142 research outputs found
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
Evidence of biomass smoke exposure as a causative factor for the development of COPD
© 2017 by the authors. Chronic obstructive pulmonary disease (COPD) is a progressive disease of the lungs characterised by chronic inflammation, obstruction of airways, and destruction of the parenchyma (emphysema). These changes gradually impair lung function and prevent normal breathing. In 2002, COPD was the fifth leading cause of death, and is estimated by theWorld Health Organisation (WHO) to become the third by 2020. Cigarette smokers are thought to be the most at risk of developing COPD. However, recent studies have shown that people with life-long exposure to biomass smoke are also at high risk of developing COPD. Most common in developing countries, biomass fuels such as wood and coal are used for cooking and heating indoors on a daily basis. Women and children have the highest amounts of exposures and are therefore more likely to develop the disease. Despite epidemiological studies providing evidence of the causative relationship between biomass smoke and COPD, there are still limited mechanistic studies on how biomass smoke causes, and contributes to the progression of COPD. This review will focus upon why biomass fuels are used, and their relationship to COPD. It will also suggest methodological approaches to model biomass exposure in vitro and in vivo
Redirecting benefits to genuine fishermen: Bangladesh's new fisheries management policy
Policies, Fishery management, Bangladesh,
Ensino de Ciências e Matemática: elementos didáticos para teoria e experimentação
O livro Ensino de Ciências e Matemática — elementos didáticos para teoria e experimentação visa ser um referencial para professores e estudantes destas duas áreas do conhecimento pautado na ideia da busca de novas práticas pedagógicas para o desenvolvimento de competências por meio da aprendizagem dialógica, cuja base é o correto trabalho da teoria e da experimentação — elementos norteadores do pensamento científico.
Seu conteúdo é resultante de notas de aula da Especialização em Ensino de Ciências e Matemática ofertada por docentes da Universidade Federal da Integração Latino-Americana (UNILA) como curso de formação continuada (Comfor/MEC) para professores da educação básica no Oeste do Paraná
Perturbations of Dark Matter Gravity
Until recently the study of the gravitational field of dark matter was
primarily concerned with its local effects on the motion of stars in galaxies
and galaxy clusters. On the other hand, the WMAP experiment has shown that the
gravitational field produced by dark matter amplifies the higher acoustic modes
of the CMBR power spectrum, more intensely than the gravitational field of
baryons. Such a wide range of experimental evidences from cosmology to local
gravity suggests the necessity of a comprehensive analysis of the dark matter
gravitational field per se, regardless of any other attributes that dark matter
may eventually possess.
In this paper we introduce and apply Nash's theory of perturbative geometry
to the study of the dark matter gravitational field alone, in a
higher-dimensional framework. It is shown that the dark matter gravitational
perturbations in the early universe can be explained by the extrinsic curvature
of the standard cosmology. Together with the estimated presence of massive
neutrinos, such geometric perturbation is compatible not only with the observed
power spectrum in the WMAP experiment but also with the most recent data on the
accelerated expansion of the universe.
It is possible that the same structure formation exists locally, such as in
the cases of young galaxies or in cluster collisions. In most other cases it
seems to have ceased when the extrinsic curvature becomes negligible, leading
to Einstein's equations in four dimensions. The slow motion of stars in
galaxies and the motion of plasma substructures in nearly colliding clusters
are calculated with the geodesic equation for a slowly moving object in a
gravitational field of arbitrary strength.Comment: 10 pages, 5 figure
The Deformable Universe
The concept of smooth deformations of a Riemannian manifolds, recently
evidenced by the solution of the Poincar\'e conjecture, is applied to
Einstein's gravitational theory and in particular to the standard FLRW
cosmology. We present a brief review of the deformation of Riemannian geometry,
showing how such deformations can be derived from the Einstein-Hilbert
dynamical principle. We show that such deformations of space-times of general
relativity produce observable effects that can be measured by four-dimensional
observers. In the case of the FLRW cosmology, one such observable effect is
shown to be consistent with the accelerated expansion of the universe.Comment: 20 pages, LaTeX, 3 figure
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