1,721,050 research outputs found
Coach Ken Visser, Matt Hertzler and; Ernie Chapman
No full textFootball coach Ken Visser, Matt Hertzler and Ernie Chapman. Hertzler was awarded ESPN the Magazine Academic All-District VIII Offensive line in 1997 and All-American Offensive Line in 1997 and 1998.https://digitalcommons.chapman.edu/chapman_family/1036/thumbnail.jp
Phenomenological aspects of black holes beyond general relativity
Full text linkWhile singularities are inevitable in the classical theory of general relativity, it is commonly believed that they will not be present when quantum gravity effects are taken into account in a consistent framework. In particular, the structure of black holes should be modified in frameworks beyond general relativity that aim at regularizing singularities. Being agnostic on the nature of such theory, in this paper we classify the possible alternatives to classical black holes and provide a minimal set of phenomenological parameters that describe their characteristic features. The introduction of these parameters allows us to study, in a largely model-independent manner and taking into account all the relevant physics, the phenomenology associated with these quantum-modified black holes. We perform an extensive analysis of different observational channels and obtain the most accurate characterization of the viable constraints that can be placed using current data. Aside from facilitating a critical revision of previous work, this analysis also allows us to highlight how different channels are capable of probing certain features but are oblivious to others, and pinpoint the theoretical aspects that should be addressed in order to strengthen these tests
Causal hierarchy in modified gravity
Full text linkWe investigate the causal hierarchy in various modified theories of gravity. In general relativity the standard causal hierarchy, (key elements of which are chronology, causality, strong causality, stable causality, and global hyperbolicity), is well-established. In modified theories of gravity there is typically considerable extra structure, (such as: multiple metrics, aether fields, modified dispersion relations, Hořava-like gravity, parabolic propagation, etcetera), requiring a reassessment and rephrasing of the usual causal hierarchy. We shall show that in this extended framework suitable causal hierarchies can indeed be established, and discuss the implications for the interplay between “superluminal” propagation and causality. The key distinguishing feature is whether the signal velocity is finite or infinite. Preserving even minimal notions of causality in the presence of infinite signal velocity requires the aether field to be both unique and hypersurface orthogonal, leading us to introduce the notion of global parabolicity
Non-perturbative results for the luminosity and area distances
Full text linkThe notion of luminosity distance is most often defined in purely FLRW (Friedmann-Lemaitre-Robertson-Walker) cosmological spacetimes, or small perturbations thereof. However, the abstract notion of luminosity distance is actually much more robust than this, and can be defined non-perturbatively in almost arbitrary spacetimes. Some quite general results are already known, in terms of dAobserver/dΩsource, the cross-sectional area per unit solid angle of a null geodesic spray emitted from some source and subsequently detected by some observer. We shall reformulate these results in terms of a suitably normalized null geodesic affine parameter and the van Vleck determinant, ΔvV. The contribution due to the null geodesic affine parameter is effectively the inverse square law for luminosity, and the van Vleck determinant can be viewed as providing a measure of deviations from the inverse square law. This formulation is closely related to the so-called Jacobi determinant, but the van Vleck determinant has somewhat nicer analytic properties and wider and deeper theoretical base in the general relativity, quantum physics, and quantum field theory communities. In the current article we shall concentrate on non-perturbative results, leaving near-FLRW perturbative investigation for future work
Vorticity in analogue spacetimes
Full text linkAnalogue spacetimes can be used to probe and study physically interesting spacetime geometries by constructing, either theoretically or experimentally, some notion of an effective Lorentzian metric [geff(g,V,Ξ)]ab. These effective metrics generically depend on some physical background metric gab, often flat Minkowski space ηab, some "medium" with 4-velocity Va, and possibly some additional background fields and parameters Ξ. (These might include signal propagation speeds and the like.) Analogue spacetimes based on electromagnetic media date back to Gordon's work in the 1920s, analogue spacetimes based on acoustics in fluids date back to Unruh's work in the 1980s, and BEC-based analogue spacetimes date back to various authors in the 1990s. The analogue spacetimes based on acoustic propagation in bulk fluids have perhaps the most rigorous mathematical formulation, and these acoustics-based analogue models really work best in the absence of vorticity, when the medium has an irrotational flow. This physical restriction makes it difficult to mimic the particularly interesting case of rotating astrophysical spacetimes, spacetimes with nonzero angular momentum, and in the current article we explore the extent to which one might hope to be able to develop an analogue model for astrophysical spacetimes with angular momentum (thereby implying vorticity in the 4-velocity of the medium). We shall focus on two particular analogue models: (1) the use of a charged BEC as the background medium, where new results concerning the interplay between healing length and London penetration depth are a key technical improvement, and (2) new results regarding the Gordon metric associated with an isotropic fluid medium
Perturbative treatment of the luminosity distance
Full text linkWe derive a generalized luminosity distance versus redshift relation for a linearly perturbed Friedmann-Lemaitre-Robertson-Walker (FLRW) metric with two scalar mode excitations. We use two equivalent approaches, based on the Jacobi map and the van Vleck determinant, respectively. We apply the resultant formula to two simple models - an exact FLRW universe and an approximate FLRW universe perturbed by a single scalar mode sinusoidally varying with time. For both models, we derive a cosmographic expansion for dL in terms of z. We comment on the interpretation of our results and their possible application to more realistic cosmological models
Towards a Gordon form of the Kerr spacetime
No full textIt is not currently known how to put the Kerr spacetime metric into the so-called Gordon form, although the closely related Kerr-Schild form of the Kerr metric is well known. A Gordon form for the Kerr geometry, if it could be found, would be particularly useful in developing analogue models for the Kerr spacetime, since the Gordon form is explicitly given in terms of the 4-velocity and 'refractive index' of an effective medium. In the current article we report progress toward this goal. First we present the Gordon form for an approximation to Kerr spacetime in the slow-rotation limit, obtained by suitably modifying the well-known Lense-Thirring form of the slow-rotation metric. Second we present the Gordon form for the Kerr spacetime in the near-null limit, (the 4-velocity of the medium being close to null). That these two perturbative approximations to the Kerr spacetime in Gordon form exist gives us some confidence that ultimately one might be able to write the exact Kerr spacetime in this form
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
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