26,946 research outputs found
Heterotic Braneworld Gravity
No full textIn the past decade, there has been considerable interest in braneworld scenarios where the universe lives on a brane in a higher-dimensional bulk and gravity is modified. The heterotic braneworld scenario of Lukas, Ovrut, Stelle and Waldram (LOSW) is derived from Horava-Witten M-theory, where six of the eleven dimensions have been compactified on a Calabi-Yau manifold. The solution consists of two parallel three-branes separated by the 11th dimension with a scalar field in the bulk. In this dissertation we review some of the alternative theories of gravity, including the Kaluza-Klein model as an early example of a theory featuring extra dimensions, and the more recent braneworld models, in particular the models proposed by Randall and Sundrum, based on which many braneworld techniques were developed. We use these techniques to study gravity in the LOSW model, and explore the possibilities for a black hole solution. Using perturbation theory, we find that the zero mode sector consists of the graviton and the radion which is coupled to the bulk scalar field, and there is a continuum of massive states. The brane gravity is scalar-tensor with a Brans-Dicke parameter of ω = 0.5. Then we show that although it is possible to have a black string between the two branes, it suffers from a Gregory-Laflamme instability. We also show that it is not possible to obtain spherically symmetric solutions, so we solve the coupled brane and bulk Einstein equations for an axisymmetric metric. We obtain a solution which asymptotes the LOSW vacuum and resembles the black string. The solution looks like the Schwarzshild solution at a large distance, but the interbrane distance is not constant and the string becomes infinite as it reaches the Schwarzshild radius
The innermost regions of Young Stellar Objects as seen by VLTI/GRAVITY
No full textUnderstanding how the protoplanetary disks around young stars are structured and evolve is crucial for star and planet formation theories. We observed in an homogeneous way several tens of Herbig Ae/Be and T Tauri disks with the VLTI/GRAVITY instrument in the near-infrared K-band. We aim at spatially resolving the sub astronomical unit region to gain a statistical understanding of their morphological and compositional properties, and at looking for correlations with stellar parameters, such as luminosity, mass, and age. Thanks to its spectral resolution coupled with high angular resolution, VLTI/GRAVITY allows us to probe the location of the warm and hot gas in the inner disk. In a near future, the GRAVITY+ instrument will have a drastically improved sensitivity that will give us access to a much wider sample of young stars, including lower-mass stars and younger embedded targets
The Geographical and Institutional Proximity of Scientific Collaboration Networks
No full textThe geography of innovation has established itself as a central subject in economic geography. Geographical proximity to firms and organizations like universities is supposed to have a positive effect on a firms’ innovative performance. One of the reasons causing these positive agglomeration effects is the fact that collaboration is eased by geographical proximity. Although the role of proximity for collaboration is a well researched theme with regard to innovation, less is known about the role of proximity in scientific collaboration and how this affects the probability and nature of networking among research institutions. This is surprising given the fact that collaboration in science has become a central policy issue. In this paper we set out a number of theoretical considerations about the role of geography for innovation and see whether these apply for science as well. The empirical part will focus on the geography of collaboration in scientific knowledge production, testing the hypothesis that collaboration between different kinds of organizations is geographically more localized than collaboration between the same kinds of organizations due to institutional or organizational proximity. Besides this we will analyze the importance of spatial proximity for various forms of collaboration (such as university-university and university-firm collaboration) using the concept of the gravity model. Finally we will look at the spatial structure of these collaboration networks using insights from social network methodology. Based on co-publications, central nodes of collaborative interaction and network structures are analysed over time. On the network-level we conclude on differences in the fields of life- and physical sciences and on differences on the type of relations according to university-firm, university-university and university-governmental institution linkages. On the regional level we conclude on the centrality and spatial extent of scientific collaboration hubs over time
The Structure of Scientific Collaboration Networks in Scientometrics
No full textThe structure of scientific collaboration networks in scientometrics was investigated at the level of individuals by using bibliographic data of all papers published in the international journal Scientometrics retrieved from the Science Citation Index (SCI) during 1978 to 2004. Combined analysis of social network analysis (SNA), co-occurrence analysis, cluster analysis and frequency analysis of words was explored to reveal: (1) The microstructure of the collaboration network on scientists’ aspects of scientometrics; (2) The major collaborative fields of the collaborative sub-networks; (3) The collaborative center of the collaboration network in scientometrics
Collaboration in Iranian Scientific Publications
No full textThis study looks at international collaboration in Iranian scientific publications through the ISI Science Citation Index® (SCI) for the years 1995-1999, inclusive. These results are compared to and contrasted with the earlier findings for the periods covering 1985-1994 (Osareh & Wilson 2000). The results of Iran's increasing productivity over a 15-year period are presented. Iran doubled its output in the first two five-year periods and increased 2.8-fold from the second to the third five-year period. The rise in Iran's scientific publication output is due mainly to factors such as the ending of the war, better economic conditions, recent changes in the Iranian government's policy, basic changes in the political environment brought about by the Reformers, expansion of the Iranian presses for national publications, and the recent return of a large number of students trained overseas through government scholarships. External changes also account for the increased productivity, e.g., the acceptance of three Iranian source journals by the SCI, increased access to international databases through the Internet and better electronic communication facilities for international collaboration. One of the most important and significant factors that caused this dramatic rise seems to be the government's research policies in the last few years. Since 1999, the Iran Science, Research and Technology Ministry, has encouraged researchers to publish their non-Farsi language articles in highly ranked international scientific journals, for example, by giving prizes to researchers who publish their articles in ISI-ranked journals
ATLAS search for new phenomena in dijet mass and angular distributions using pp collisions at s√=7 TeV
No full textMass and angular distributions of dijets produced in LHC proton-proton collisions at a centre-of-mass energy s√=7 TeV have been studied with the ATLAS detector using the full 2011 data set with an integrated luminosity of 4.8 fb−1. Dijet masses up to ~ 4.0 TeV have been probed. No resonance-like features have been observed in the dijet mass spectrum, and all angular distributions are consistent with the predictions of QCD. Exclusion limits on six hypotheses of new phenomena have been set at 95% CL in terms of mass or energy scale, as appropriate. These hypotheses include excited quarks below 2.83 TeV, colour octet scalars below 1.86 TeV, heavy W bosons below 1.68 TeV, string resonances below 3.61 TeV, quantum black holes with six extra space-time dimensions for quantum gravity scales below 4.11 TeV, and quark contact interactions below a compositeness scale of 7.6 TeV in a destructive interference scenario
Vector meson production at low x from gauge/gravity duality
No full textWe use gauge/gravity duality to study vector meson (J/?, ? 0 , ?, ?) production in electron-proton scattering, in the limit of high center of mass energy at fixed momentum transfer, corresponding to the limit of low Bjorken x, where the process is dominated by pomeron exchange. Our approach considers the pomeron at strong coupling, described by the graviton Regge trajectory in AdS space with a hard-wall to mimic confinement effects. Both the proton and vector mesons are described by simple holographic wave functions in AdS. This model agrees with HERA H1 data with a ? 2 per degree of freedom below one on total cross-sections, and below two on differential cross-sections, confirming the success of previous studies that model low x DIS and DVCS using gauge/gravity duality
Quantum Gravity: A Primer for Philosophers.
No full text‘Quantum Gravity’ does not denote any existing theory: the field of quantum gravity is very much a ‘work in progress’. As you will see in this chapter, there are multiple lines of attack each with the same core goal: to find a theory that unifies, in some sense, general relativity (Einstein’s classical field theory of gravitation) and quantum field theory (the theoretical framework through which we understand the behaviour of particles in non-gravitational fields). Quantum field theory and general relativity seem to be like oil and water, they don’t like to mix—it is fair to say that combining them to produce a theory of quantum gravity constitutes the greatest unresolved puzzle in physics. Our goal in this chapter is to give the reader an impression of what the problem of quantum gravity is; why it is an important problem; the ways that have been suggested to resolve it; and what philosophical issues these approaches, and the problem itself, generate. This review is extremely selective, as it has to be to remain a manageable size: generally, rather than going into great detail in some area, we highlight the key features and the options, in the hope that readers may take up the problem for themselves—however, some of the basic formalism will be introduced so that the reader is able to enter the physics and (what little there is of) the philosophy of physics literature prepared. I have also supplied references for those cases where I have omitted some important facts. Hence, this chapter is intended primarily as a catalyst for future research projects by philosophers of physics, both budding and well-matured
The methodological status of co-authorship networks
No full textA powerful strategy within the study of collaboration
in science is to posit that co-authorship patterns
represent social networks.
It is prerequisite to an application of Social
Network Analysis (SNA) to define the network
entities. A network analysis of the inter-institutional
collaboration in COLLNET on the basis
of co-authorships was conducted. The study reveals
that it is crucial whether the co-authorship
itself is seen as an author's relational property or
as a social event that brings the authors together.
The former possibility is represented by a onemode
network in which each author can be related
to each other author. Quite distinct from
that are two-mode networks, the latter approach.
They consist of two single data sets in which relations
are only possible between different sets.
Different modes of representations require
different network approaches. One is that co-authorship
networks are seen as one-mode networks,
which has the advantage of the application
of a variety of measures. In contrast, twomode
networks, the other option, cannot be analysed
by standard techniques but its distinctive
features demand a new conceptualisation of
measures. In conclusion, the two-mode perspective
is more promising because it allows a dual
perspective on collaboration in science which includes
researchers as well as their scientific output
Co-authorship Network of Scientometrics Research Collaboration
No full textThis paper examines the co-authorship network in the field of scientometrics using social network analysis techniques with the aim of developing an understanding of research collaboration in this scientific community. Using co-authorship data from 3125 articles published in the journal Scientometrics with a time span of more than three decades (1980-2012), we construct an evolving co-authorship network and calculate three centrality measures (closeness, betweenness, and degree) for 3024 authors, 1207 institutions, 68 countries and 22 academic fields in this network. This paper also discusses the usability of centrality measures in author ranking, and suggests that centrality measures can be useful indicators for impact analysis. Findings revealed that scientometrics was not dominated by a couple of key researchers as quite a significant number of popular researchers were identified. The United States occupies the topmost position in all measures except for degree centrality. The most active, central and collaborative academic discipline in scientometrics is Information & Library Science
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