Gravitational Repulsion in an Expanding Ball of Dust

In general relativity, there is a velocity dependent term in the gravitational acceleration of a test particle for an observer at infinity. Depending on the direction of motion and the speed, that term can be repulsive. We show that this is also the case in the Parametrized Post-Newtonian (PPN) formalism. We compute the magnitude of that repulsive term for an expanding sphere of dust observed at infinity, and find that it could mimic the effect of a cosmological constant. The time evolution of such an expanding ball of dust for an observer at infinity is calculated, and compared with the standard $\Lambda$CDM model. We find that the so-called coincidence problem does not exist for such a model as the energy density attributed to the expansion is always of the same order as the matter energy density.Comment: 8 pages, 2 figures. Accepted for publication in Class. Quantum Gra

A viable relativistic scalar theory of gravitation

We build a self-consistent relativistic scalar theory of gravitation on a flat Minkowski spacetime from a general field Lagrangian. It is shown that, for parameters that satisfy the Equivalence Principle, this theory predicts the same outcome as general relativity for every classical solar-system test. This theory also admits gravitational waves that propagate at the speed of light, and the gravitational radiation energy loss in a binary system is shown to be very similar to the GR prediction. We then analyze the strong gravity regime of the theory for a spherically symmetric configuration and find that there is an effective "singularity" near the Schwarzschild radius. The main goal of this work is to show that, contrary to what is commonly believed, there are relativistic scalar theories of gravitation defined on a Minkowski spacetime that are not ruled out by the classical solar system tests of general relativity.Comment: 15 pages, 4 figures. Accepted for publication in Classical and Quantum Gravit

Ejercicio físico y estilo de vida sedentario: consecuencias para la salud

Perhaps the greatest barriers for achieving major advances in public health in the twenty-first century result from the paralysis of the pandemic paradigm or from the widespread inability to envision alternative or new models of thought. Human movement represents a complex behavior that is influenced by personal motivation, health and mobility problems, genetic factors, and social and physical environments in which people live. These factors influence the propensity to engage in sedentary behaviors or in physical activity. However, the biological, social, and environmental pathways leading to sedentary behavior versus physical activity may be different. In addition, the health effects associated with sedentary behavior and physical activity may be the result of different biological mechanisms. Thus, our objective was to discuss the importance of physical exercise on health-related outcomes and the consequences of sedentary lifestyles. Research on sedentary behavior has been growing;however, the evidence for its determinants is relatively sparse. More studies are needed to obtain more conclusive results because it is fundamental to understand these complex relationships related to the practice and the acquisition of active and healthy lifestyles as opposed to a sedentary lifestyle.info:eu-repo/semantics/publishedVersio

Economics of Robust Surveillance on Exotic Animal Diseases: the Case of Bluetongue

Control of emerging animal diseases critically depends on their early detection. However, designing surveillance programs for exotic and emerging diseases is very challenging because of knowledge gaps on the probability of incursion and mechanisms of spread. Using the example of Bluetongue Virus, which is exotic to the UK, we develop a metapopulation epidemic-economic modelling framework that considers the incursion, detection, spread and control of a disease in a livestock production system composed of heterogeneous subpopulations. The model is then embedded in an information gap (info-gap) framework to assess the robustness of surveillance and vaccination policies to unacceptable outbreaks losses and applied to the case of Bluetongue in the UK. The results show that active reporting of suspect clinical signs by farmers is a very robust way to reduce unacceptable outcomes. Vaccination of animals in high risk regions led to robustly protective programs. If vaccines are not available, surveillance targeted to the high risk region is very robust even if the extent of the high risk region is not known and effectiveness of detection is very low. Surveillance programs focusing in all regions with the same intensity are in general not robust unless the dispersal of the vector connecting both regions is very high.compartmental epidemic model, emergent animal disease, Knightian uncertainty, sentinel surveillance system, Livestock Production/Industries,

A new perspective on the quaternionic numerical range of normal matrices

A new geometric proof of a known result characterizing the quaternionic numerical range of normal matrices is proposed. Our proof can be interpreted in probabilistic terms.info:eu-repo/semantics/acceptedVersio

On the convexity and circularity of the numerical range of nilpotent quaternionic matrices

We provide a sufficient condition for the numerical range of a nilpotent matrix N to be circular in terms of the existence of cycles in an undirected graph associated with N. We prove that if we add to this matrix N a diagonal real matrix D, the matrix D+N has convex numerical range. For 3 x 3 nilpotent matrices, we strength further our results and obtain necessary and sufficient conditions for circularity and convexity of the numerical range.info:eu-repo/semantics/publishedVersio

A transsignaling strategy for QoS support in heterogeneous networks

The increasing usage of multiple signalling mechanisms, with associated QoS extensions, creates several problems to commercial data networks. New and scalable approaches are required for the network operator to support this diversity. This paper discusses a highly flexible, scalable architecture for processing QoS Admission Control in public networks. The architecture relies on the cooperation of two different entities, an agent and a manager, with fully distributed implementation, and able to perform the required signalling, authorization, and admission control decisions. If required, the agent is capable of interfacing with different signalling mechanisms. Early implementation conclusions are also presented. This architecture is capable of operating with multiple QoS frameworks, with minimal added overhead