promising direction combining optimization and machine learning techniques for inverse problems

    Effects of curvature and interactions on the dynamics of the deconfinement phase transition

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    We study the dynamics of first-order cofinement-deconfinement phase transition through nucleation of hadronic bubbles in an expanding quark gluon plasma in the context of heavy ion collisions for interacting quark and hadron gas and by incorporating the effects of curvature energy. We find that the interactions reduce the delay in the phase transition whereas the curvature energy has a mixed behavior. In contrast to the case of early Universe phase transition, here lower values of surface tension increase the supercooling and slow down the hadronization process. Higher values of bag pressure tend to speed up the transition. Another interesting feature is the start of the hadronization process as soon as the QGP is created.Comment: LaTeX, 17 pages including 14 postscript figure

    A New Redshift Interpretation

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    A nonhomogeneous universe with vacuum energy, but without spacetime expansion, is utilized together with gravitational and Doppler redshifts as the basis for proposing a new interpretation of the Hubble relation and the 2.7K Cosmic Blackbody Radiation.Comment: 9 pages LaTeX, no figure

    What is the Homogeneity of our Universe Telling Us?

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    The universe we observe is homogeneous on super-horizon scales, leading to the ``cosmic homogeneity problem''. Inflation alleviates this problem but cannot solve it within the realm of conservative extrapolations of classical physics. A probabilistic solution of the problem is possible but is subject to interpretational difficulties. A genuine deterministic solution of the homogeneity problem requires radical departures from known physics.Comment: 6 pages. Awarded Honorable Mention in the 1999 Gravity Research Foundation Essay Competitio

    Cosmic acceleration: Inhomogeneity versus vacuum energy

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    In this essay, I present an alternative explanation for the cosmic acceleration which appears as a consequence of recent high redshift Supernova data. In the usual interpretation, this cosmic acceleration is explained by the presence of a positive cosmological constant or vacuum energy, in the background of Friedmann models. Instead, I will consider a Local Rotational Symmetric (LRS) inhomogeneous spacetime, with a barotropic equation of state for the cosmic matter. Within this framework the kinematical acceleration of the cosmic fluid or, equivalently, the inhomogeneity of matter, is just the responsible of the SNe Ia measured cosmic acceleration. Although in our model the Cosmological Principle is relaxed, it maintains local isotropy about our worldline in agreement with the CBR experiments.Comment: LATEX, 7 pags, no figs, Honorable Mention in the 1999 Essay Competition of the Gravity Research Foundatio

    When to stop - a cardinal secretary search experiment

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    The cardinal secretary search problem confronts the decision-maker with more or less candidates who have identically and independently distributed values and appear successively in a random order without recall of earlier candidates. Its benchmark solution implies monotonically decreasing sequences of optimal value aspirations (acceptance thresholds) for any number of remaining candidates. We compare experimentally observed aspirations with optimal ones for different numbers of (remaining) candidates and methods of experimental choice elicitation: “hot” collects play data, “warm” asks for an acceptance threshold before confronting the next candidate, and “cold” for a complete profile of trial-specific acceptance thresholds. The initially available number of candidates varies across elicitation methods to obtain more balanced data. We find that actual search differs from benchmark behavior, in average search length and success, but also in some puzzling qualitative aspects

    Non-stationary de Sitter cosmological models

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    In this note it is proposed a class of non-stationary de Sitter, rotating and non-rotating, solutions of Einstein's field equations with a cosmological term of variable function.Comment: 11 pages, Latex. International Journal of Modern Physics D (accepted for publication

    Partial versus general compulsory solidarity: an experimental analysis

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    We focus on ways and means of solidarity and their more or less voluntary and involuntary character. Alternative ways of redistribution are modeled by combining redistribution as emergent from a non-discriminatory voluntary contribution mechanism, VCM, with an outside option for a “super-rich”, R, participant to donate to VCM participants. The outsider may discriminate between participants of the VCM on the basis of information accessible at a cost to her. Inclusion in and exclusion from the VCM are involuntary while contributions in it are voluntary. How involuntary inclusion of R in VCM affects her discriminatory voluntary donations and contribution behavior is explored experimentally