1,720,966 research outputs found
Reference level of the vacuum energy density of the universe and astrophysical data
No full textAn extended framework of gravity, in which the first Friedmann equation is satisfied up to some constant due to violation of gauge invariance, is tested against astrophysical data: Supernovae Type-Ia, Cosmic Chronometers, and Gamma-ray bursts. A generalized expression for the Friedmann equation, including the possible vacuum contributions, is suggested, and two particular cosmological models with two independent parameters are considered within this framework and compared on the basis of the likelihood analysis. One of the models considered includes contribution of the residual vacuum fluctuations to the energy density and places the limit on the UV cutoff scale as (Formula presented.), where (Formula presented.) is the number of minimally coupled scalar fields. Model comparison using the Akaike information criteria and Bayesian evidence shows a preference for the conventional ΛCDM over the extended models. A more general model with three parameters is considered within which an anti-correlated behavior between the dynamical vacuum fluctuations contribution and a negative cosmological constant was found. The result is an upper limit of (Formula presented.) at 95% C.L., which is only mildly disfavored ((Formula presented.)) with respect to ΛCDM
Constraining beyond ΛcDM models with 21cm intensity mapping forecasted observations combined with latest CMB data
No full textWe explore constraints on dark energy and modified gravity with forecasted 21cm intensity mapping measurements using the Effective Field Theory approach. We construct a realistic mock data set forecasting a low redshift 21cm signal power spectrum P 21(z,k) measurement from the MeerKAT radio-telescope. We compute constraints on cosmological and model parameters through Monte-Carlo Markov-Chain techniques, testing both the constraining power of P 21(k) alone and its effect when combined with the latest Planck 2018 CMB data. We complement our analysis by testing the effects of tomography from an ideal mock data set of observations in multiple redshift bins. We conduct our analysis numerically with the codes EFTCAMB/EFTCosmoMC, which we extend by implementing a likelihood module fully integrated with the original codes. We find that adding P 21(k) to CMB data provides significantly tighter constraints on Ωc h 2 and H 0, with a reduction of the error with respect to Planck results at the level of more than 60%. For the parameters describing beyond ΛCDM theories, we observe a reduction in the error with respect to the Planck constraints at the level of ≲ 10%. The improvement increases up to ∼ 35% when we constrain the parameters using ideal, tomographic mock observations. We conclude that the power spectrum of the 21cm signal is sensitive to variations of the parameters describing the examined beyond ΛCDM models and, thus, P 21(k) observations could help to constrain dark energy. The constraining power on such theories is improved significantly by tomography
Sources of H0 -tension in dark energy scenarios
Full text linkBy focusing on a simple extension of Lambda cold dark matter (ΛCDM) in which the dark energy equation of state is allowed to vary, we assess which epoch(s) possibly source the H0-tension. We consider cosmic microwave background (CMB) data in three possible ways: (i) complete CMB data; (ii) excluding the low-l+lowE (l<30 temperature and polarization) likelihoods; (iii) imposing early Universe priors, which allow us to disentangle early- and late-time physics. Through a joint analysis with low-redshift supernovae type-Ia and gravitationally lensed time delay datasets, and neglecting galaxy clustering baryonic acoustic oscillation (BAO) data, we find that the inclusion of early Universe CMB priors is consistent with local estimate of H0, while excluding the low-l+lowE likelihoods mildly relaxes the tension. This is in contrast to joint analyses with the complete CMB data. Our simple implementation of contrasting the effect of different CMB priors on the H0 estimate shows that the early Universe information from the CMB data when decoupled from late-time physics could be in agreement with a higher value of H0, even for ΛCDM model with no necessary modification. We also find no evidence for the early dark energy model using only the early Universe physics within the CMB data. Finally, using the BAO data in different redshift ranges to perform inverse distance ladder analysis, we find that the early Universe modifications, while perfectly capable of alleviating the H0-tension when including the BAO galaxy clustering data, would be at odds with the Ly-α BAO data due to the difference in rd vs H0 correlation between the two BAO datasets. We therefore infer and speculate that source for the H0-tension between CMB and local estimates could possibly originate in the modeling of late-time physics within the CMB analysis. This in turn recasts the H0-tension as an effect of late-time physics in CMB, instead of the current early-time CMB vs local late-time physics perspective
Testing generalized scalar-tensor theories of gravity with clusters of galaxies
Full text linkWe test the generalized scalar tensor theory in static systems, namely galaxy clusters. The degenerate higher-order scalar-tensor (DHOST) theory modifies the Newtonian potential through effective Newtonian constant and Ξ1 parameter in the small scale, which modifies the hydrostatic equilibrium. We utilize the well-compiled X-COP catalog consisting of 12 clusters with intracluster medium (ICM) pressure profile by Sunyaev-Zel'dovich effect data and temperature profile by x-ray data for each cluster. We perform a fully Bayesian analysis modeling Navarro-Frenk-White (NFW) for the mass profile, and the simplified Vikhlinin model for the electron density. Carefully selecting suitable clusters to present our results, we find a mild to moderate, i.e., ∼2σ significance for a deviation from the standard scenario in four of the clusters. However, in terms of Bayesian evidence, we find either equivalent or mild preference for GR. We estimate a joint constraint of Ξ1=-0.030±0.043 using eight clusters, for a modification from a ΛCDM scenario. This limit is in very good agreement with theoretical ones and an order of magnitude more stringent than the previous constraint obtained using clusters. We also quote a more conservative limit of Ξ1=-0.061±0.074. Finally, we comment on the tentative redshift dependence of the Ξ1 parameter
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
- …
