4,485 research outputs found
Coupled quintessence and vacuum decay
We discuss observational consequences of a class of cosmological models
characterized by the dilution of pressureless matter attenuated with respect to
the usual scaling due to the decay of vacuum energy. We carry out a
joint statistical analysis of observational data from the new \emph{gold}
sample of 182 SNe Ia, recent estimates of the CMB shift parameter, and BAO
measurements from the SDSS to show that such models favor the decay of vacuum
only into the dark matter sector, and that the separately conserved baryons
cannot be neglected. In order to explore ways to more fundamentally motivated
models, we also derive a coupled scalar field version for this general class of
vacuum decay scenarios.Comment: 6 pages, 3 figures, LaTe
The Seyfert Population in the Local Universe
The magnitude-limited catalog of the Southern Sky Redshift Survey (SSRS2), is
used to characterize the properties of galaxies hosting Active Galactic Nuclei.
Using emission-line ratios, we identify a total of 162 (3%) Seyfert galaxies
out of the parent sample with 5399 galaxies. The sample contains 121 Seyfert 2
galaxies and 41 Seyfert 1. The SSRS2 Seyfert galaxies are predominantly in
spirals of types Sb and earlier, or in galaxies with perturbed appearance as
the result of strong interactions or mergers. Seyfert galaxies in this sample
are twice as common in barred hosts than the non-Seyferts. By assigning
galaxies to groups using a percolation algorithm we find that the Seyfert
galaxies in the SSRS2 are more likely to be found in binary systems, when
compared to galaxies in the SSRS2 parent sample. However, there is no
statistically significant difference between the Seyfert and SSRS2 parent
sample when systems with more than 2 galaxies are considered. The analysis of
the present sample suggests that there is a stronger correlation between the
presence of the AGN phenomenon with internal properties of galaxies
(morphology, presence of bar, luminosity) than with environmental effects
(local galaxy density, group velocity dispersion, nearest neighbor distance).Comment: 35 pages, 13 figures, Accepted to be publised in Astronomical Journa
Dynamic model of gene regulation for the lac operon
Gene regulatory network is a collection of DNA which interact with each other and with other matter in the cell. The lac operon is an example of a relatively simple genetic network and is one of the best-studied structures in the Escherichia coli bacteria. In this work we consider a deterministic model of the lac operon with a noise term, representing the stochastic nature of the regulation. The model is written in terms of a system of simultaneous first order differential equations with delays. We investigate an analytical and numerical solution and analyse the range of values for the parameters corresponding to a stable solution
On the Nature of the Cosmological Constant Problem
General relativity postulates the Minkowski space-time to be the standard
flat geometry against which we compare all curved space-times and the
gravitational ground state where particles, quantum fields and their vacuum
states are primarily conceived. On the other hand, experimental evidences show
that there exists a non-zero cosmological constant, which implies in a deSitter
space-time, not compatible with the assumed Minkowski structure. Such
inconsistency is shown to be a consequence of the lack of a application
independent curvature standard in Riemann's geometry, leading eventually to the
cosmological constant problem in general relativity.
We show how the curvature standard in Riemann's geometry can be fixed by
Nash's theorem on locally embedded Riemannian geometries, which imply in the
existence of extra dimensions. The resulting gravitational theory is more
general than general relativity, similar to brane-world gravity, but where the
propagation of the gravitational field along the extra dimensions is a
mathematical necessity, rather than being a a postulate. After a brief
introduction to Nash's theorem, we show that the vacuum energy density must
remain confined to four-dimensional space-times, but the cosmological constant
resulting from the contracted Bianchi identity is a gravitational contribution
which propagates in the extra dimensions. Therefore, the comparison between the
vacuum energy and the cosmological constant in general relativity ceases to be.
Instead, the geometrical fix provided by Nash's theorem suggests that the
vacuum energy density contributes to the perturbations of the gravitational
field.Comment: LaTex, 5 pages no figutres. Correction on author lis
Crescimento do coqueiro jovem no nordeste de São Paulo.
bitstream/item/81095/1/CPATC-COM.-TEC.-21-98.pd
Germinação da semente e desenvolvimento da plântula de coqueiro anão verde no Nordeste de São Paulo.
Avalia o comportamento germinativo da semente e o desenvolvimento da plântula de coqueiro Anão, em condições definidas.bitstream/item/79800/1/CPATC-COM.-TEC.-12-97.pd
Capacidade predatória e aspectos biológicos de Chrysoperla externa (Hagen , 1861) (Neuroptera: Chrysopidae) alimentada com Rhopalosiphum, maidis (Fitch, 1856) (Hemiptera: Aphididae).
Objetivou-se estudar alguns aspectos biológicos. a capacidade predatória. e a resposta funcional de larvas de Chrysoperla externa (Hagen) alimentadas com ninfas de segundo e terceiro ínstares do pulgão Rhopalosiphum maidis (Fitch). em cinco densidades. Os ensaios foram conduzidos em câmaras climatizadas reguladas a 25 +/- 1°C. UR de 70 +/- 10% e fotofase de 12 horas. Para o estudo de biologia e capacidade predatória. o delineamento foi inteiramente casualizado. com cinco repetições e oito indivíduos/repetição. totalizando 40 larvas e. para a resposta funcional. os tratamentos corresponderam a cinco densidades de presa e cinco repetições com dois indivíduos/repetição. As observações foram realizadas às 24. 48 horas e no final de cada instar. O consumo aumentou proporcionalmente em função do estádio de desenvolvimento da larva. sendo maior no terceiro instar. com 279.1 pulgões. representando aproximadamente 82.0% do consumo total. Observaram-se uma duração de 11 dias para. a fase larva! e uma viabilidade de 100% em todos os estádios de desenvolvimento. Para os três ínstares e fase larval. constatou-se maior consumo e duração em função do aumento da densidade de presas. Verificou-se uma resposta funcional Tipo II para. todos os ínstares. Constatou-se. para larvas de segundo e terceiro instares. o menor tempo de busca. e o primeiro instar apresentou a maior tempo de manuseio
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