143 research outputs found
Topologica Defects and Corrections to the Nambu Action
The effective action of a (1+2)-dimensional defect is obtained as an
expansion in powers of the thickness.Considering non-straight solutions as the
zero order term, the corrections to the Nambu action are found to depend on the
curvature scalar and on the gaussian curvature .Comment: UNB.FIS.FM-002/92, Marcos@FNAL, 12 pages, Late
The Deformable Universe
The concept of smooth deformations of a Riemannian manifolds, recently
evidenced by the solution of the Poincar\'e conjecture, is applied to
Einstein's gravitational theory and in particular to the standard FLRW
cosmology. We present a brief review of the deformation of Riemannian geometry,
showing how such deformations can be derived from the Einstein-Hilbert
dynamical principle. We show that such deformations of space-times of general
relativity produce observable effects that can be measured by four-dimensional
observers. In the case of the FLRW cosmology, one such observable effect is
shown to be consistent with the accelerated expansion of the universe.Comment: 20 pages, LaTeX, 3 figure
Localization of gravity in brane world with arbitrary extra dimensions
We study the induced 4-dimensional linearized Einstein field equations in an
m-dimensional bulk space by means of a confining potential. It is shown that in
this approach the mass of graviton is quantized. The cosmological constant
problem is also addressed within the context of this approach. We show that the
difference between the values of the cosmological constant in particle physics
and cosmology stems from our measurements in two different scales, small and
large.Comment: 8 pages. arXiv admin note: substantial text overlap with
arXiv:gr-qc/0408004, arXiv:gr-qc/0607067, arXiv:0704.1035, arXiv:0707.3558,
arXiv:0710.266
Interaction generalisation and demographic feedbacks drive the resilience of plant–insect networks to extinctions
Understanding the processes driving ecological resilience, that is the extent to which systems retain their structure while absorbing perturbations, is a central challenge for theoretical and applied ecologists. Plant–insect assemblages are well-suited for the study of ecological resilience as they are species-rich and encompass a variety of ecological interactions that correspond to essential ecosystem functions. Mechanisms affecting community response to perturbations depend on both the natural history and structure of ecological interactions. Natural history attributes of the interspecific interactions, for example whether they are mutualistic or antagonistic, may affect the ecological resilience by controlling the demographic feedbacks driving ecological dynamics at the community level. Interaction generalisation may also affect resilience, by defining opportunities for interaction rewiring, the extent to which species are able to switch interactions in fluctuating environments. These natural history attributes may also interact with network structure to affect ecological resilience. Using adaptive network models, we investigated the resilience of plant–pollinator and plant–herbivore networks to species loss. We specifically investigated how fundamental natural history differences between these systems, namely the demographic consequences of the interaction and their level of generalisation—mediating rewiring opportunities—affect the resilience of dynamic ecological networks to extinctions. We also create a general benchmark for the effect of network structure on resilience simulating extinctions on theoretical networks with controlled structures. When network structure was static, pollination networks were less resilient than herbivory networks; this is related to their high levels of nestedness and the reciprocally positive feedbacks that define mutualisms, which made co-extinction cascades more likely and longer in plant–pollinator assemblages. When considering interaction rewiring, the high generalisation and the structure of pollination networks boosted their resilience to extinctions, which approached those of herbivory networks. Simulation results using theoretical networks suggested that the empirical structure of herbivory networks may protect them from collapse. Elucidating the ecological and evolutionary processes driving interaction rewiring is key to understanding the resilience of plant–insect assemblages. Accounting for rewiring requires ecologists to combine natural history with network models that incorporate feedbacks between species abundances, traits and interactions. This combination will elucidate how perturbations propagate at community level, reshaping biodiversity structure and ecosystem functions
Geometry of Brane-Worlds
The most general geometrical scenario in which the brane-world program can be
implemented is investigated. The basic requirement is that it should be
consistent with the confinement of gauge interaction, the existence of quantum
states and the embedding in a bulk with arbitrary dimensions, signature and
topology.
It is found that the embedding equations are compatible with a wide class of
Lagrangians, starting with a modified Einstein-Hilbert Lagrangian as the
simplest one, provided minimal boundaries are added to the bulk.
A non-trivial canonical structure is derived, suggesting a canonical
quantization of the brane-world geometry relative to the extra dimensions,
where the quantum states are set in correspondence with high frequency
gravitational waves. It is shown that in the cases of at least six dimensions,
there exists a confined gauge field included in the embedding structure. The
size of extra dimensions compatible with the embedding is calculated and found
to be different from the one derived with product topology.Comment: Minor changes and a correction to equation (22). 9 pages twocolumn
Revte
The accelerating universe in brane-world cosmology
The standard Friedmann universe embedded in a five dimensional and constant
curvature bulk is examined without any a priori junction condition between the
brane and the bulk. A geometrical explanation for the accelerated expansion of
the universe is derived by using a minimum set of assumptions consistent with
the brane-world program. It is shown that the extrinsic curvature of the brane
can be associated to the dark energy which presumably drives the universe
expansion.Comment: Revtex, 4 page
Brane-world Quantum Gravity
The Arnowitt-Deser-Misner canonical formulation of general relativity is
extended to the covariant brane-world theory in arbitrary dimensions. The
exclusive probing of the extra dimensions makes a substantial difference,
allowing for the construction of a non-constrained canonical theory. The
quantum states of the brane-world geometry are defined by the
Tomonaga-Schwinger equation, whose integrability conditions are determined by
the classical perturbations of submanifolds contained in the Nash's
differentiable embedding theorem. In principle, quantum brane-world theory can
be tested by current experiments in astrophysics and by near future laboratory
experiments at Tev energy. The implications to the black-hole information loss
problem, to the accelerating cosmology, and to a quantum mathematical theory of
four-sub manifolds are briefly commented.Comment: 14 pages, no figures, JHEP format, to appear in JHEP April 200
Quaternion-Loop Quantum Gravity
It is shown that the Riemannian curvature of the 3-dimensional hypersurfaces
in space-time, described by the Wilson loop integral, can be represented by a
quaternion quantum operator induced by the SU(2) gauge potential, thus
providing a justification for quaternion quantum gravity at the Tev energy
scale.Comment: 5 pages, latex, no figures. Improved pdf. To appear in the
Foundations of Physic
Uso da adubação orgânica e cobertura morta na cultura da calêndula (Calendula officinalis L.).
A calêndula representa fonte alternativa econômica principalmente para a agricultura familiar. O objetivo do presente trabalho foi avaliar os efeitos da adubação orgânica e do uso da cobertura morta sobre a produção de biomassa e teor de flavonóides totais em plantas de calêndula. O experimento foi realizado no Campus de Montes Claros da Universidade Federal de Minas Gerais, utilizando-se o delineamento em blocos casualizados em esquema fatorial 2x2x4 com quatro repetições: duas épocas de colheita, presença ou ausência de cobertura morta e quatro doses de composto orgânico (0, 30, 60 e 90 t ha-1). As maiores produções de biomassa e flavonóides totais foram verificadas com o aumento das doses de composto orgânico. Para os flavonóides totais o maior teor foi obtido na segunda colheita na presença de cobertura morta
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