7,150 research outputs found
Astrophysical signatures of boson stars: quasinormal modes and inspiral resonances
Compact bosonic field configurations, or boson stars, are promising dark
matter candidates which have been invoked as an alternative description for the
supermassive compact objects in active galactic nuclei. Boson stars can be
comparable in size and mass to supermassive black holes and they are hard to
distinguish by electromagnetic observations. However, boson stars do not
possess an event horizon and their global spacetime structure is different from
that of a black hole. This leaves a characteristic imprint in the
gravitational-wave emission, which can be used as a discriminant between black
holes and other horizonless compact objects. Here we perform a detailed study
of boson stars and their gravitational-wave signatures in a fully relativistic
setting, a study which was lacking in the existing literature in many respects.
We construct several fully relativistic boson star configurations, and we
analyze their geodesic structure and free oscillation spectra, or quasinormal
modes. We explore the gravitational and scalar response of boson star
spacetimes to an inspiralling stellar-mass object and compare it to its black
hole counterpart. We find that a generic signature of compact boson stars is
the resonant-mode excitation by a small compact object on stable quasi-circular
geodesic motion.Comment: 20 pages, 8 figures. v2: minor corrections, version to be published
in Phys. Rev. D. v3: final versio
Slowly Rotating Anisotropic Neutron Stars in General Relativity and Scalar-Tensor Theory
Some models (such as the Skyrme model, a low-energy effective field theory
for QCD) suggest that the high-density matter prevailing in neutron star
interiors may be significantly anisotropic. Anisotropy is known to affect the
bulk properties of nonrotating neutron stars in General Relativity. In this
paper we study the effects of anisotropy on slowly rotating stars in General
Relativity. We also consider one of the most popular extensions of Einstein's
theory, namely scalar-tensor theories allowing for spontaneous scalarization (a
phase transition similar to spontaneous magnetization in ferromagnetic
materials). Anisotropy affects the moment of inertia of neutron stars (a
quantity that could potentially be measured in binary pulsar systems) in both
theories. We find that the effects of scalarization increase (decrease) when
the tangential pressure is bigger (smaller) than the radial pressure, and we
present a simple criterion to determine the onset of scalarization by
linearizing the scalar-field equation. Our calculations suggest that binary
pulsar observations may constrain the degree of anisotropy or even, more
optimistically, provide evidence for anisotropy in neutron star cores.Comment: 19 pages, 7 figures, 1 table. Matches version in press in CQG. Fixed
small typo
The process of agricultural technology generation in Brazil: a social audit.
The focus of agricultural technology studies in Brazil has been on technology diffusion or adoption. This approach stresses the neutrality of technology and its adoption depends on farmers' psychological and individual values. The agricultural technology generation process and the organisations in which technology is generated have not been considered as active factors. This thesis regards both as highly significant in farmers' adoption or rejection of technology. Approaches to development, modernisation and underdevelopment, along with agricultural globalisation, are the applied theoretical perspectives used to understand what happens in the underdeveloped countries in an integrated world system. This is an ex-post facto and cross-sectional study. The empirical data, based on a case study, was collected in Brazil, in and around the Brazilian Agricultural Research Organisation (EMBRAPA), a topdown state-owned organisation. Agricultural technology generation, its adoption, as well as the attitudes of users, clients, policy-makers, politicians and unions to the agricultural technology generation process were investigated. The fieldwork was conducted with eighty-seven agricultural researchers from four national agricultural research centres, one hundred and forty-four farmers, and eighty individuals and organisations' representatives. Qualitative and quantitative analyses indicated that the agricultural technology generation process is related more to scientific issues than to farmers' demands. The technology adopted by farmers was determined primarily by developments within the process of technology generation rather than through any persuasion. The thesis concludes that as a result of the process of technology generation in EMBRAP A, organised and capitalist farmers have been targeted rather than small or subsistence farmers. Therefore, the new farm as a whole research model is recommended, which explores the whole production system rather than specific agricultural products.Dissertation (Doctor in Philosophy)- School of Social Sciences, University of Sussex. Tese de doutorado
Isotretinoin and inflammatory bowel disease
[Excerpt] We read with great interest the recent
review published in your Journal by Reddy et al., which
suggests a possible association between isotretinoin use and
inflammatory bowel disease (IBD) (1). Although IBD is described as a possible adverse drug reaction in the product
information, little attention has been given in the literature to
this association.[...
Relation between BCG coverage rate and COVID-19 infection worldwide
The available numbers on COVID-19 have shown a huge difference
in incidence rates worldwide [1]. The explanation for these differences
is certainly multifactorial and more time is needed for accurate data
analysis and to a complete understanding.info:eu-repo/semantics/publishedVersio
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