1,126 research outputs found
A Spinor Theory of Gravity and the Cosmological Framework
Recently we have presented a new formulation of the theory of gravity based
on an implementation of the Einstein Equivalence Principle distinct from
General Relativity. The kinetic part of the theory - that describes how matter
is affected by the modified geometry due to the gravitational field - is the
same as in General Relativity. However, we do not consider the metric as an
independent field. Instead, it is an effective one, constructed in terms of two
fundamental spinor fields and and thus the metric does not
have a dynamics of its own, but inherits its evolution through its relation
with the fundamental spinors. In the first paper it was shown that the metric
that describes the gravitational field generated by a compact static and
spherically symmetric configuration is very similar to the Schwarzschild
metric. In the present paper we describe the cosmological framework in the
realm of the Spinor Theory of Gravity
A nongravitational wormhole
Using the effective metric formalism for photons in a nonlinear
electromagnetic theory, we show that a certain field configuration in
Born-Infeld electromagnetism in flat spacetime can be interpreted as an
ultrastatic spherically symmetric wormhole. We also discuss some properties of
the effective metric that are valid for any field configuration.Comment: LaTex, 9 pages with 5 figures, minor changes, accepted for
publication in Class. Quantum Gra
A Note on the Local Cosmological Constant and the Dark Energy Coincidence Problem
It has been suggested that the Dark Energy Coincidence Problem could be
interpreted as a possible link between the cosmological constant and a massive
graviton. We show that by using that link and models for the graviton mass a
dark energy density can be obtained that is indeed very close to measurements
by WMAP. As a consequence of the models, the cosmological constant was found to
depend on the density of matter. A brief outline of the cosmological
consequences such as the effect on the black hole solution is given
Geometrical aspects of light propagation in nonlinear electrodynamics
We analyze the propagation of light in the context of nonlinear
electrodynamics, as it occurs in modified QED vacua. We show that the
corresponding characteristic equation can be described in terms of a
modification of the effective geometry of the underlying spacetime structure.
We present the general form for this effective geometry and exhibit some new
consequences that result from such approach.Comment: LaTex, 11 pages, accepted for publication in Phys. Rev.
The sonic analogue of black hole radiation
A microscopic description of Hawking radiation in sonic black holes has been
recently presented (Giovanazzi S 2005 Phys. Rev. Lett. 94 061302). This exactly
solvable model is formulated in terms of one-dimensional scattering of a Fermi
gas. In this paper, the model is extended to account possible finite size
effects of a realistic geometry. The flow of particles is maintained by a
piston (i.e. an impenetrable barrier) moving slowly towards the sonic horizon.
Using existing technologies the Hawking temperature can be of order of a few
microkelvin in a realistic experiment.Comment: 14 pages, 7 figures, submitted to Journal of Physics B: Atomic,
Molecular & Optical Physic
Influence of cane girdling and plastic covering on leaf gas exchange, water potential and viticultural performance of table grape cv. Matilde
Canes of field-grown uncovered and covered (plastic film) table grapes, cv. Matilde, were girdled at veraison. Leaves of girdled vines displayed lower rates of transpiration on a leaf area basis and lower rates of CO2 uptake; stem and leaf water potentials were decreased. Both, covering and cane girdling stimulated vegetative growth and increased leaf area per vine. However, sugar accumulation in berries and fruit quality were not affected by cane girdling and were slightly reduced by covering. Therefore, harvesting dates were not advanced. Cane girdling appeared to influence carbohydrate partitioning by stimulating shoot growth at the expense of fruit production. It is concluded that the stage of rapid sugar accumulation was not yet reached by the time girdling took place. The development of a larger transpiring leaf surface area per vine is supposed to have lowered the vine water status
Reduction of Evapotranspiration in Microenvironment Conditions of Table Grape Vineyards Protected by Different Types of Plastic Covers
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