1,720,965 research outputs found
Rotation of Linear Polarization Plane and Circular Polarization from Cosmological Pseudoscalar Fields
No full textWe discuss the rotation of the linear polarization plane and the production of circular polarization generated by a cosmological pseudoscalar field. We compute analytically and numerically the propagation of the Stokes parameters from the last scattering surface for an oscillating and a monotonic decreasing pseudoscalar field. For the models studied in this paper, we show the comparison between the widely used approximation in which the linear polarization rotation angle is constant in time and the exact result. © 2009 The American Physical Society
Photon Propagation as a Probe for Fundamental Physics
No full textIn this thesis we show how light propagation can be used in order to
constrain particle physics models beyond the Standard Model. We study in
particular the effects of non standard physics on the polarization of cosmic
microwave background radiation and on the flux of ultrahigh energy γ-rays
(Eγ > 1019 eV).
In the first part we discuss the effects on cosmic microwave background
polarization of coupling between photons and pseudoscalar fields acting as
dark matter (e.g. axions) or as dark energy (e.g. ultralight pseudo Nambu-
Goldstone bosons). In particular we describe how the public code CAMB
can be modified in order to take into account the rotation of the linear polarization
plane from last scattering surface to nowadays, produced by photon
propagation in a cosmological background of pseudoscalar particles. Polarization
power spectra are compared with the ones obtained in the widely
used approximation in which the rotation angle is assumed constant in time.
We show how polarization-polarization and temperature-polarization angular
power spectra can be very useful to constrain the coupling constant gφ
between photons and pseudoscalars.
In the second part of this thesis we use the current upper limits on the flux
of ultrahigh-energy photons in order to constrain Lorentz invariance violating
terms in the dispersion relations for elementary particles. Theories trying to
unify quantum mechanics with general relativity and many supersymmetry
models predict indeed that Lorentz symmetry has to be modified at energies
of the order of the Planck scale (1028 eV). If standard dispersion relations of
elementary particles are modified, then the propagation and therefore also
the energy spectrum of ultrahigh-energy cosmic rays can be considerably
changed. We study in particular how it is possible to constrain Lorentz
invariance violating terms for photons and electrons (suppressed both at
first and second order of the Planck mass) improving current constraints by
several orders of magnitude.
The main results are summarized in the following papers:
• F. Finelli and M. Galaverni, “Rotation of Linear Polarization Plane
and Circular Polarization from Cosmological Pseudoscalar Fields,”
arXiv:0802.4210 [astro-ph], submitted.
• M. Galaverni, F. Finelli, “Systematics of Cosmic Microwave Background
Polarization,” Internal Report IASF-BO 454/2007.
• M. Galaverni and G. Sigl, “Lorentz Violation in the Photon Sector and
Ultra-High Energy Cosmic Rays,” Phys. Rev. Lett. 100, 021102 (2008)
[arXiv:0708.1737 [astro-ph]].
• M. Galaverni and G. Sigl, “Lorentz Violation and Ultrahigh-Energy
Photons,” Phys. Rev. D 78, 063003 (2008) [arXiv:0807.1210 [astroph]]
Lorentz Violation in the Photon Sector and Ultra-High Energy Cosmic Rays
No full textLorentz symmetry breaking at very high energies may lead
to photon dispersion relations of the form
with new terms suppressed by a power of the Planck mass . We show
that first and second order terms of size
and ,
respectively, would lead to a photon component in cosmic rays above
eV that should already have been detected.
This suggests that LI breaking suppressed up to second order
in the Planck scale are unlikely to be phenomenologically viable
for photon
Light Bending as a Probe of the Nature of Dark Energy
No full textWe study the bending of light for static spherically symmetric (SSS) space-times which include a dark energy contribution. Geometric dark energy models generically predict a correction to the Einstein angle written in terms of the distance to the closest approach, whereas a cosmological constant does not.
While dark energy is associated with a repulsive force in cosmological context, its effect on null geodesics in SSS space-times can be attractive as for the Newtonian term. This dark energy contribution may not be negligible with respect to the Einstein prediction in lensing involving clusters of galaxies. Strong lensing may therefore be useful to distinguish from other dark energy models
Lorentz Violation and Ultrahigh-Energy Photons.
No full textThe propagation of photons, electrons and positrons at ultra-high energies
above eV can be changed considerably if the dispersion relations
of these particles are modified by terms suppressed by powers of the Planck scale.
We recently pointed out that the current non-observation of photons in the
ultra-high energy cosmic ray flux at such energies can put strong constraints
on such modified dispersion relations. In the present work we generalize these
constraints to all three Lorentz invariance breaking parameters that can occur
in the dispersion relations for photons, electrons and positrons at first and second
order suppression with the Planck scale. We also show how the excluded regions in these three-dimensional
parameter ranges would be extended if ultra-high energy photons are detected in the future
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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