Some remarks on the attractor behaviour in ELKO cosmology

Recent results on the dynamical stability of a system involving the interaction of the ELKO spinor field with standard matter in the universe have been reanalysed, and the conclusion is that such system does not exhibit isolated stable points that could alleviate the cosmic coincidence problem. When a constant parameter $\delta$ related to the potential of the ELKO field is introduced in the system however, stable fixed points are found for some specific types of interaction between the ELKO field and matter. Although the parameter $\delta$ is related to an unknown potential, in order to satisfy the stability conditions and also that the fixed points are real, the range of the constant parameter $\delta$ can be constrained for the present time and the coincidence problem can be alleviated for some specific interactions. Such restriction on the ELKO potential opens possibility to apply the ELKO field as a candidate to dark energy in the universe, and so explain the present phase of acceleration of the universe through the decay of the ELKO field into matter.Comment: 17 pages, section III with minor changes and section IV rewritten with a new analysi

Braneworlds scenarios in a gravity model with higher order spatial three-curvature terms

In this work we study a Horava-like five-dimensional model in the context of braneworld theory. To begin with, the equations of motion of such model are obtained and, within the realm of warped geometry, we show that the model is consistent if and only if $\lambda$ takes its relativistic value 1. Furthermore, since the first derivative of the warp factor is discontinuous over the branes, we show that the elimination of problematic terms involving the square of the warp factor second order derivatives are eliminated by imposing detailed balance condition in the bulk. Afterwards, the Israel's junction conditions are computed, allowing the attainment of an effective Lagrangian in the visible brane. In particular, for a (4+1)-dimensional Horava-like model defined in the bulk without cosmological constant, we show that the resultant effective Lagrangian in the brane corresponds to a (3+1)-dimensional Horava-like model with an emergent positive cosmological constant but without detailed balance condition. Now, restoration of detailed balance condition, at this time imposed over the brane, plays an interesting role by fitting accordingly the sign of the arbitrary constant $\beta$ that labels the extra terms in the model, insuring a positive brane tension and a real energy for the graviton within its dispersion relation. To end up with, the brane consistency equations are obtained and, as a result, we show that the detailed balance condition again plays an essential role in eliminating bad behaving terms and that the model admits positive brane tensions in the compactification scheme if, and only if, $\beta$ is negative, what is in accordance with the previous result obtained for the visible brane.Comment: 23 pages, 1 figure, title modifie

Notes on the Two-brane Model with Variable Tension

Motivated by possible extensions of the braneworld models with two branes, we investigate some consequences of a variable brane tension using the well established results on consistency conditions. By a slight modification of the usual stress-tensor used in order to derive the braneworld sum rules, we find out some important constraints obeyed by time dependent brane tensions. In particular it is shown that the tensions of two Randall-Sundrum like branes obeying, at the same time, an Eotvos law, aggravate the fine tuning problem. Also, it is shown that if the hidden brane tension obeys an Eotvos law, then the visible brane has a mixed behavior allowing a bouncing-like period at early times while it is dominated by an Eotvos law nowadays. To finalize, we discuss some qualitative characteristics which may arise in the scope of dynamical brane tensions, as anisotropic background and branons production.Comment: 7 pages, 1 figure, accepted for publication in Physical Review

Impacto do aquecimento global na aptidão da laranjeira no Estado da Bahia.

O Brasil produziu aproximadamente 18,5 milhões de toneladas de frutos de laranjeira no ano de 2008, sendo que a Bahia contribuiu como segundo maior produtor brasileiro (6%), seguido pelos Estados de Sergipe (4%) e Minas Gerais (3%) e atrás apenas de São Paulo (78%) (IBGE, 2008). As produtividades médias obtidas na Bahia (17 t/ha) inferiores às alcançadas em São Paulo (24 t/ha) refletem tanto a aptidão das regiões onde estão situados os pomares no Estado, apresentando transição acentuada do clima úmido para o subúmido seco, bem como as deficiências nas práticas agrícolas e manejo agronômico da cultura (plantio de mudas de qualidade, espaçamentos adequados, diversificação de porta-enxertos, adubação, monitoramento e manejo fitossanitário, irrigação, dentre outros).O cultivo da laranjeira na Bahia é caracterizado por ser desenvolvimento predominantemente em condições de sequeiro, e o estresse por deficiência de água no solo é o maior limitador da expansão, sendo este o principal critério utilizadonos trabalhos de zoneamentos climático no Nordeste brasileiro. A distribuição irregular das chuvas interfere na qualidade e produtividade de frutos e esses riscos podem ser aumentados com as previsões futuras de ocorrência de eventos extremos (seca e enchentes) e maior restrição hídrica no solo, devido ao aumento da evapotranspiração das culturas, relacionadas ao aquecimento global (Assad & Pinho, 2008; Semenov, 2009).O presente trabalho teve o objetivo de avaliar os impactos do aquecimento global (aumento de temperatura) na aptidão da cultura da laranja no Estado da Bahia.pdf 236