305,174 research outputs found
Modelli Cosmologici e Sottovarieta' Spaziotemporali
No full textTesi di Laurea quadriennale di E. Dimastrogiovanni. Relatore C. Coriano
Stability analysis of chromo-natural inflation and possible evasion of Lyth's bound
No full textWe perform the complete stability study of the model of chromo-natural inflation by Adshead and Wyman [Phys. Rev. Lett. 108, 261302 (2012)], where, due to its coupling to a SU(2) vector, a pseudoscalar inflaton χ slowly rolls on a steep potential. As a typical example, one can consider an axion with a sub-Planckian decay constant f. The model was recently studied by Dimastrogiovanni, Fasiello, and Tolley [J. Cosmol. Astropart. Phys. 02, 046 (2013)] in the mg ≫ H limit, where mg is the mass of the fluctuations of the vector field, and H the Hubble rate. We show that the inflationary solution is stable for
mg > 2 H, while it otherwise experiences a strong instability due to scalar perturbations in the subhorizon regime. The tensor perturbations are instead enhanced at large mg, while the vector ones remain perturbatively small. Depending on the parameters, this model can give a chiral gravity wave signal that can be detected in ongoing or forthcoming cosmic microwave background experiments. This detection can occur even if, during inflation, the inflaton spans an interval of size Δχ=O(f) which is some orders of magnitude below the Planck scale, evading a well known bound that holds for a free inflaton by Lyth [Phys. Rev. Lett. 78, 1861 (1997)]. The spectral tilt of the scalar perturbations typically decreases with decreasing mg. Therefore the simultaneous requirements of stability, sufficiently small tensor-to-scalar ratio, and sufficiently flat scalar spectrum can pose nontrivial bounds on the parameters of the model
Cosmological correlation functions in scalar and vector inflationary models
Get PDFThis thesis is centered on three main subjects within the theory of inflation and cosmological perturbations: loop corrections to the power spectrum of curvature fluctuations generated during inflation; evolution of cosmological fluctuations in anisotropic pre-inflationary cosmologies; statistical anisotropy and non-Gaussianity predictions of models of inflation populated with vector fields. Currently, what makes even more interesting the study of 2-nd and higher order corrections to cosmological correlation functions as well as the computation of higher-than-two order correlators, is the almost unprecedented chance to confront theories with new and increasingly accurate experimental data that will shed more light in the physics of the early Universe. In the context of loop calculations, we have computed the corrections arising from scalar-tensor interactions in models of single-field inflation (both for the standard slow-roll model and for models described by Lagrangians with non-canonical kinetic terms). In the context of anisotropic cosmologies, also motivated by the observation of ``anomalies'' in the Cosmic Microwave Background (CMB) fluctuations, we have computed the bispectrum and the trispectrum of the curvature fluctuations in inflationary models with SU(2) vector fields, analyzing the statistical anisotropy features of the correlators in these models; finally, we have studied cosmological perturbations for a Universe with a Bianchi type-I background metric, with an energy density dominated by a pressureless fluid and in the presence of a cosmological constant.Questa tesi e' incentrata su tre argomenti principali che si collocano nella teoria dell'inflazione e delle perturbazioni cosmologiche: correzioni ad un loop per il power spectrum delle fluttuazioni di curvatura scalare prodotte durante l'inflazione; evoluzione delle perturbazioni in cosmologie anisotrope pre-inflazionarie; predizioni di non-Gaussianita' e di anisotropia statistica per modelli d'inflazione in presenza di campi vettoriali. Al momento, lo studio di correzioni al secondo ordine e oltre per funzioni di correlazioni cosmologiche, cosi' come il calcolo di correlatori di ordine superiore al secondo, sono resi ancora piu' interessanti dalla possibilita' di un confronto della teoria con nuovi e mai cosi' accurati dati sperimentali, che si spera aiuteranno nella comprensione della fisica dell'Universo primordiale. Nel contesto di calcoli a un loop, abbiamo calcolato le correzioni dovute alle interazioni scalari-gravitoni in modelli di single-field inflation (sia nel caso standard di slow-roll inflation sia per modelli con Lagrangiane dai termini cinetici non canonici). Nel constesto delle cosmologie anisotrope, motivate anche dal rilevamento di ``anomalie'' nelle fluttuazioni del Fondo Cosmico a Microonde (CMB), abbiamo calcolato il bispettro e il trispettro delle fluttuazioni della curvatura scalare in modelli inflazionari con campi vettoriali di tipo SU(2), studiandone le caratteristiche di anisotropia statistica; infine, abbiamo studiato le perturbationi cosmologiche per un Universo con una metrica di background di tipo Bianchi-I, con una densita' di energia dominata da un fluido non-collisionale e in presenza di una costante cosmologica
Gauge-flation confronted with Planck
No full textGauge-flation is a recently proposed model in which inflation is driven solely by a non-Abelian gauge field thanks to a specific higher order derivative operator. The nature of the operator is such that it does not introduce ghosts. We compute the cosmological scalar and tensor perturbations for this model, improving over an existing computation. We then confront these results with the Planck data. The model is characterized by the quantity γ ≡ g2Q2/H2 (where g is the gauge coupling constant, Q the vector vev, and H the Hubble rate). For γ < 2, the scalar perturbations show a strong tachyonic instability. In the stable region, the scalar power spectrum ns is too low at small γ, while the tensor-to-scalar ratio r is too high at large γ. No value of γ leads to acceptable values for ns and r, and so the model is ruled out by the CMB data. The same behavior with γ was obtained in Chromo-natural inflation, a model in which inflation is driven by a pseudo-scalar coupled to a non-Abelian gauge field. When the pseudo-scalar can be integrated out, one recovers the model of Gauge-flation plus corrections. It was shown that this identification is very accurate at the background level, but differences emerged in the literature concerning the perturbations of the two models. On the contrary, our results show that the analogy between the two models continues to be accurate also at the perturbative level
Gravitational waves and scalar perturbations from spectator fields
No full textThe most conventional mechanism for gravitational waves (gw) production during inflation is the amplification of vacuum metric fluctuations. In this case the gw production can be uniquely related to the inflationary expansion rate H. For example, a gw detection close to the present experimental limit (tensor-to-scalar ratio r ~ 0.1) would indicate an inflationary expansion rate close to 1014 GeV. This conclusion, however, would be invalid if the observed gw originated from a different source. We construct and study one of the possible covariant formulations of the mechanism suggested in [1], where a spectator field σ with a sound speed cs Lt 1 acts as a source for gw during inflation. In our formulation σ is described by a so-called P(X) Lagrangian and a non-minimal coupling to gravity. This field interacts only gravitationally with the inflaton, which has a standard action. We compute the amount of scalar and tensor density fluctuations produced by σ and find that, in our realization, r is not enhanced with respect to the standard result but it is strongly sensitive to cs, thus breaking the direct r ↔ H connection
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
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
Author, publisher and bookseller : a tripartite synergy in Nigerian book industry
Get PDFThis work is about the roles of Author, Publisher and Bookseller in Book development in
Nigeria. The paper started by delving into the history of Book Publishing in Nigeria after
which it proceeded by defining who an author, a publisher, and a bookseller is and
expatiated on the indispensable roles of these key actors in Nigerian Book Industry and in
the emerging Information Society. Furthermore, the various constraints to book
development were identified while the paper advised on how the Book Industry can be
further promoted in Nigeria. However, the paper concluded and made recommendations
on how the Book sector can help in enhancing scholarship in the country
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
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