1,721,028 research outputs found
General Relativistic Dynamics of Irrotational Dust: Cosmological Implications
No full textThe nonlinear dynamics of cosmological perturbations of an irrotational collisionless fluid is analyzed within general relativity. Relativistic and Newtonian solutions are compared, stressing the different role of boundary conditions in the two theories. Cosmological implications of relativistic effects, already present at second order in perturbation theory, are studied and the dynamical role of the magnetic part of the Weyl tensor is elucidated
A Relativistic Approach to Gravitational Instability in the Expanding Universe: Second Order Lagrangian Solutions
No full textA Lagrangian relativistic approach to the non-linear dynamics of cosmological perturbations of an irrotational collisionless fluid is considered. Solutions are given at second order in perturbation theory for the relevant fluid and geometric quantities and compared with the corresponding ones in the Newtonian approximation. Specifically, we compute the density, the volume expansion scalar, the shear, the 'electric' part, or tide, and the 'magnetic' part of the Weyl tenser. The evolution of the shear and the tide beyond the linear regime strongly depends on the ratio of the characteristic size of the perturbation to the cosmological horizon distance. For perturbations on sub-horizon
perturbations of an irrotational collisionless fluid is considered. Solutions are given at second order in perturbation theory for the relevant fluid and geometric quantities and compared with the corresponding ones in the Newtonian approximation. Specifically, we compute the density, the volume expansion scalar, the shear, the 'electric' part, or tide, and the 'magnetic' part of the Weyl tenser. The evolution of the shear and the tide beyond the linear regime strongly depends on the ratio of the characteristic size of the perturbation to the cosmological horizon distance. For perturbations on sub-horizon scales the usual Newtonian approximation applies, at least at the considered perturbative order; on super-horizon scales, instead, a new picture emerges, which we call the 'silent universe', as each fluid element evolves independently of the environment, being unable to exchange signals with the surrounding matter through either sound waves or gravitational radiation. For perturbations inside the Hubble radius, particular attention is paid to singling out non-local effects during the non-linear evolution of fluid elements. These non-local effects are shown to be carried by a traceless and divergence-free tenser, contained in the magnetic part of the Weyl tenser, which is dynamically generated as soon as the system evolves away from the linear regime
General Relativistic Approach to the Non-Linear Evolution of Collisionless Matter
No full textA new general-relativistic algorithm is developed to study the nonlinear evolution of scalar (density) perturbations of an irrotational collisionless fluid up to shell crossing, under the approximation of neglecting the interaction with tensor (gravitational-wave) perturbations. The dynamics of each fluid element is separately followed in its own inertial rest frame by a system of twelve coupled first-order ordinary differential equations, which can be further reduced to six under very general conditions. Initial conditions are obtained in a cosmological framework, from linear theory, in terms of a single gauge-invariant potential. Physical observables, which are expressed in the Lagrangian form at different times, can be traced back to the Eulerian picture by solving supplementary first-order differential equations for the relative position vectors of neighboring fluid elements. Similarly to the Zel'dovich approximation, in our approach the evolution of each fluid element is completely determined by the local initial conditions and can be independently followed up to the time when it enters a multistream region. Unlike the Zel'dovich approximation, however, our approach is correct also in three dimensions (except for the possible role of gravitational waves). The accuracy of our numerical procedure is tested by integrating the nonlinear evolution of a spherical perturbation in an otherwise spatially flat Friedmann-Robertson-Walker universe and comparing the results with the exact Tolman-Bondi solution for the same initial profile. An exact solution for the planar symmetric case is also given, which turns out to be locally identical to the Zel'dovich solution
Non-Linear Relativistic Evolution of Cosmological Perturbations in Irrotational Dust
No full textWe report
here second order perturbative Lagrangian computations of the collapse in irrotational dust fluid models and show that,
on scales smaller than the horizon size, where the Newtonian Theory should apply, the magnetic
tensor cannot be neglected in the general case. Only for perturbations on super-horizon scales the condition Hab = 0 condition would apply ( where Hab is the magnetic part of the Weyl tensor), leading to the preferential collapse of fluid elements to spindles, independently from the environmental conditions
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
A Frozen-Flow Approximation to the Evolution of Large-Scale Structures in the Universe
No full textThe study proposes a new approximation to the evolution of large-scale structures in the Universe which is based on neglecting the role of particle inertia compared to the damping implied by the Hubble drag. This assumption makes it possible to prevent the occurrence of orbit crossing, which represents the main drawback of the Zel'dovich approximation. Owing to this property, an approximate description of the Eulerian density field at later times and/or on smaller scales compared to the classical Zel'dovich algorithm is obtained. The approximation scheme is applied to follow the evolution of structures within the standard CDM model, where it gives a fairly accurate representation of the density pattern from a resolution scale of about 500 km/s, while the two-point correlation function fits the true nonlinear result quite well, even on small scales. This method offers a strong reduction in the computational time, without a significant loss of accuracy
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
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