1,720,967 research outputs found
Testing deviations from the ΛCDM model with electromagnetic and gravitational waves
Full text linkThe ΛCDM model has been extensively tested over the past decades and has been established as the standard model of cosmology. Despite its huge successes, it faces
some serious theoretical problems, especially related to the nature of dark matter and dark energy. Different possible modifications and extensions have been considered in the past in order to solve these problems, and a question of central importance is how these modifications can be tested and experimentally distinguished from the standard case. The aim of this thesis is twofold. First, it presents two different modifications of the dark sector. A model in which dark matter forms a Bose-Einstein condensate in high density regions and in the process forms a non-minimal coupling to the metric is considered as a possible deviation from the cold dark matter scenario, while the possibility that dark energy is inhomogeneous in space is discussed as a possible deviation from a pure cosmological constant scenario. Second, it examines ways to test these possibilities via two main observational channels - gravitational waves and electromagnetic waves. The gravitational wave event GW170817 is used to test the dark matter model and to put constraints on the mass of the dark matter field and the strength of the nonminimal coupling. The luminosity distance and the redshift of light are highlighted as important observables for dark energy, and generalised theoretical formulae for these observables are derived for conformally FLRW and perturbed FLRW spacetimes. The luminosity distance and the redshift are finally used to test for possible anisotropies of the accelerated expansion of the universe
Perturbative treatment of the luminosity distance
Full text linkWe derive a generalized luminosity distance versus redshift relation for a linearly perturbed Friedmann-Lemaitre-Robertson-Walker (FLRW) metric with two scalar mode excitations. We use two equivalent approaches, based on the Jacobi map and the van Vleck determinant, respectively. We apply the resultant formula to two simple models - an exact FLRW universe and an approximate FLRW universe perturbed by a single scalar mode sinusoidally varying with time. For both models, we derive a cosmographic expansion for dL in terms of z. We comment on the interpretation of our results and their possible application to more realistic cosmological models
Non-perturbative results for the luminosity and area distances
Full text linkThe notion of luminosity distance is most often defined in purely FLRW (Friedmann-Lemaitre-Robertson-Walker) cosmological spacetimes, or small perturbations thereof. However, the abstract notion of luminosity distance is actually much more robust than this, and can be defined non-perturbatively in almost arbitrary spacetimes. Some quite general results are already known, in terms of dAobserver/dΩsource, the cross-sectional area per unit solid angle of a null geodesic spray emitted from some source and subsequently detected by some observer. We shall reformulate these results in terms of a suitably normalized null geodesic affine parameter and the van Vleck determinant, ΔvV. The contribution due to the null geodesic affine parameter is effectively the inverse square law for luminosity, and the van Vleck determinant can be viewed as providing a measure of deviations from the inverse square law. This formulation is closely related to the so-called Jacobi determinant, but the van Vleck determinant has somewhat nicer analytic properties and wider and deeper theoretical base in the general relativity, quantum physics, and quantum field theory communities. In the current article we shall concentrate on non-perturbative results, leaving near-FLRW perturbative investigation for future work
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
Macroeconomic Challenges and Risks Posed by the Global Coronavirus Crisis
Full text linkThe article presents an analysis of the current global economic crisis caused by the coronavirus pandemic in 2020. The author’s objecttives were to define and discuss the characteristics of the current crisis and its external causes as well as the macroeconomic, financial, institutional and social imbalances in the period after the global financial crisis of 2008 that led to its outbreak. Using this analysis, the author is identifying some longer-term macroeconomic challenges and risks that would deepen and prolong the current crisis as on a global scale as well as in the economies of Europe and Bulgaria. The main thesis to which the author adheres is that although the contemporary crises that occurred in 1929, during the 70s, and in 2008 had typical common characteristics and provided us with valuable lessons to be learnt, each of those crises had its own character and features. Based on this thesis, the article draws attention to the essential critical problems of the global economy in addition to those caused by the coronavirus shock
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
- …
