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Cosmologies with a Dynamical Vacuum Energy
No full textThe possibility of a nonzero cosmological constant has been invoked several times in the past, both
for theoretical and observational motivations. It has been often discarded by particle physicists, due
to the huge difficulties in justifying a value of vacuum energy tiny enough to allow the universe to
survive more than 10-41 s after the Big Bang. At present, the cosmological constant problem is still,
probably, the most ununderstood issue of the physics.
However, in recent times, it has again come into vogue, and again as a consequence of a number of
observational evidences. Despite their apparent simplicity, the results of observations of more than 40
distant type lA supernovae seem to converge on the astonishing evidence that the present expansion
of the universe is accelerating; this fact, together with many other experimental evidences of a lowdensity
universe, and with the most recent CMB data indicating that the universe is very near to
the flatness, has opened again the difficult question of what is the unobserved energy component that
would balance a low-density universe with a flat one.
At the same time, a new branch of cosmology has been opened, involving scalar fields as candidates for
such "missing" energy. Motivated by the difficulties of cosmological constant models and by the most
recent observational case of an accelerating universe, many alternative scenarios have been proposed:
amongst them, the Quintessence scenarios, which are the subject of this thesis.
In these models, the "missing energy" should reside in a dynamical scalar field rather than in a pure
vacuum state; the dynamics of the field plays an important role, since the field energy density can
adjust in a way that it comes to dominate at late times.
The most important and distinctive feature of a scalar field vs. a cosmological constant, is that a field
will develop fluctuations, that interact gravitationally with those of matter. To obtain the correct
predictions of their impact on the Cosmic Microwave Background radiation and on the evolution of
perturbations, the formalism of linear perturbation theory must be widely used.
In this thesis we will focus on some basic issues connected with the attempt to build predictions on
the cosmological impact of such scalar fields, following the results discussed in refs. [173, 174, 10, 15].
The first chapter is introductory and aims at giving an overview of the current observational evidences
from which the case of a positive non-zero vacuum energy arises, motivating the consideration of the
cosmological constant problem. In Chapter 2, the gauge-invariant formalism of linear cosmological
perturbation theory is described, with particular attention to quantities such as the gravitational
potentials, entropy and curvature perturbations, which are used in the following of the thesis.
In Chapter 3 we recall some basic ideas on the mechanisms that generated the observed Cosmic
Microwave Background of radiation, whose small but detectable anisotropies contain a large amount
of information on the history of the Universe. In particular, CMB anisotropies turned out to be a
very rich ground of investigation for discriminating between Quintessence and cosmological constant
scenarios.
These chapters set the framework for the results that will be presented in the following chapters,
containing the original work of the thesis.
In Chapter 4, focusing on scalar-type perturbations, we settled the analytical initial conditions that
must be imposed on the components of cosmic fluid involving a minimally-coupled scalar field, in order
to produce purely adiabatic or purely isocurvature initial conditions on super-horizon scales. Thus,
an interesting comparison with the "standard" pure CDM flat model is performed. The distinctive
imprints of Quintessence on large scale structure and on CMB anisotropies, both of polarization and
temperature, are extensively analyzed.
Chapter 5 extends the concept of Quintessence to a larger class of scalar fields, having an explicit
coupling with the Ricci scalar in the Lagrangian. These more general models, here named "Extended
Quintessence", are shown to enrich the phenomenology with respect to the simple minimally-coupled Quintessence. In fact, the predictions for the CMB anisotropies show new distinctive features, directly
related to the presence of a non-zero coupling of the field with the gravitational sector of the Lagrangian
and, ultimately, with the time-variation of the gravitational constant.
A problem of "fine tuning" is however inherent both to cosmological constant and quintessence models:
in order to have today an amount of vacuum energy comparable with that of matter, the vacuum energy
density should have been initially vanishingly small.
A way out to such fine tuning problem is possible in Quintessence scenarios, where one can select a
subclass of models which admit "tracking solutions". This means that the present value of scalar field
energy density, once fixed, can be determined starting from a very wide set of initial conditions, even
though the tracking solutions are not perfect attractors and do not solve the problem of why the field
energy density should have this value just today.
Chapter 6 considers tracking behaviors in Extended Quintessence scenarios and presents a description
of the rich phenomenology that arises from the corresponding dynamics; in particular, we show that
the coupling with the Ricci scalar can act initially as an effective potential pushing the field in the
tracking trajectory ( "R-boost"). The dependence of the phenomenology on the sign of the coupling
constant is also described.
Finally, Chapter 7 presents the conclusions and faces the future observational perspectives, on the
light of the most recent data from MAXIMA and BOOMERANG-98 balloon experiments and of the
future satellite missions MAP and Planck
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Instability of Chaplygin Gas Trajectories in Unified Dark Matter Models
No full textIn the past few years, the Chaplygin gas has been considered as an appealing candidate for unifying dark matter and dark energy into a single substance. This picture had to face several problems when trying to fit its predictions with cosmological observations. We point out another potential problem of this model, showing that, when the Chaplygin gas is described through a classical scalar field, the corresponding trajectories are strongly unstable. This implies that an extreme fine-tuning must apply to the initial values of the field in order to end up today with values allowed by observational data
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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