16,992 research outputs found

    Cosmological Solutions of Higher-Curvature String Effective Theories with Dilatons

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    We study the effect of higher-curvature terms in the string low-energy effective actions on the cosmological solutions of the theory, up to corrections quartic in the curvatures, for the bosonic and heterotic strings as well as the type II superstring. We find that cosmological solutions exist for all string types but they always disappear when the dilaton field is included, a conclusion that can be avoided if string-loop effects are taken into account.Comment: 7 pages, plain Tex with panda.tex macro (included), no figure

    Chaplygin inspired Inflation

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    We discuss chaotic inflation in the context of a phenomenological modification of gravity inspired by the Chaplygin gas equation of state. We find that all observationalconstraints can be satisfied provided the Chaplygin scale is smaller than 6.9×10−3M6.9 \times 10^{-3} M and the inflaton mass is smaller than 4.7×10−6M4.7 \times 10^{-6} M, respectively, where $M^2\equiv(8 \pi G)^{-1} is the reduced Planck mass.Comment: Revtex4, 5 pages. Version to match the one published in Physics Letters

    The electromagnetic coupling and the dark side of the Universe

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    We examine the properties of dark energy and dark matter through the study of the variation of the electromagnetic coupling. For concreteness, we consider the unification model of dark energy and dark matter, the generalized Chaplygin gas model (GCG), characterized by the equation of state p=−Aραp=-\frac{A}{\rho^\alpha}, where pp is the pressure, ρ\rho is the energy density and AA and α\alpha are positive constants. The coupling of electromagnetism with the GCG's scalar field can give rise to such a variation. We compare our results with experimental data, and find that the degeneracy on parameters α\alpha and AsA_s, As≡A/ρch01+αA_s \equiv A / \rho_{ch0}^{1+\alpha}, is considerable.Comment: Revtex 4, 5 pages and 5 figure

    Building and interconnecting hydrogen networks : insights from the electricity and gas experience in Europe

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    This paper aims to investigate the transition to a new energy system based on hydrogen in the European liberalized framework. After analyzing the literature on the hydrogen infrastructure needs in Europe, we estimate the size and scope of the transition challenge. We take the theoretical framework of network economics to analyze early hydrogen infrastructure needs. Therefore, several concepts are applied to hydrogen economics such as demand club effects, scale economies on large infrastructures, scope economies, and positive socio-economical externalities. On the examples of the electricity and natural gas industry formation in Europe, we argue for public intervention in order to create conditions to reach more rapidly the critical size of the network and to prompt network externalities allowing for the market diffusion of and, thus, an effective transition to the new energy system.Network economics ; infrastructure ; hydrogen

    Primordial Density Fluctuations in a Dual Supergravity Cosmology

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    We analyse the spectrum of energy density fluctuations of a dual supergravity model where the dilaton and the moduli are stabilized and sucessful inflation is achieved inside domain walls that separate different vacua of the theory. Constraints on the parameters of the superpotential are derived from the amplitude of the primordial energy density fluctuations as inferred from COBE and it is shown that the scale dependence of the tensor perturbations nearly vanishes.Comment: 13 pages, Latex, 3 figures (uses epsf.sty

    Prospects for the hydrogen transition based on the network economic approach : Insights from the electricity and gas experience in Europe

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    This paper aims to investigate the transition to a new energy system based on hydrogen in the European liberalized framework. After analyzing the literature on the hydrogen infrastructure needs in Europe, we estimate the size and scope of the transition challenge.We take the theoretical framework of network economics to analyze early hydrogen infrastructure needs. Therefore, several concepts are applied to hydrogen economics such as demand club effects, scale economies on large infrastructures, scope economies, and positive socio-economical externalities. On the examples of the electric and natural gas industry formation in Europe, we argue for public intervention in order to create conditions to reach more rapidly the critical size of the network and to prompt network externalities allowing for the market diffusion of and, thus, an effective transition to the new energy system.Economics of regulation ; network economics ; technological change ; energy economics ; hydrogen ; electricity ; natural gas
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