1,720,977 research outputs found
Interacting dark sector from the trace-free Einstein equations: cosmological perturbations with no instability
No full textIn trace-free Einstein gravity, the stress-energy tensor of matter is not
necessarily conserved and so the theory offers a natural framework for
interacting dark energy models where dark energy has a constant equation of
state . We derive the equations of motion for linear cosmological
perturbations in interacting dark energy models of this class, focusing on the
scalar sector. Then, we consider a specific model where the energy-momentum
transfer potential is proportional to the energy density of cold dark matter;
this transfer potential has the effect of inducing an effective equation of
state for cold dark matter. We analyze in detail the
evolution of perturbations during radiation domination on super-Hubble scales,
finding that the well-known large-scale instability that affects a large class
of interacting dark energy models is absent in this model. To avoid a gradient
instability, energy must flow from dark matter to dark energy. Finally, we show
that interacting dark energy models with are equivalent to a class of
generalized dark matter models.Comment: 23 pages + references; accepted for publication in PR
Bouncing universes analytic and effective dynamics in reduced-symmetry models of loop quantum cosmology
Full text linkThe Singularity Problem in classical General Relativity and the need of a quantum theory for the gravitational field are two of the reasons that brought to the formulation of Loop Quantum Gravity, one of the most accredited theories of Quantum Gravity. In this work we analyze the main features of the theory, from the Kinematical Hilbert space to the physical one passing through the quantization of the constraints, and then we focus on two Minisuperspaces of its cosmological sector. We see explicitally how the theory solves the classical singularity problem at the analytic and effective level, and replaces the physical singularity with a quantum bounce in the case of an homogeneous and isotropic model (FRW with k=0), with multiple bounces in the Bianchi I Universe (only homogeneity with commuting Killing vectors)
Emergent Lorentzian geometries from spin-foams and group field theories
Full text linkThe emergence of Lorentzian geometries is investigated within the spin-foam, Part I, and group field theory (GFT), Part II, approach to quantum gravity. The spectral dimension of periodic Euclidean spin-foam frusta is studied. At large scales, the spectral dimension is generically four. At lower scales, a non-trivial flow of the spectral dimension is observed, sensitive to quantum effects, curvature induced oscillations and the parameters of the theory. The removal of numerical cutoffs and a thermodynamic limit is discussed, suggesting a phase transition from zero to four large-scale dimensions. Lorentzian Regge calculus for (3+1) cosmology, modelled with Lorentzian 4-frusta, coupled to a massless free scalar field is studied. It is shown that causal regularity, solutions to the Regge equations and a continuum limit only exist if the cells connecting neighboring slices are timelike. The dynamics can be expressed relationally only in the small deficit angle limit. Effective (2+1) spin-foam cosmology with a minimally coupled massive scalar field is investigated. The scalar field mass is shown to ensure convergence of the path integral. The classicality of expectation values is shown to be intimately connected to causal regularity and the path integral measure. A causal completion of the Barrett-Crane group field theory model is developed. Its amplitudes are explicitly computed in spin representation using methods from integral geometry. A Landau-Ginzburg analysis is applied to the complete Barrett-Crane (BC) group field theory model. It is shown that mean-field theory is generically self-consistent, and that timelike faces do not contribute to the critical behavior. Employing the complete BC model, a physical Lorentzian reference frame is coupled, and scalar cosmological perturbations are extracted from entangled GFT coherent states. The dynamics of these perturbations are shown to agree with classical results up to quantum corrections
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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