Dark Interactions and Cosmological Fine-Tuning

Cosmological models involving an interaction between dark matter and dark energy have been proposed in order to solve the so-called coincidence problem. Different forms of coupling have been studied, but there have been claims that observational data seem to narrow (some of) them down to something annoyingly close to the $\Lambda$CDM model, thus greatly reducing their ability to deal with the problem in the first place. The smallness problem of the initial energy density of dark energy has also been a target of cosmological models in recent years. Making use of a moderately general coupling scheme, this paper aims to unite these different approaches and shed some light as to whether this class of models has any true perspective in suppressing the aforementioned issues that plague our current understanding of the universe, in a quantitative and unambiguous way.Comment: 13 pages, 9 figures, accepted for publication in JCAP. Minor corrections, one figure replaced, references adde

Efeitos da dispersão e deposição de arsénio, cobre, chumbo e zinco na zona envolvente à mina de Neves-Corvo

A região de Neves-Corvo (37°34'13,5"N, 7°58’19,1”O) possui o mais importante depósito de cobre de Portugal, sendo ainda um dos maiores do mundo. Situada a 15 km de Castro Verde, no Baixo Alentejo, esta mina produz essencialmente cobre, estanho e zinco. A mineração constitui uma atividade extremamente poluente alterando a composição da atmosfera, do solo, e águas quer subterrâneas ou superficiais. O objetivo deste trabalho é estudar a dispersão e deposição dos elementos (arsénio, cobre, chumbo e zinco) provenientes da mineração, em folhas de plantas hortícolas locais e em 44 amostras de solo num raio de 1,5 km. O material vegetal foi dividido em dois grupos, com igual número de folhas de cada planta. Todas as folhas de um dos grupos foram lavadas por água destilada, para remoção das poeiras. As amostras de solo foram recolhidas até 10 cm de profundidade. No laboratório, as amostras de solo foram peneiradas para retirar o material grosseiro e serem homogeneizadas. Todas as amostras foram seguidamente secas em estufa a 60ºC até obtenção de peso constante. A quantificação e análise dos elementos foi feita através de um Thermo Scientific™ Niton™ XL3t GOLDD + XRF. Observa-se que na maior parte das folhas não lavadas a quantidade de elementos é maior que nas folhas lavadas. No entanto é nas folhas perenes da laranjeira que a diferença se faz notar em todos os elementos, com diferenças de 45 % para o arsénio, 23% para o cobre, 50% para o chumbo e 20% para o zinco. As 42 amostras de solo provenientes de solos de zonas agrícolas mostraram uma média de 28.3 mg de As kg-1, 77.7 mg de Cu kg-1, 21.9 mg de Pb kg-1 e 59.4 mg de Zn kg-1. Os valores médios de arsénio e cobre estão acima dos valores de referências usados pela Agencia Portuguesa do Ambiente (APA) (11 mg de As kg-1 e 62 mg de Cu kg-1). Em duas amostras provenientes de duas aldeias próximas da mina (A-do-Corvo e Neves da Graça), os valores de arsénio são superiores aos valores de referência para zonas residenciais (18 mg de As kg-1). No caso da amostra de solo proveniente da aldeia de A-do-Corvo, os valores de cobre são 9,77 vezes superiores aos valores de referência (92 mg de Cu kg-1). Na observação de valores superiores aos estipulados pela APA e sabendo que a Faixa Piritosa Ibérica (FPI) apresentam valores de fundo geoquímico superiores, calcularam-se valores de referência locais. Com estes valores de referência obtiveram-se valores de corte, dos quais podemos dizer efetivamente quais as zonas contaminadas. Foram realizados mapas morfológicos onde estão visíveis as áreas com os valores de arsénio e cobre. Estas áreas foram divididas em não contaminadas (Azul), incerteza (Verde) e contaminadas (Vermelho). Concluiu-se que dos quatro elementos analisados nas amostras de solo, o arsénio em especial e também o cobre apresentam concentrações em alguns lugares, acima do valor de corte. Essas zonas poderão ser passíveis de uma futura monotorização ou remediação.info:eu-repo/semantics/publishedVersio

Adiabatic decaying vacuum model for the universe

We study a model that the entropy per particle in the universe is constant. The sources for the entropy are the particle creation and a lambda decaying term. We find exact solutions for the Einstein field equations and show the compatibilty of the model with respect to the age and the acceleration of the universe.Comment: 10 pages, 2 figure

Taub's plane-symmetric vacuum spacetime revisited

The gravitational properties of the {\em only} static plane-symmetric vacuum solution of Einstein's field equations without cosmological term (Taub's solution, for brevity) are presented: some already known properties (geodesics, weak field limit and pertainment to the Schwarzschild family of spacetimes) are reviewed in a physically much more transparent way, as well as new results about its asymptotic structure, possible matchings and nature of the source are furnished. The main results point to the fact that the solution must be interpreted as representing the exterior gravitational field due to a {\em negative} mass distribution, confirming previous statements to that effect in the literature. Some analogies to Kasner's spatially homogeneous cosmological model are also referred to.Comment: plain LaTex, four Postscript figure

Cosmological particle production, causal thermodynamics, and inflationary expansion

Combining the equivalence between cosmological particle creation and an effective viscous fluid pressure with the fact that the latter represents a dynamical degree of freedom within the second-order Israel-Stewart theory for imperfect fluids, we reconsider the possibility of accelerated expansion in fluid cosmology. We find an inherent self-limitation for the magnitude of an effective bulk pressure which is due to adiabatic (isentropic) particle production. For a production rate which depends quadratically on the Hubble rate we confirm the existence of solutions which describe a smooth transition from inflationary to noninflationary behavior and discuss their interpretation within the model of a decaying vacuum energy density. An alternative formulation of the effective imperfect fluid dynamics in terms of a minimally coupled scalar field is given. The corresponding potential is discussed and an entropy equivalent for the scalar field is found.Comment: 16 pages, revtex file, submitted to Phys. Rev.

Thermodynamics of Decaying Vacuum Cosmologies

The thermodynamic behavior of vacuum decaying cosmologies is investigated within a manifestly covariant formulation. Such a process corresponds to a continuous irreversible energy flow from the vacuum component to the created matter constituents. It is shown that if the specific entropy per particle remains constant during the process, the equilibrium relations are preserved. In particular, if the vacuum decays into photons, the energy density $\rho$ and average number density of photons $n$ scale with the temperature as $\rho \sim T^{4}$ and $n \sim T^{3}$. The temperature law is determined and a generalized Planckian type form of the spectrum, which is preserved in the course of the evolution, is also proposed. Some consequences of these results for decaying vacuum FRW type cosmologies as well as for models with ``adiabatic'' photon creation are discussed.Comment: 21 pages, uses LATE

Cosmic anti-friction and accelerated expansion

We explain an accelerated expansion of the present universe, suggested from observations of supernovae of type Ia at high redshift, by introducing an anti-frictional force that is self-consistently exerted on the particles of the cosmic substratum. Cosmic anti-friction, which is intimately related to ``particle production'', is shown to give rise to an effective negative pressure of the cosmic medium. While other explanations for an accelerated expansion (cosmological constant, quintessence) introduce a component of dark energy besides ``standard'' cold dark matter (CDM) we resort to a phenomenological one-component model of CDM with internal self-interactions. We demonstrate how the dynamics of the LambdaCDM model may be recovered as a special case of cosmic anti-friction. We discuss the connection with two-component models and obtain an attractor behavior for the ratio of the energy densities of both components which provides a possible phenomenological solution to the coincidence problem.Comment: 19 pages, 7 (3 new) figures, new derivation of kinetic equation with force term, accepted by Physical Review