1,720,956 research outputs found
Supernova constraints on multi-coupled dark energy
No full textThe persisting consistency of ever more
accurate observational data with
the predictions of the standard ΛCDM cosmological model puts
severe constraints on possible alternative scenarios, but still does not
shed any light on the fundamental nature of the cosmic dark sector. As
large deviations from a ΛCDM cosmology are ruled out by data, the
path to detect possible features of alternative models goes necessarily
through the definition of cosmological scenarios that leave almost
unaffected the background and — to a lesser extent — the
linear perturbations evolution of the universe. In this context, the
Multi-coupled DE (McDE) model was proposed by Baldi [9] as a particular
realization of an interacting Dark Energy field characterized by an
effective screening mechanism capable of suppressing the effects of the
coupling at the background and linear perturbation level. In the present
paper, for the first time, we challenge the McDE scenario through a
direct comparison with real data, in particular with the luminosity
distance of Type Ia supernovae. By studying the existence and stability
conditions of the critical points of the associated background dynamical
system, we select only the cosmologically consistent solutions, and
confront their background expansion history with data. Confirming
previous qualitative results, the McDE scenario appears to be fully
consistent with the adopted sample of Type Ia supernovae, even for
coupling values corresponding to an associated scalar fifth-force about
four orders of magnitude stronger than standard gravity. Our analysis
demonstrates the effectiveness of the McDE background screening, and
shows some new non-trivial asymptotic solutions for the future evolution
of the universe. Clearly, linear perturbation data and, even more,
nonlinear structure formation properties are expected to put much
tighter constraints on the allowed coupling range. Nonetheless, our
results show how the background expansion history might be highly
insensitive to the fundamental nature and to the internal complexity of
the dark sector
Linear perturbation constraints on multi-coupled dark energy
No full textThe Multi-coupled Dark Energy (McDE) scenario
has been recently proposed
as a specific example of a cosmological model characterized by a
non-standard physics of the dark sector of the universe that
nevertheless gives an expansion history which does not significantly
differ from the one of the standard ΛCDM model. Thanks to a
dynamical screening mechanism, in fact, the interaction between the Dark
Energy field and the Dark Matter sector is effectively suppressed at the
background level during matter domination. As a consequence, background
observables cannot discriminate a McDE cosmology from ΛCDM for a
wide range of model parameters. On the other hand, linear perturbations
are expected to provide tighter bounds due to the existence of
attractive and repulsive fifth-forces associated with the dark
interactions. In this work, we present the first constraints on the McDE
scenario obtained by comparing the predicted evolution of linear density
perturbations with a large compilation of recent data sets for the
growth rate fσ8, including 6dFGS, LRG, BOSS, WiggleZ
and VIPERS. Confirming qualitative expectations, growth rate data
provide much tighter bounds on the model parameters as compared to the
extremely loose bounds that can be obtained when only the background
expansion history is considered. In particular, the 95% confidence level
on the coupling strength |β| is reduced from |β| <= 83
(background constraints only) to |β| <= 0.88 (background and
linear perturbation constraints). We also investigate how these
constraints further improve when using data from future wide-field
surveys such as supernova data from LSST and growth rate data from
Euclid-type missions. In this case the 95% confidence level on the
coupling further reduce to |β| <= 0.85. Such constraints are in
any case still consistent with a scalar fifth-force of gravitational
strength, and we foresee that tighter bounds might be possibly obtained
from the investigation of nonlinear structure formation in McDE
cosmologies
Limits on the Reconstruction of a Single Dark Energy Scalar Field Potential from SNe Ia Data
No full textIn this paper we perform a reconstruction of the scalar field potential responsible for cosmic acceleration using SNe Ia data. After describing the method, we test it with real SNe Ia data—Union2.1 and JLA SNe datasets. We demonstrate that with the current data precision level, the full reconstruction is not possible. We discuss the problems which arise during the reconstruction process and the ways to overcome them
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
Limits on the Reconstruction of a Single Dark Energy Scalar Field Potential from SNe Ia Data
No full textVariations 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
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