4,148 research outputs found
Evolving wormhole geometries within nonlinear electrodynamics
In this work, we explore the possibility of evolving (2+1) and
(3+1)-dimensional wormhole spacetimes, conformally related to the respective
static geometries, within the context of nonlinear electrodynamics. For the
(3+1)-dimensional spacetime, it is found that the Einstein field equation
imposes a contracting wormhole solution and the obedience of the weak energy
condition. Nevertheless, in the presence of an electric field, the latter
presents a singularity at the throat, however, for a pure magnetic field the
solution is regular. For the (2+1)-dimensional case, it is also found that the
physical fields are singular at the throat. Thus, taking into account the
principle of finiteness, which states that a satisfactory theory should avoid
physical quantities becoming infinite, one may rule out evolving
(3+1)-dimensional wormhole solutions, in the presence of an electric field, and
the (2+1)-dimensional case coupled to nonlinear electrodynamics.Comment: 17 pages, 1 figure; to appear in Classical and Quantum Gravity. V2:
minor corrections, including a referenc
On thin-shell wormholes evolving in flat FRW spacetimes
We analize the stability of a class of thin-shell wormholes with spherical
symmetry evolving in flat FRW spacetimes. The wormholes considered here are
supported at the throat by a perfect fluid with equation of state
and have a physical radius equal to , where is a
time-dependent function describing the dynamics of the throat and is the
background scale factor. The study of wormhole stability is done by means of
the stability analysis of dynamic systems.Comment: 8 pages; to appear in MPL
Melhoramento genético como ferramenta para o crescimento e o desenvolvimento da ovinocultura de corte.
Resumo: Dentre as ações relacionadas à promoção do crescimento e do desenvolvimento de uma atividade pecuária, o melhoramento genético é uma das que mais merece atenção. Para que a ovinocultura de corte no Brasil possa se desenvolver, são necessárias a seleção e a multiplicação de genótipos apropriados aos diversos ecossistemas encontrados no país. Com a finalidade de aperfeiçoar a capacidade de produção dos animais, as ferramentas básicas do melhoramento genético são a seleção e os métodos de acasalamento. Neste trabalho são apresentadas algumas definições fundamentais do melhoramento genético e discutidas formas de sua contribuição para o crescimento da ovinocultura de corte. [Animal breeding as a tool for upgrowth and development of meat sheep sector]. Abstract: Among the actions to promote the growing and the development of an animal breeding activity, the genetic improvement is one of that more attention deserve. Selection and multiplication of genotypes appropriated to several ecossystems found in the country are necessary to the development of meat sheep sector in Brazil. To optimize the production ability of the animals, the basic tools of animal breeding are the selection and the mating methods. In this paper fundamental definitions of animal breeding are presented and forms of its contribution for the upgrowth of meat sheep sector are discussed.Palestra apresentada no XVII Congresso Brasileiro de Reprodução Animal, 31 de maio a 02 de junho de 2007, Curitiba, PR
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