16,992 research outputs found
Cosmological Solutions of Higher-Curvature String Effective Theories with Dilatons
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
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 and the inflaton mass is smaller
than , 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
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
, where is the pressure, is the energy
density and and 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 and , , is
considerable.Comment: Revtex 4, 5 pages and 5 figure
Building and interconnecting hydrogen networks : insights from the electricity and gas experience in Europe
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
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
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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