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Abstract: We revisit the theory of null shells in general relativity, with a particular emphasis on null shells placed at horizons of black holes. We study in detail the considerable freedom that is available in the case that one solders two metrics together across null hypersurfaces (such as Killing horizons) for which the induced metric is invariant under translations along the null generators. In this case the group of soldering transformations turns out to be infinite dimensional, and these solderings create non-trivial horizon shells containing both massless matter and impulsive gravitational wave components. We also rephrase this result in the language of Carrollian symmetry groups. To illustrate this phenomenon we discuss in detail the example of shells on the horizon of the Schwarzschild black hole (with equal interior and exterior mass), uncovering a rich classical structure at the horizon and deriving an explicit expression for the general horizon shell energy- momentum tensor. In the special case of BMS-like soldering supertranslations we find a conserved shell-energy that is strikingly similar to the standard expression for asymptotic BMS supertranslation charges, suggesting a direct relation between the physical properties of these horizon shells and the recently proposed BMS supertranslation hair of a black hole.
Found in: ključnih besedah
Summary of found: ...Black Holes, Classical Theories of Gravity, Space-Time Symmetries...
Keywords: Black Holes, Classical Theories of Gravity, Space-Time Symmetries
Published: 10.03.2016; Views: 3804; Downloads: 0
Fulltext (333,17 KB)

Found in: ključnih besedah
Summary of found: ...... are significantly cutting into the parameter space of one of the most popular candidates,...
Keywords: We live in a golden age for astro-particle physics, with a significant number of experiments actively monitoring high-energy Universe. Many of these probes provide excellent tests of particle physics models of dark matter particles. In particular, experiments such as Fermi -LAT, AMS-02, Ice Cube, ... are significantly cutting into the parameter space of one of the most popular candidates, the WIMPs. In this talk I will describe some of the strategies and methods used to search for dark matter with astrophysical data. Special attention will be given to the latest indications of an unaccounted gamma-ray excess at few GeV in the Fermi-LAT data in the region around the Galactic Centre, which steered lots of attention as it was shown to be consistent with putative signals of WIMP dark matter particles and complementary constraints provided by other experiments. Finally I will discuss projections of the expected sensitivities with upcoming experiments and continued data taking with current ones.
Published: 17.07.2017; Views: 3992; Downloads: 0
Fulltext (50,35 MB)

Found in: ključnih besedah
Summary of found: ...polygroup, fuzzy topological space,(relatively) fuzzy continuity, lower (upper) fuzzy topological subpolygroup...
Keywords: polygroup, fuzzy topological space, (relatively) fuzzy continuity, lower (upper) fuzzy topological subpolygroup
Published: 21.08.2018; Views: 2603; Downloads: 0
Fulltext (120,69 KB)

Abstract: Space weather refers to environmental conditions in the interplanetary space and Earth’s magnetosphere, ionosphere and exosphere and can influence the performance and reliability of electronics based technological systems. The major role in space weather changes plays the solar wind, a stream of charged particles (mostly electrons and protons) with energies of approximately 1 keV, that can cause geomagnetic storms and auroras. During their entry into the atmosphere, high energy cosmic rays collide with atomic nuclei of atmospheric gasses. When scattering occurs extensive air showers are created. Those cascades of secondary particles create flashes of light due to the Cherenkov effect as well as excite molecules of nitrogen gas in atmosphere, which then glow in fluorescent light. In order to observe the light created by air showers, it has to be collected with telescopes. The particles from the cascades that reach ground can be detected with surface detectors. The Pierre Auger Observatory is the largest observatory for cosmic ray measurements. It is located in Argentinian pampas covering an area of 3000 km2. It consists of 1660 surface detectors and 27 fluorescence telescopes. For cosmic rays with energies above few 1017 eV, a precise reconstruction of energy and direction of primary particle is achievable. Observatory also allows measurement of flux of incoming particles down to primary energies in ca. 10 GeV - 10 TeV interval, with a median energy ca. 80-90 GeV. This measurement capability is called "scaler" mode, since the corresponding data consist of scaler counted cascade particles with deposited energy between 15 and 100 MeV, at the average rate of 2 kHz per individual surface detector. For the purpose of this master thesis I compared the data from scaler mode measurements with measurements of neutron monitors, which are commonly used for space weather observations. With the correlation received from the comparison, I showed that scaler mode operation of Pierre Auger observatory can be used to monitor space weather events such as solar cycle and the decreases in the observed galactic cosmic ray intensity due to solar wind (Forbush decrease).
Found in: ključnih besedah
Keywords: Pierre Auger Observatory, cosmic rays, space weather, Forbush decrease
Published: 17.09.2019; Views: 3902; Downloads: 167
Fulltext (5,21 MB)

Abstract: In this paper I consider examples of urban public inscriptions which appeared during the Gezi Park protests of Istanbul in 2013. I consider them both as a form of expression permitting the creation and circulation of utopian messages of resistance, and as relating to the inclusion of a multiplicity of identities in the protest arena, such as Kurdish, Armenian and LGBT identities. I insist on the role the written word plays in forging a specific and non-exclusivist imaginary of the protest, positioned within complex power dynamics and national narratives. I view the physical presence of specific graphic expressions referencing “marginal identities” as part of a set of revolutionary actions, which demonstrate the importance of written-in spaces and surfaces in processes of “symbolic reterritorialization”. This theme emerges as one of the central questions in interpretations of events relating to the Gezi resistance.
Found in: ključnih besedah
Keywords: public inscriptions, reappropriation of space, protest, Gezi Park, minorities
Published: 22.06.2020; Views: 1738; Downloads: 0
Fulltext (853,82 KB)