1,721,124 research outputs found
Perihelion Precession and Generalized Uncertainty Principle
No full textWe compute the corrections to the Schwarzschild metric necessary to reproduce the Hawking temperature derived from a Generalized Uncertainty Princi- ple (GUP), so that the GUP deformation parameter is directly linked to the deforma- tion of the metric. Using this modified Schwarzschild metric, we compute corrections to the standard General Relativistic predictions for the perihelion precession for plan- ets in the solar system. This analysis allows us to set bounds for the GUP deformation parameter from well-known astronomical measurements
CMB acoustic scale in the entropic-like accelerating universe
No full textWe consider generalizations of the entropic accelerating universe recently proposed in [D. A. Easson, P. H. Frampton, and G. F. Smoot, Phys. Lett. B 696, 273 (2011).] and [D. A. Easson, P. H. Frampton, and G. F. Smoot, arXiv:1003.1528.] and show that their background equations can be made equivalent to a model with a dark energy component with a constant parameter of state wX=-1+2γ/3, where γ is related to the coefficients of the new terms in the Friedmann equations. After discussing all the Friedmann equations for an arbitrary γ, we show how to recover the standard scalings for dust and radiation. The acoustic scale ℓA, related to the peak positions in the pattern of the angular power spectrum of the cosmic microwave background anisotropies, is also computed and yields the stringent bound |γ|≪1. We then argue that future data might be able to distinguish this model from pure ΛCDM (corresponding to γ=0)
Bubble dynamics: (Nucleating) radiation inside dust
No full textWe consider two spatially flat Friedmann-Robertson-Walker spacetimes divided by a timelike thin shell in the nontrivial case in which the inner region of finite extension contains radiation and the outer region is filled with dust. We will then show that, while the evolution is determined by a large set of constraints, an analytical description for the evolution of the bubble radius can be obtained by formally expanding for short times after the shell attains its minimum size. In particular, we will find that a bubble of radiation, starting out with vanishing expansion speed, can be matched with an expanding dust exterior, but not with a collapsing dust exterior, regardless of the dust energy density. The former case can then be used to describe the nucleation of a bubble of radiation inside an expanding dust cloud, although the final configuration contains more energy than the initial dust, and the reverse process, with collapsing radiation transforming into collapsing dust, is therefore energetically favored. We however speculate a (small) decaying vacuum energy or cosmological constant inside dust could still trigger nucleation. Finally, our perturbative (yet analytical) approach can be easily adapted to different combinations of matter inside and outside the shell, as well as to more general surface density, of relevance for cosmology and studies of defect formation during phase transitions
A Quantum Cosmic Conjecture
No full textFor a quantum mechanically Gaussian shaped, electrically charged,
massive particle, we compute the Horizon Wave-function(s) in order to study (a) the
existence of the inner Cauchy horizon of the corresponding Reissner–Nordström
space-time when the charge-to-mass ratio 0 < α < 1 and (b) the survival of a naked
singularity when the charge-to-mass ratio α > 1. Our results suggest that any semi-
classical instability one expects near the inner horizon may not occur in quantum
black holes, with a mass around the Planck scale, and that no states with charge-to-
mass ratio greater than a critical value (of the order of 2) should exist
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
Minimum black hole mass from colliding Gaussian packets
No full textWe study the formation of a black hole in the col- lision of two Gaussian packets. Rather than following their dynamical evolution in detail, we assume a horizon forms when the mass function for the two packets becomes larger than half the flat areal radius, as it would occur in a spher- ically symmetric geometry. This simple approximation al- lows us to determine the existence of a minimum black hole mass solely related to the width of the packets. We then com- ment on the possible physical implications, both in classical and quantum physics, and models with extra spatial dimen- sions
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
Boundaries and the Casimir effect in noncommutative space-time
No full textWe calculate modifications to the scalar Casimir force between two parallel plates due to space-time noncommutativity. We devise a heuristic approach to overcome the difficulties of describing boundaries in noncommutative theories and predict that boundary corrections are of the same order as noncommutative volume corrections. Further, both corrections have the form of more conventional finite surface effects
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