5,754 research outputs found
Caracterização morfológica, agronômica e fenológica de arroz-vermelho coletados nos Estados da Paraíba e Ceará.
O objetivo deste trabalho é informar à comunidade científica em geral especialmente aos melhoristas, os resultados da avaliação morfológica, agronômica e fenológica de acessos de arroz-vermelho e de cariopse branca, coletados naqueles estados
Pound-Rebka experiment and torsion in the Schwarzschild spacetime
We develop some ideas discussed by E. Schucking [arXiv:0803.4128] concerning
the geometry of the gravitational field. First, we address the concept
according to which the gravitational acceleration is a manifestation of the
spacetime torsion, not of the curvature tensor. It is possible to show that
there are situations in which the geodesic acceleration of a particle may
acquire arbitrary values, whereas the curvature tensor approaches zero. We
conclude that the spacetime curvature does not affect the geodesic
acceleration. Then we consider the the Pound-Rebka experiment, which relates
the time interval of two light signals emitted at a position
, to the time interval of the signals received at a
position , in a Schwarzschild type gravitational field. The experiment is
determined by four spacetime events. The infinitesimal vectors formed by these
events do not form a parallelogram in the (t,r) plane. The failure in the
closure of the parallelogram implies that the spacetime has torsion. We find
the explicit form of the torsion tensor that explains the nonclosure of the
parallelogram.Comment: 16 pages, two figures, one typo fixed, one paragraph added in section
Low redshift constraints on energy-momentum-powered gravity models
There has been recent interest in the cosmological consequences of
energy-momentum-powered gravity models, in which the matter side of Einstein's
equations is modified by the addition of a term proportional to some power,
, of the energy-momentum tensor, in addition to the canonical linear term.
In this work we treat these models as phenomenological extensions of the
standard CDM, containing both matter and a cosmological constant. We
also quantitatively constrain the additional model parameters using low
redshift background cosmology data that are specifically from Type Ia
supernovas and Hubble parameter measurements. We start by studying specific
cases of these models with fixed values of which lead to an analytic
expression for the Friedmann equation; we discuss both their current
constraints and how the models may be further constrained by future
observations of Type Ia supernovas for WFIRST complemented by measurements of
the redshift drift by the ELT. We then consider and constrain a more extended
parameter space, allowing to be a free parameter and considering scenarios
with and without a cosmological constant. These models do not solve the
cosmological constant problem per se. Nonetheless these models can
phenomenologically lead to a recent accelerating universe without a
cosmological constant at the cost of having a preferred matter density of
around instead of the usual . Finally we
also briefly constrain scenarios without a cosmological constant, where the
single component has a constant equation of state which needs not be that of
matter; we provide an illustrative comparison of this model with a more
standard dynamical dark energy model with a constant equation of state.Comment: 13+2 pages, 12+1 figures; A&A (in press
The electromagnetic field in accelerated frames
We develop a geometrical framework that allows to obtain the electromagnetic
field quantities in accelerated frames. The frame of arbitrary accelerated
observers in space-time is defined by a suitable set of tetrad fields, whose
timelike components are adapted to the worldlines of a field of observers. We
consider the Faraday tensor and Maxwell's equations as abstract tensor
quantities in space-time, and make use of tetrad fields to project the
electromagnetic field quantities in the accelerated frames. As an application,
plane and spherical electromagnetic waves are projected in linearly accelerated
frames in Minkowski space-time. We show that the amplitude, frequency and the
wave vector of the plane wave in the accelerated frame vary with time, while
the light speed remains constant. We also obtain the variation of the Poynting
vector with time in the accelerated frame.Comment: 19 pages, no figures, written as a chapter of the open access book
"Electromagnetic Radiation", ISBN 979-953-307-332-2 (Intech, 2012
On reference frames in spacetime and gravitational energy in freely falling frames
We consider the interpretation of tetrad fields as reference frames in
spacetime. Reference frames may be characterized by an antisymmetric
acceleration tensor, whose components are identified as the inertial
accelerations of the frame (the translational acceleration and the frequency of
rotation of the frame). This tensor is closely related to
gravitoelectromagnetic field quantities. We construct the set of tetrad fields
adapted to observers that are in free fall in the Schwarzschild spacetime, and
show that the gravitational energy-momentum constructed out of this set of
tetrad fields, in the framework of the teleparallel equivalent of general
relatrivity, vanishes. This result is in agreement with the principle of
equivalence, and may be taken as a condition for a viable definition of
gravitational energy.Comment: 19 pages, no figures, accepted by Classical and Quantum Gravit
First transgenic geminivirus-resistant plant in the field.
Bean golden mosaic virus (BGMV) belongs to the genus Begomovirus, whose genome is composed of two single-stranded DNA molecules, designated DNA-A and DNA-B, both of which are essential for infectivity. BGMV is transmitted by the whitefly Bemisia tabaci (Gennadius) in a persistent, circulative manner, causing golden mosaic in common bean. This disease is characterized by yellow-green mosaic of leaves, stunted growth and distorted pods, which may vary among genotypes
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