461 research outputs found

    Classical analogous of quantum cosmological perfect fluid models

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    Quantization in the mini-superspace of a gravity system coupled to a perfect fluid, leads to a solvable model which implies singularity free solutions through the construction of a superposition of the wavefunctions. We show that such models are equivalent to a classical system where, besides the perfect fluid, a repulsive fluid with an equation of state pQ=ρQp_Q = \rho_Q is present. This leads to speculate on the true nature of this quantization procedure. A perturbative analysis of the classical system reveals the condition for the stability of the classical system in terms of the existence of an anti-gravity phase.Comment: Latex file, 10 pages, 3 figure

    Quantum cosmological perfect fluid model and its classical analogue

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    The quantization of gravity coupled to a perfect fluid model leads to a Schr\"odinger-like equation, where the matter variable plays the role of time. The wave function can be determined, in the flat case, for an arbitrary barotropic equation of state p=αρp = \alpha\rho; solutions can also be found for the radiative non-flat case. The wave packets are constructed, from which the expectation value for the scale factor is determined. The quantum scenarios reveal a bouncing Universe, free from singularity. We show that such quantum cosmological perfect fluid models admit a universal classical analogue, represented by the addition, to the ordinary classical model, of a repulsive stiff matter fluid. The meaning of the existence of this universal classical analogue is discussed. The quantum cosmological perfect fluid model is, for a flat spatial section, formally equivalent to a free particle in ordinary quantum mechanics, for any value of α\alpha, while the radiative non-flat case is equivalent to the harmonic oscillator. The repulsive fluid needed to reproduce the quantum results is the same in both cases.Comment: Latex file, 13 page

    On the Dynamics of Bianchi IX cosmological models

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    A cosmological description of the universe is proposed in the context of Hamiltonian formulation of a Bianchi IX cosmology minimally coupled to a massless scalar field. The classical and quantum results are studied with special attention to the case of closed Friedmann-Robertson-Walker model.Comment: 11 pages, 1 figur

    On exact solutions for quintessential (inflationary) cosmological models with exponential potentials

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    We first study dark energy models with a minimally-coupled scalar field and exponential potentials, admitting exact solutions for the cosmological equations: actually, it turns out that for this class of potentials the Einstein field equations exhibit alternative Lagrangians, and are completely integrable and separable (i.e. it is possible to integrate the system analytically, at least by quadratures). We analyze such solutions, especially discussing when they are compatible with a late time quintessential expansion of the universe. As a further issue, we discuss how such quintessential scalar fields can be connected to the inflationary phase, building up, for this class of potentials, a quintessential inflationary scenario: actually, it turns out that the transition from inflation toward late-time exponential quintessential tail admits a kination period, which is an indispensable ingredient of this kind of theoretical models. All such considerations have also been done by including radiation into the model.Comment: Revtex4, 10 figure

    Surto alimentar por Salmonella Enteritidis no Noroeste do Estado de SĂŁo Paulo, Brasil

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    A foodborne outbreak which affected 211 persons occurred, in a School, in 1993. The epidemiological data obtained by interviewing the affected and non affected persons sampled showed as predominant symptoms: diarrhoea, fever (77.7%), abdominal cramps (67.7%), vomiting (65.8%), hot-and-cold sensations (54.5%) and headache (44.5%). The median incubation period was of 17 hours, the limits being 3 and 29 hours. The disease period was of from 3 to 4 days. The food concerned was a kind of patĂ©, a mayonnaise mixture prepared with fresh eggs with boiled potatoes that was consumed with bread. The biological material analysis - 3 coprocultures, and leftovers of the food revealed the presence of one and the same organism: Salmonella Enteritidis. In the food, the numbers of this bacterium per gram were sufficient to account for the manifestation of the disease (104 and 105/g). The antibiogram of all the isolates showed the same sensibility pattern. The preparation related to this outbreak suggests the endogenous contamination of the eggs; the cross contamination - the outbreak affected three school periods, as the food was prepared separately for each school period; and the conditions under which the food was kept during the time from preparation to consumption. The observation of the 3 food handlers, by successive coprocultures, for one week, indicates that they were not asymptomatic carriers nor were they affected as a result of this outbreak by the causal bacteria.Em 1993 ocorreu um surto alimentar em escola, com 211 afetados. Os dados epidemiolĂłgicos levantados por entrevista de amostragem de afetados e nĂŁo afetados mostraram que os sintomas predominantes foram diarrĂ©ia, febre (77,7%), dor abdominal (67,7%), vĂŽmito (65,8%), calafrios (54,5%) e cefalĂ©ia (44,5%). A mediana de incubação foi de 17 horas, com limites entre 3 e 29 horas. A duração da doença foi de 3 a 4 dias. O alimento consumido foi um tipo de patĂȘ, mistura de molho de maionese preparada com ovos crus com batata cozida, passado em pĂŁo. A anĂĄlise de material biolĂłgico (3 coproculturas) e de restos de alimentos revelou a presença do mesmo microrganismo, a Salmonella Enteritidis. No caso dos alimentos, o nĂșmero encontrado desta bactĂ©ria por gramo de produto era compatĂ­vel com a quantidade de cĂ©lulas necessĂĄria para desencadear a doença (10(4)e 10(5)/g). O antibiograma de todas as cepas isoladas revelou o mesmo padrĂŁo de sensibilidade. As falhas no preparo do alimento relacionadas com o levantamento indicam a possibilidade de contaminação endĂłgena dos ovos; contaminação cruzada - o surto afetou trĂȘs perĂ­odos escolares, sendo que para cada um o alimento foi preparado em separado - e as condiçÔes de manutenção do alimento apĂłs preparo e atĂ© o consumo. A observação por uma semana seguida das 3 merendeiras envolvidas, atravĂ©s de coprocultura, nĂŁo indicaram que as mesmas fossem portadoras assintomĂĄticas desta bactĂ©ria ou que tivessem sido envolvidas no surto em questĂŁo

    Primordial perturbations in a non singular bouncing universe model

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    We construct a simple non singular cosmological model in which the currently observed expansion phase was preceded by a contraction. This is achieved, in the framework of pure general relativity, by means of a radiation fluid and a free scalar field having negative energy. We calculate the power spectrum of the scalar perturbations that are produced in such a bouncing model and find that, under the assumption of initial vacuum state for the quantum field associated with the hydrodynamical perturbation, this leads to a spectral index n=-1. The matching conditions applying to this bouncing model are derived and shown to be different from those in the case of a sharp transition. We find that if our bounce transition can be smoothly connected to a slowly contracting phase, then the resulting power spectrum will be scale invariant.Comment: 11 pages, RevTeX 4, 8 figures, submitted to Phys. Rev.

    Spin chains with dynamical lattice supersymmetry

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    Spin chains with exact supersymmetry on finite one-dimensional lattices are considered. The supercharges are nilpotent operators on the lattice of dynamical nature: they change the number of sites. A local criterion for the nilpotency on periodic lattices is formulated. Any of its solutions leads to a supersymmetric spin chain. It is shown that a class of special solutions at arbitrary spin gives the lattice equivalents of the N=(2,2) superconformal minimal models. The case of spin one is investigated in detail: in particular, it is shown that the Fateev-Zamolodchikov chain and its off-critical extension admits a lattice supersymmetry for all its coupling constants. Its supersymmetry singlets are thoroughly analysed, and a relation between their components and the weighted enumeration of alternating sign matrices is conjectured.Comment: Revised version, 52 pages, 2 figure

    Magnetization and dimerization profiles of the cut two-leg spin ladder and spin-1 chain

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    The physical properties of the edge states of the cut two-leg spin ladder are investigated by means of the bosonization approach. By carefully treating boundary conditions, we derive the existence of spin-1/2 edge states in the spin ladder with a ferromagnetic rung exchange and for the open spin-1 Heisenberg chain. In contrast, such states are absent in the antiferromagnetic rung coupling case. The approach, based on a mapping onto decoupled semi-infinite off-critical Ising models, allows us to compute several physical quantities of interest. In particular, we determine the magnetization and dimerization profiles of the cut two-leg spin ladder and of the open biquadratic spin-1 chain in the vicinity of the SU(2)2_2 WZNW critical point.Comment: RevTeX 4, no figure, 26 page
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