5,558 research outputs found

    Origin of FRW cosmology in slow-roll inflation from noncompact Kaluza-Klein theory

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    Using a recently introduced formalism we discuss slow-roll inflaton from Kaluza-Klein theory without the cylinder condition. In particular, some examples corresponding to polynomic and hyperbolic Ď•\phi-potentials are studied. We find that the evolution of the fifth coordinate should be determinant for both, the evolution of the early inflationary universe and the quantum fluctuations.Comment: (final version) to be published in EPJ

    Energy-Momentum Restrictions on the Creation of Gott Time Machines

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    The discovery by Gott of a remarkably simple spacetime with closed timelike curves (CTC's) provides a tool for investigating how the creation of time machines is prevented in classical general relativity. The Gott spacetime contains two infinitely long, parallel cosmic strings, which can equivalently be viewed as point masses in (2+1)-dimensional gravity. We examine the possibility of building such a time machine in an open universe. Specifically, we consider initial data specified on an edgeless, noncompact, spacelike hypersurface, for which the total momentum is timelike (i.e., not the momentum of a Gott spacetime). In contrast to the case of a closed universe (in which Gott pairs, although not CTC's, can be produced from the decay of stationary particles), we find that there is never enough energy for a Gott-like time machine to evolve from the specified data; it is impossible to accelerate two particles to sufficiently high velocity. Thus, the no-CTC theorems of Tipler and Hawking are enforced in an open (2+1)-dimensional universe by a mechanism different from that which operates in a closed universe. In proving our result, we develop a simple method to understand the inequalities that restrict the result of combining momenta in (2+1)-dimensional gravity.Comment: Plain TeX, 41 pages incl. 9 figures. MIT-CTP #225

    A Relativistic Description of Gentry's New Redshift Interpretation

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    We obtain a new expression of the Friedmann-Robertson-Walker metric, which is an analogue of a static chart of the de Sitter space-time. The reduced metric contains two functions, M(T,R)M(T,R) and Ψ(T,R)\Psi(T,R), which are interpreted as, respectively, the mass function and the gravitational potential. We find that, near the coordinate origin, the reduced metric can be approximated in a static form and that the approximated metric function, Ψ(R)\Psi(R), satisfies the Poisson equation. Moreover, when the model parameters of the Friedmann-Robertson-Walker metric are suitably chosen, the approximated metric coincides with exact solutions of the Einstein equation with the perfect fluid matter. We then solve the radial geodesics on the approximated space-time to obtain the distance-redshift relation of geodesic sources observed by the comoving observer at the origin. We find that the redshift is expressed in terms of a peculiar velocity of the source and the metric function, Ψ(R)\Psi(R), evaluated at the source position, and one may think that this is a new interpretation of {\it Gentry's new redshift interpretation}.Comment: 11 pages. Submitted to Modern Physics Letters

    Towards Resolution of Hierarchy Problems in a Cosmological Context

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    A cosmological scenario is proposed, which simultaneously solves the mass hierarchy and the small dark energy problem. In the present scenario an effective gravity mass scale (inverse of the Newton's constant) increases during the inflationary period. The small cosmological constant or the dark energy density in the present universe is dynamically realized by introducing two, approximately O(2) symmetric dilatons, taking the fundamental mass scale at TeV.Comment: 12 pages, no figur

    Geometry and Destiny

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    The recognition that the cosmological constant may be non-zero forces us to re-evaluate standard notions about the connection between geometry and the fate of our Universe. An open Universe can recollapse, and a closed Universe can expand forever. As a corollary, we point out that there is no set of cosmological observations we can perform that will unambiguously allow us to determine what the ultimate destiny of the Universe will be.Comment: 7 pages, Gravity Research Foundation Essa

    Post-Newtonian expansion for Gauss-Bonnet Gravity

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    The Parametrized Post-Newtonian expansion of gravitational theories with a scalar field coupled to the Gauss-Bonnet invariant is performed and confrontation of such theories with Solar system experiments is discussed.Comment: 4 pages; typos corrected, published versio

    Persistent Topology of Syntax

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    We study the persistent homology of the data set of syntactic parameters of the world languages. We show that, while homology generators behave erratically over the whole data set, non-trivial persistent homology appears when one restricts to specific language families. Different families exhibit different persistent homology. We focus on the cases of the Indo-European and the Niger-Congo families, for which we compare persistent homology over different cluster filtering values. We investigate the possible significance, in historical linguistic terms, of the presence of persistent generators of the first homology. In particular, we show that the persistent first homology generator we find in the Indo-European family is not due (as one might guess) to the Anglo-Norman bridge in the Indo-European phylogenetic network, but is related to the position of Ancient Greek and the Hellenic branch within the network.Comment: 15 pages, 25 jpg figure

    Metric fluctuations and its evolution during inflation

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    W discuss the evolution of the fluctuations in a symmetric ϕc\phi_c-exponential potential which provides a power-law expansion during inflation using both, the gauge invariant field Φ\Phi and the Sasaki-Mukhanov field.Comment: version accepted in EPJ

    Large Scale Inhomogeneities from the QCD Phase Transition

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    We examine the first-order cosmological QCD phase transition for a large class of parameter values, previously considered unlikely. We find that the hadron bubbles can nucleate at very large distance scales, they can grow as detonations as well as deflagrations, and that the phase transition may be completed without reheating to the critical temperature. For a subset of the parameter values studied, the inhomogeneities generated at the QCD phase transition might have a noticeable effect on nucleosynthesis.Comment: 15 LaTeX pages + 6 PostScript figures appended at the end of the file, HU-TFT-94-1
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