1,721,026 research outputs found
The Universe at the MeV era : neutrino evolution and cosmological observables
No full textLa physique des neutrinos dans l’Univers primordial est un élément clé pour notre compréhension des étapes cosmologiques ultérieures, telles que la nucléosynthèse primordiale (BBN) ou la formation des grandes structures. La décennie qui s’ouvre annonce de nouveaux résultats expérimentaux permettant d’explorer et de contraindre encore plus précisément les modèles cosmologiques - ce qui nécessite des prédictions théoriques robustes. Cette thèse de doctorat présente une étude de l’évolution des neutrinos dans les premières secondes après le Big Bang, plus précisément lorsque la température de l’Univers est de l’ordre du mégaélectron-volt. Cette évolution est obtenue numériquement en résolvant des équations cinétiques dont nous proposons une nouvelle dérivation. Une première application est le calcul du découplage dit "standard" afin de calculer le paramètre cosmologique quantifiant la densité d’énergie des espèces relativistes primordiales, Neff, à une précision de quelques dix-millièmes. Cette étude a mis en évidence la possibilité de décrire de manière effective le phénomène d’oscillations de saveur, en tirant profit de la large séparation d’échelles temporelles en jeu. Une telle approximation est ensuite adaptée et validée dans le cas d’asymétries non-nulles entre les neutrinos et les antineutrinos. Enfin, nous étudions semi-analytiquement les conséquences du découplage incomplet des neutrinos sur la BBN, afin de comprendre comment les abondances primordiales en hélium et deutérium sont affectées par cette physique.Neutrino physics in the early Universe is key to our understanding of later cosmological stages, such as primordial nucleosynthesis (BBN) or the formation of large-scale structures. The coming decade promises new experimental results to explore and constrain cosmological models even more precisely - which requires robust theoretical predictions. This PhD thesis presents a study of the evolution of neutrinos in the first seconds after the Big Bang, more precisely when the temperature of the Universe is of the order of one mega-electronvolt. This evolution is obtained numerically by solving kinetic equations for which we propose a new derivation. A first application is the calculation of the so-called “standard” decoupling in order to calculate the cosmological parameter quantifying the energy density of the primordial relativistic species, Neff, to a precision of a few ten-thousandths. This study has highlighted the possibility of effectively describing the phenomenon of flavour oscillations, taking advantage of the large separation of time scales involved. Such an approximation is then adapted and validated in the case of non-zero asymmetries between neutrinos and antineutrinos. Finally, we study semi-analytically the consequences of incomplete neutrino decoupling on BBN, in order to understand how the primordial abundances of helium and deuterium are affected by this physics
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
Radiative transport of relativistic species in cosmology
No full textWe review the general construction of distribution functions for gases of fermions and bosons (photons), emphasizing the similarities and differences between both cases. The central object which describes polarization for photons is a tensor-valued distribution function, whereas for fermions it is a vector-valued one. The collision terms of Boltzmann equations for fermions and bosons also possess the same general structure and differ only in the quantum effects associated with the final state of the reactions described. In particular, neutron-proton conversions in the early universe, which set the primordial Helium abundance, enjoy many similarities with Compton scattering which shapes the cosmic microwave background and we show that both can be handled with a Fokker-Planck type expansion. For neutron-proton conversions, this allows to obtain the finite nucleon mass corrections, required for precise theoretical predictions, whereas for Compton scattering it leads to the thermal and recoil effects which enter the Kompaneets equation. We generalize the latter to the general case of anisotropic and polarized photon distribution functions. Finally we discuss a parameterization of the photon spectrum based on logarithmic moments which allows for a neat separation between temperature shifts and spectral distortions
Graph representation of balance sheets: from exogenous to endogenous money
Full text linkA graph representation of the financial relations in a
given monetary structure is proposed. It is argued that the graph of debt-liability relations is naturally organized and simplified into a tree structure, around banks and a central bank. Indeed, this optimal graph allows to perform payments very easily as it amounts to the suppression of loops introduced by pending payments. Using this language of graphs to analyze the monetary system, we first examine the systems based on commodity money and show their incompatibility with credit. After dealing with the role of the state via its ability to spend and raise taxes, we discuss the chartalist systems based on pure fiat
money, which are the current systems. We argue that in those cases, the Treasury and the central bank can be meaningfully consolidated. After describing the interactions of various autonomous currencies, we
argue that fixed exchanged rates can never be maintained, and we discuss the controversial role of the IMF in international financial relations.
We finally use graph representations to give our interpretation on open problems, such as the monetary aggregates, the sectoral financial balances and the endogenous nature of money. Indeed, once appropriately consolidated, graphs of financial relations allow to formulate easily unambiguous statements about the monetary arrangements
Radiative transport of relativistic species in cosmology
No full textInternational audienceWe review the general construction of distribution functions for gases of fermions and bosons (photons), emphasizing the similarities and differences between both cases. The central object which describes polarization for photons is a tensor-valued distribution function, whereas for fermions it is a vector-valued one. The collision terms of Boltzmann equations for fermions and bosons also possess the same general structure and differ only in the quantum effects associated with the final state of the reactions described. In particular, neutron-proton conversions in the early universe, which set the primordial Helium abundance, enjoy many similarities with Compton scattering which shapes the cosmic microwave background and we show that both can be handled with a Fokker-Planck type expansion. For neutron-proton conversions, this allows to obtain the finite nucleon mass corrections, required for precise theoretical predictions, whereas for Compton scattering it leads to the thermal and recoil effects which enter the Kompaneets equation. We generalize the latter to the general case of anisotropic and polarized photon distribution functions. Finally we discuss a parameterization of the photon spectrum based on logarithmic moments which allows for a neat separation between temperature shifts and spectral distortions
Dynamique non-linéaire et anisotropie primordiale en cosmologie
No full textThe high accuracy in the measurements of the cosmic microwave background requires to understand in details the underlying physics in order to draw relevant conclusions for the primordial phase of the universe. In this thesis, we study the theory of non-linear perturbations in the framework of general relativity. Our goal is to determine the transfer of the metric perturbations together with the matter content perturbations, between the inflationary primordial phase of the universe and the observations performed today. We first consider the fluid approximation in order to extract the general leading behaviours. We then turn to the kinetic theory at second order, which is necessary in order to obtain the non-linear transfer function for radiation, in order to determine the non-Gaussianity in the cosmic microwave background.We also study the linear perturbation theory around anisotropic universes. We set up the gauge invariant perturbation theory around a Bianchi I space-time, and we study the observational signatures of a model of inflation having this symmetry.La grande précision des mesures du fond diffus cosmologique nécessitent de comprendre avec finesse la physique sous-jacente afin d'en tirer des conclusions pertinentes sur la phase primordiale de l'univers. Dans cette thèse nous étudions la théorie des perturbations non-linéaires dans le cadre de la relativité générale. Notre but est de déterminer le transfert des perturbations de la métrique ainsi que des perturbations du contenu matériel,entre la phase primordiale de l'univers et les observations réaliséesaujourd'hui. Nous nous plaçons tout d'abord dans l'approximation fluide afin d'appréhender les comportements généraux attendus. Ensuite nous étudions la théorie cinétique au second ordre, nécessaire pour obtenir le transfert radiatif non-linéaire, dans le but de déterminer la non-gaussianité dans lefond diffus cosmologique. Nous étudions également la théorie des perturbations linéairesautour d'espaces anisotropes. Nous élaborons la théorie des perturbations invariantes de jauge autour d'un espace de Bianchi I, puis nous étudions les signatures observationnelles d'une phase primordiale d'inflation possédant cette symétrie
- …
