1,720,985 research outputs found
Dangerous angular Kaluza-Klein/glueball relics in string theory cosmology
No full textThe presence of Kaluza-Klein (KK) particles in the universe is a potential manifestation of string theory cosmology. In general, they can be present in the high temperature bath of the early universe. In particular examples, string theory inflation often ends with brane-antibrane annihilation followed by the energy cascading through massive closed string loops to KK modes which then decay into lighter standard model particles. However, massive KK modes in the early universe may become dangerous cosmological relics if the inner manifold contains warped throat(s) with approximate isometries. In the complimentary picture, in the AdS/CFT dual gauge theory with extra isometries, massive glueballs of various spins become the dangerous cosmological relics. The decay of these angular KK modes/glueballs, located around the tip of the throat, is caused by isometry breaking which results from gluing the throat to the compact Calabi-Yau (CY) manifold. We address the problem of these angular KK particles/glueballs, studying their interactions and decay channels, from the theory side, and the resulting cosmological constraints on the warped compactification parameters, from the phenomenology side. The abundance and decay time of the long-lived nonrelativistic angular KK modes depend strongly on the parameters of the warped geometry, so that observational constraints rule out a significant fraction of the parameter space. In particular, the coupling of the angular KK particles can be weaker than gravitational
Exact identification of the radion and its coupling to the observable sector
No full textBraneworld models in extra dimensions can be tested in laboratory by the coupling of the radion to the standard model fields. The identification of the radion as a canonically normalized field involves a careful general relativity treatment: if a bulk scalar is responsible for the stabilization of the system, its fluctuations are entangled with the perturbations of the metric and they also have to be taken into account (similarly to the well-developed theory of scalar metric perturbations in 4D cosmology with a scalar field). Extracting a proper dynamical variable in a warped geometry/scalar setting is a nontrivial task, performed so far only in the limit of negligible backreaction of the scalar field on the background geometry. We perform the general calculation, diagonalizing the action up to second order in the perturbations and identifying the physical eigenmodes of the system for any amplitude of the bulk scalar. This computation allows us to derive a very simple expression for the exact coupling of the eigenmodes to the standard model fields on the brane, valid for an arbitrary background configuration. As an application, we discuss the Goldberger-Wise mechanism for the stabilization of the radion in the Randall-Sundrum-type models. The existing studies, limited to small amplitude of the bulk scalar field, are characterized by a radion mass which is significantly below the physical scale at the observable brane. We extend them beyond the small backreaction regime. For intermediate amplitudes, the radion mass approaches the electroweak scale, while its coupling to the observable brane remains nearly constant. At very high amplitudes, the radion mass instead decreases, while the coupling sharply increases. Severe experimental constraints are expected in this regime
Gravity waves signatures from anisotropic preinflation
No full textWe show that expanding or contracting Kasner universes are unstable due to the amplification of gravitational waves (GW). As an application of this general relativity effect, we consider a preinflationary anisotropic geometry characterized by a Kasner-like expansion, which is driven dynamically towards inflation by a scalar field. We investigate the evolution of linear metric fluctuations around this background, and calculate the amplification of the long-wavelength GW of a certain polarization during the anisotropic expansion (this effect is absent for another GW polarization, and for scalar fluctuations). These GW are superimposed to the usual tensor modes of quantum origin from inflation, and are potentially observable if the total number of inflationary e-folds exceeds the minimum required to homogenize the observable universe only by a small margin. Their contribution to the temperature anisotropy angular power spectrum decreases with the multipole l as l^{−p}, where p depends on the slope of the initial GW power spectrum. Constraints on the long-wavelength GW can be translated into limits on the total duration of inflation and the initial GW amplitude. The instability of classical GW (and zero-vacuum fluctuations of gravitons) during Kasner-like expansion (or contraction) may have other interesting applications. In particular, if GW become nonlinear, they can significantly alter the geometry before the onset of inflation
Gravitational instability of de Sitter compactifications
No full textWe consider warped compactifications in ( 4 + d)-dimensional theories, with four-dimensional (4d) de Sitter dS(4) vacua ( with Hubble parameter H) and with a compact internal space. After introducing a gauge-invariant formalism for the generic metric perturbations of these backgrounds, we focus on modes which are scalar with respect to dS(4). The physical eigenmasses of these modes acquire a large universal tachyonic contribution - 12d/(d + 2) H^2, independently of the stabilization mechanism for the compact space, in addition to the usual KK masses, which instead encode the effects of the stabilization. General arguments, as well as specific examples, lead us to conjecture that, for sufficiently large dS curvature, the compactified geometry becomes gravitationally unstable due to the tachyonic growth of the scalar perturbations. This means that for any stabilization mechanism the curvature of the dS geometry cannot exceed some critical value. We relate this effect to the anisotropy of the bulk geometry and suggest the end points of the instability. Of relevance for inflationary cosmology, the perturbations of the bulk metric inevitably induce a new modulus field, which describes the conformal fluctuations of the 4d metric. If this mode is light during inflation, the induced conformal fluctuations will be amplified with a scale free spectrum and with an amplitude which is disentangled from the standard result of slow-roll inflation. The conformal 4d metric fluctuations give rise to a very generic realization of the mechanism of modulated cosmological fluctuations, related to spatial variation of couplings during ( p) reheating after inflation
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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