1,720,961 research outputs found
The Evolution of a Higher-Dimensional FRW Universe with Variable <i>G</i> and Λ and Particle Creation
No full textUsing an open thermodynamic systems theory, the effect of particle creation on the evolution and dynamics of the standard cosmological FLRW model in a higher-dimensional spacetime with functionally dependent cosmological and gravitational constants Λ and G is investigated. The gravitational field equations have been transformed into a dimensionless system of non-linear, first-order, coupled differential equations (DEs) as functions of the universe’s density parameters Ωi and rate of particle creation Ψ in redshift space, which can be numerically casted. Two cosmological models are obtained, depending on the choice of particle creation rate—Ψ∼H2 and Ψ∼n2 for dust-, radiation- and dark-energy-dominated universes, respectively. The dynamic behaviour of each model is discussed
A Generalized Solution of Bianchi Type-V Models with Time-Dependent G and Λ
No full textWe study the homogeneous but anisotropic Bianchi type-V cosmological model with time-dependent gravitational and cosmological “constants”. Exact solutions of the Einstein field equations (EFEs) are presented in terms of adjustable parameters of quantum field theory in a spatially curved and expanding background. It has been found that the general solution of the average scale factor a as a function of time involved the hypergeometric function. Two cosmological models are obtained from the general solution of the hypergeometric function and the Emden–Fowler equation. The analysis of the models shows that, for a particular choice of parameters in our first model, the cosmological “constant” decreases whereas the Newtonian gravitational “constant” increases with time, and for another choice of parameters, the opposite behaviour is observed. The models become isotropic at late times for all parameter choices of the first model. In the second model of the general solution, both the cosmological and gravitational “constants” decrease while the model becomes more anisotropic over time. The exact dynamical and kinematical quantities have been calculated analytically for each model
The cosmology of f(R, Lm) gravity: constraining the background and perturbed dynamics
Full text linkCentre for Space Research, North-West University, Potchefstroom 2520, South AfricaThis paper delves into the late-time accelerated
expansion of the universe and the evolution of cosmic structures within the context of a specific f (R, Lm) gravity model,
formulated as f (R, Lm) = λR + βLα
m + η. To study the
cosmological viability of the model, we employed the latest cosmic measurement datasets: (i) 57 observational Hubble parameter data points (OHD); (ii) 1048 distance moduli
data points (SNIa); (iii) a combined dataset (OHD+SNIa);
and large scale structure datasets, including (iv) 14 growth
rate data points (f); and (v) 30 redshift space distortion data
points (fσ8). These datasets facilitated the constraint of the
f (R, Lm)-gravity model via MCMC simulations, followed
by a comparative analysis with the ΛCDM model. A comprehensive statistical analysis has been conducted to evaluate the f (R, Lm)-gravity model’s efficacy in explaining both
the accelerated expansion of the universe and the growth of
cosmic structures. Using large-scale structure data, we find
the best-fit values of Ωm = 0.242+0.016 −0.032, α = 1.15+0.20 −0.20,
β = 1.12+0.13 −0.30, λ = 0.72+0.30 −0.13 and γ = 0.555 ± 0.014
using f-data and Ωm = 0.284+0.035 −0.049, σ8 = 0.799+0.045 −0.086,
α = 0.766+0.026 −0.064, β = 1.08+0.40 −0.16, and λ = 0.279+0.078 −0.11
using fσ8-data at the 1σ and 2σ confidence levels, respectively, with the model showing substantial observational support based on ΔAIC values but less observational support
based on the ΔBIC values on Jeffreys’ statistical criteria. On
the other hand, from the joint analysis of the OHD+SNIa
data, we obtain α = 1.091+0.035 −0.042, β = 1.237+0.056 −0.16 and
ae-mails: [email protected];
[email protected] (corresponding author)
be-mail: [email protected]
ce-mail: [email protected]
λ = 0.630+0.031
−0.050 with the Jeffreys’ scale statistical criteria showing the f (R, Lm) model having substantial support
when using OHD data, less observational support with the
joint analysis OHD+SNIa, and rejected using SNIa data,
compared with ΛCDM at the background level
The cosmology of f ( R , L m ) gravity: constraining the background and perturbed dynamics
No full textAbstract This paper delves into the late-time accelerated expansion of the universe and the evolution of cosmic structures within the context of a specific f ( R , L m ) gravity model, formulated as f ( R , L m ) = λ R + β L m α + η . To study the cosmological viability of the model, we employed the latest cosmic measurement datasets: (i) 57 observational Hubble parameter data points (OHD); (ii) 1048 distance moduli data points (SNIa); (iii) a combined dataset (OHD+SNIa); and large scale structure datasets, including (iv) 14 growth rate data points (f); and (v) 30 redshift space distortion data points (f σ 8 ). These datasets facilitated the constraint of the f ( R , L m ) -gravity model via MCMC simulations, followed by a comparative analysis with the Λ CDM model. A comprehensive statistical analysis has been conducted to evaluate the f ( R , L m ) -gravity model’s efficacy in explaining both the accelerated expansion of the universe and the growth of cosmic structures. Using large-scale structure data, we find the best-fit values of Ω m = 0 . 242 - 0.032 + 0.016 , α = 1 . 15 - 0.20 + 0.20 , β = 1 . 12 - 0.30 + 0.13 , λ = 0 . 72 - 0.13 + 0.30 and γ = 0.555 ± 0.014 using f-data and Ω m = 0 . 284 - 0.049 + 0.035 , σ 8 = 0 . 799 - 0.086 + 0.045 , α = 0 . 766 - 0.064 + 0.026 , β = 1 . 08 - 0.16 + 0.40 , and λ = 0 . 279 - 0.11 + 0.078 using f σ 8 -data at the 1 σ and 2 σ confidence levels, respectively, with the model showing substantial observational support based on Δ AIC values but less observational support based on the Δ BIC values on Jeffreys’ statistical criteria. On the other hand, from the joint analysis of the OHD+SNIa data, we obtain α = 1 . 091 - 0.042 + 0.035 , β = 1 . 237 - 0.16 + 0.056 and λ = 0 . 630 - 0.050 + 0.031 with the Jeffreys’ scale statistical criteria showing the f ( R , L m ) model having substantial support when using OHD data, less observational support with the joint analysis OHD+SNIa, and rejected using SNIa data, compared with Λ CDM at the background level
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
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