5,432 research outputs found

    On the Dirac field in the Palatini form of 1/R gravity

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    In recent papers (astro-ph/0306630, gr-qc/0312041) I have argued that the observed cosmological acceleration can be accounted for by the inclusion of a 1/R term in the gravitational action in the Palatini formalism. Subsequently, Flanagan (astro-ph/0308111, gr-qc/0403063) argued that this theory is equivalent to a scalar-tensor theory which produces corrections to the standard model that are ruled out experimentally. In this article I examine the Dirac field coupled to 1/R gravity. The Dirac action contains the connection which was taken to be the Christoffel symbol, not an independent quantity, in the papers by Flanagan. Since the metric and connection are taken to be independent in the Palatini approach it is natural to allow the connection that appears in the Dirac action to be an independent quantity. This is the approach that is taken in this paper. The resulting theory is very different and much more complicated than the one discussed in Flanagan's papers.Comment: 6 pages, LaTe

    Spintessence: a possible candidate as a driver of the late time cosmic acceleration

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    In this paper, it is shown completely analytically that a spintessence model can very well serve the purpose of providing an early deceleration and the present day acceleration.Comment: 5 pages, no figure. Accepted for publication in Astrophysics and Space Scienc

    Dynamics of Massive Scalar Fields in dS Space and the dS/CFT Correspondence

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    Global geometric properties of dS space are presented explicitly in various coordinates. A Robertson-Walker like metric is deduced, which is convenient to be used in study of dynamics in dS space. Singularities of wavefunctions of massive scalar fields at boundary are demonstrated. A bulk-boundary propagator is constructed by making use of the solutions of equations of motion. The dS/CFT correspondence and the Strominger's mass bound is shown.Comment: latex, 14 pages and 3 figure

    Opportunities for future supernova studies of cosmic acceleration

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    We investigate the potential of a future supernova dataset, as might be obtained by the proposed SNAP satellite, to discriminate among different ``dark energy'' theories that describe an accelerating Universe. We find that many such models can be distinguished with a fit to the effective pressure-to-density ratio, ww, of this energy. More models can be distinguished when the effective slope, dw/dzdw/dz, of a changing ww is also fit, but only if our knowledge of the current mass density, Ωm\Omega_m, is improved. We investigate the use of ``fitting functions'' to interpret luminosity distance data from supernova searches, and argue in favor of a particular preferred method, which we use in our analysis.Comment: Four pages including figures. Final published version. No significant changes from v

    Geodesics on Lie groups: Euler equations and totally geodesic subgroup

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    The geodesic motion on a Lie group equipped with a left or right invariant Riemannian metric is governed by the Euler-Arnold equation. This paper investigates conditions on the metric in order for a given subgroup to be totally geodesic. Results on the construction and characterisation of such metrics are given. The setting works both in the classical nite dimensional case, and in the category of in nite dimensional Fr echet Lie groups, in which di eomorphism groups are included. Using the framework we give new examples of both nite and in nite dimensional totally geodesic subgroups. In particular, based on the cross helicity, we construct right invariant metrics such that a given subgroup of exact volume preserving di eomorphisms is totally geodesic. The paper also gives a general framework for the representation of Euler-Arnold equations in arbitrary choice of dual pairing

    Symplectic Reduction for Semidirect Products and Central Extensions

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    This paper proves a symplectic reduction by stages theorem in the context of geometric mechanics on symplectic manifolds with symmetry groups that are group extensions. We relate the work to the semidirect product reduction theory developed in the 1980's by Marsden, Ratiu, Weinstein, Guillemin and Sternberg as well as some more recent results and we recall how semidirect product reduction finds use in examples, such as the dynamics of an underwater vehicle. We shall start with the classical cases of commuting reduction (first appearing in Marsden and Weinstein, 1974) and present a new proof and approach to semidirect product theory. We shall then give an idea of how the more general theory of group extensions proceeds (the details of which are given in Marsden, Misiołek, Perlmutter and Ratiu, 1998). The case of central extensions is illustrated in this paper with the example of the Heisenberg group. The theory, however, applies to many other interesting examples such as the Bott-Virasoro group and the KdV equation

    Speaking in Code, Mental Time Travel and Mind Reading: an exploration of the study of animal intelligence

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    Many humans believe they are separate from nature based on “unique” characteristics. For many centuries, our advanced tool use was believed to separate us from other animals. However, discoveries made by Jane Goodall in the 1960’s demonstrated that humans are not unique for this capacity.1 In light of these discoveries, the definition of human was altered to suggest that our intelligence sets us apart from other animals. This definition has been used in order to justify our exploitation of other animals by suggesting that we are unique, and therefore, superior due to this “unique” intelligence. However, humans are not alone in our capacity for complex thought. Other animals exhibit three major features of intelligence: language, episodic memory, and theory of mind. However, homocentric methodology has denied these animals the opportunity to demonstrate their intelligence by confining them to human-specific standards. Only through species-specific testing can we reveal the true intellectual capacities of other animals. Through the recognition of these commonalities, the distinction between “human” and “other animals” can be blurred, thus aiding in conservation efforts as humans recognize that we are not superior to animals and therefore are not more deserving of our Earth’s resources

    Phantom Energy Accretion by Stringy Charged Black Hole

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    We investigate the dynamical behavior of phantom energy near stringy magnetically charged black hole. For this purpose, we derive equations of motion for steady-state spherically symmetric flow of phantom energy onto the stringy magnetically charged black hole. It is found that phantom energy accreting onto black hole decreases its mass. Further, the location of critical points of accretion is explored, which yields mass to charge ratio. This ratio implies that accretion process cannot transform a black hole into an extremal black hole or a naked singularity, hence cosmic censorship hypothesis remains valid here.Comment: 7 pages, no figur
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