Isotropic singularities in shear-free perfect fluid cosmologies

We investigate barotropic perfect fluid cosmologies which admit an isotropic singularity. From the General Vorticity Result of Scott, it is known that these cosmologies must be irrotational. In this paper we prove, using two different methods, that if we make the additional assumption that the perfect fluid is shear-free, then the fluid flow must be geodesic. This then implies that the only shear-free, barotropic, perfect fluid cosmologies which admit an isotropic singularity are the FRW models.Comment: 21 pages, 1 figur

General properties of cosmological models with an Isotropic Singularity

Much of the published work regarding the Isotropic Singularity is performed under the assumption that the matter source for the cosmological model is a barotropic perfect fluid, or even a perfect fluid with a $\gamma$-law equation of state. There are, however, some general properties of cosmological models which admit an Isotropic Singularity, irrespective of the matter source. In particular, we show that the Isotropic Singularity is a point-like singularity and that vacuum space-times cannot admit an Isotropic Singularity. The relationships between the Isotropic Singularity, and the energy conditions, and the Hubble parameter is explored. A review of work by the authors, regarding the Isotropic Singularity, is presented.Comment: 18 pages, 1 figur

Surface oscillations in channeled snow flows

An experimental device has been built to measure velocity profiles and friction laws in channeled snow flows. The measurements show that the velocity depends linearly on the vertical position in the flow and that the friction coefficient is a first-order polynomial in velocity (u) and thickness (h) of the flow. In all flows, oscillations on the surface of the flow were observed throughout the channel and measured at the location of the probes. The experimental results are confronted with a shallow water approach. Using a Saint-Venant modeling, we show that the flow is effectively uniform in the streamwise direction at the measurement location. We show that the surface oscillations produced by the Archimedes's screw at the top of the channel persist throughout the whole length of the channel and are the source of the measured oscillations. This last result provides good validation of the description of such channeled snow flows by a Saint-Venant modeling

Martian magnetism with orbiting sub-millimeter sensor:simulated retrieval system

A Mars-orbiting sub-millimeter sensor can be used to retrieve the magnetic field at low altitudes over large areas of significant planetary crustal magnetism of the surface of Mars from measurements of circularly polarized radiation emitted by the 368 GHz ground-state molecular oxygen absorption line. We design a full retrieval system for one example orbit to show the expected accuracies on the magnetic field components that one realization of such a Mars satellite mission could achieve. For one set of measurements around a tangent profile, we find that the two horizontal components of the magnetic field can be measured at about 200 nT error with a vertical resolution of around 4 km from 6 up to 70 km in tangent altitude. The error is similar regardless of the true strength of the magnetic field, and it can be reduced by repeated measurements over the same area. The method and some of its potential pitfalls are described and discussed

Natural scene statistics and the structure of orientation maps in the visual cortex

Visual activity after eye-opening influences feature map structure in primary visual cortex (V1). For instance, rearing cats in an environment of stripes of one orientation yields an over-representation of that orientation in V1. However, whether such changes also affect the higher-order statistics of orientation maps is unknown. A statistical bias of orientation maps in normally raised animals is that the probability of the angular difference in orientation preference between each pair of points in the cortex depends on the angle of the line joining those points relative to a fixed but arbitrary set of axes. Natural images show an analogous statistical bias; however, whether this drives the development of comparable structure in V1 is unknown. We examined these statistics for normal, stripe-reared and dark-reared cats, and found that the biases present were not consistently related to those present in the input, or to genetic relationships. We compared these results with two computational models of orientation map development, an analytical model and a Hebbian model. The analytical model failed to reproduce the experimentally observed statistics. In the Hebbian model, while orientation difference statistics could be strongly driven by the input, statistics similar to those seen in experimental maps arose only when symmetry breaking was allowed to occur spontaneously. These results suggest that these statistical biases of orientation maps arise primarily spontaneously, rather than being governed by either input statistics or genetic mechanisms

Application of seminal plasma in sex-sorting and sperm cryopreservation

Substantial dilution of boar semen during processing decreased the concentration of seminal plasma, perhaps contributing to the decline in sperm quality after cryopreservation and sex-sorting. Results of replacing seminal plasma in investigations from many laboratories have been contradictory. Results and discussion here suggest that whereas membrane status can be influenced by seminal plasma, the action of its various components, both positive and negative, is determined in part by the membrane status of the spermatozoa to which it is being exposed. Although progress has been made in identifying components of seminal plasma responsible for its protective effect (notably PSP-I/II spermadhesin for sex-sorted boar spermatozoa), little is known (in any species) regarding how external factors may influence their levels, and their functionality, in seminal plasma. It is noteworthy that seminal plasma is beneficial to post-thaw quality of sex-sorted ram spermatozoa only when added before freezing, not after thawing. Therefore, the action of seminal plasma and its components is dependent on sperm-related factors, in particular the type of processing to which they have been previously exposed. Further research is needed to unravel these biological complexities, and then characterise and synthesise useful proteins within seminal plasma. © 2008

Proposta de plano de recuperação da Bacia Hidrográfica do Arroio Passo Fundo - Guaíba - RS

A Bacia Hidrográfica do Arroio Passo Fundo, localizada em Guaíba - RS, vem apresentando um quadro de degradação ambiental que tem piorado ao longo das últimas décadas, com prejuízo à qualidade ambiental e de vida dos moradores ribeirinhos. As ações do poder público têm se mostrado insuficientes diante deste cenário, com medidas reativas e propostas generalistas. Considerando um levantamento histórico baseado em notícias de imprensa em relação ao arroio, os planos existentes de gestão ambiental, além de debates e discussões recentes que resultaram em lei municipal de proteção ao arroio, este trabalho tem por objetivo propor um plano de recuperação de sua bacia hidrográfica. Para tal, foram identificados os principais problemas ambientais enfrentados na bacia, bem como os atores ambientais envolvidos e, através de pesquisa bibliográfica, foram elaboradas propostas de intervenção, buscando-se soluções ambientais exequíveis dentro da realidade socioeconômica do município. Foram contemplados tópicos relativos a esgotamento residencial, destinação de resíduos sólidos, lançamento de resíduos industriais, resíduos agroquímicos nas lavouras, disponibilidade hídrica, impactos na saúde da população ribeirinha, e o monitoramento da qualidade da água. Como resultado, obteve-se um plano estruturado, abordando cada um desses problemas com o detalhamento das propostas de intervenções reparadoras, trazendo iniciativas análogas e bem-sucedidas que podem servir de exemplo, além da indicação dos atores ambientais capazes de viabilizá-las. Para além do planejamento, no entanto, fica evidenciado que as medidas só serão concretizadas com participação social e organização política.The Arroio Passo Fundo Hydrographic Basin, located in Guaíba - RS, has been presenting a picture of environmental degradation that has worsened over the last decades, with damage to the environmental quality and life of the riverside residents. The actions of the public power have proved to be insufficient in this scenario, with reactive measures and generalist proposals. Considering a historical survey based on press news regarding the river, the existing plans for environmental management, as well as recent debates and discussions that resulted in a municipal law to protect the river, this work aims to propose a recovery plan for its basin hydrographic. For this, the main environmental problems faced in the basin and the environmental actors involved were identified and, through bibliographical research, intervention proposals were elaborated, seeking feasible environmental solutions within the socioeconomic reality of the municipality. Topics related to residential sewage, disposal of solid waste, release of industrial waste, agrochemical residues in agriculture, water availability, impacts on the health of the riverside population, and monitoring of water quality were covered. As a result, a structured plan was obtained, addressing each of these problems with the detailing of proposals for repairing interventions, bringing similar and successful initiatives that can serve as an example, also indicating the environmental actors capable of making them viable. In addition to planning, however, it is evident that the measures will only be implemented with social participation and political organization