1,720,959 research outputs found
Tackling conceptual problems in gravity with numerically simulated gedanken experiments
Full text linkGedanken experiments, also known as thought experiments, are an extremely powerful tool for studying conceptual problems in physics. The strategy is to devise fictitious experimental setups in which the effects one is interested in investigating clearly manifest, and can be better analyzed. In some cases it is necessary to predict the evolution of physical systems, a task that can become particularly hard when the theory is nonlinear. A possible approach to solve this issue is to perform a numerical integration of the evolution equations, thus simulating the gedanken experiment. In this thesis I will present three works, that I have carried out with different collaborators, in which such research technique has been used to tackle conceptual problems of different nature. In particular, in the first we performed extensive numerical simulations of the collapse of charged wave packets in Einstein-Maxwell and in Einstein-Maxwell-scalar theories of gravity, in the attempt of forming naked singularities and violate the weak cosmic censorship conjecture. In the second work we studied the fate of minimum mass black hole in Einstein-dilaton-Gauss-Bonnet gravity, when they undergo Hawking evaporation, by constructing a numerically simulated gedanken experiment in which we dynamically reduced the black hole mass by means of wave packets of a phantom field. Lastly, in the third work we simulated the nonlinear interaction between high-amplitude, low-frequency electromagnetic wave packets and a barrier of plasma, with the purpose of studying whether, in the scenario of the plasma-driven superradiant instability, the electromagnetic field can be confined in the vicinity of the black hole even during its exponential growth
Nonlinear photon-plasma interaction and the black hole superradiant instability
No full textElectromagnetic field confinement due to plasma near accreting black holes can trigger superradiant instabilities at the linear level, limiting the spin of black holes and providing novel astrophysical sources of electromagnetic bursts. However, nonlinear effects might jeopardize the efficiency of the confinement, rending superradiance ineffective. Motivated by understanding nonlinear interactions in this scenario, here we study the full 3 + 1 nonlinear dynamics of Maxwell equations in the presence of plasma by focusing on regimes that are seldom explored in standard plasma-physics applications, namely a generic electromagnetic wave of very large amplitude but small frequency propagating in an inhomogeneous, overdense plasma. We show that the plasma transparency effect predicted in certain specific scenarios is not the only possible outcome in the nonlinear regime: plasma blowout due to nonlinear momentum transfer is generically present and allows for significant energy leakage of electromagnetic fields above a certain threshold. We argue that such effect is sufficient to dramatically quench the plasma-driven superradiant instability around black holes even in the most optimistic scenarios
What is the fate of Hawking evaporation in gravity theories with higher curvature terms?
Full text linkDuring the final stages of black hole evaporation, ultraviolet deviations
from General Relativity eventually become dramatic, potentially affecting the
end-state. We explore this problem by performing nonlinear simulations of wave
packets in Einstein-dilaton-Gauss-Bonnet gravity, the only gravity theory with
quadratic curvature terms which can be studied at fully nonperturbative level.
Black holes in this theory have a minimum mass but also a nonvanishing
temperature. This poses a puzzle concerning the final fate of Hawking
evaporation in the presence of high-curvature nonperturbative effects. By
simulating the mass loss induced by evaporation at the classical level using an
auxiliary phantom field, we study the nonlinear evolution of black holes past
the minimum mass. We observe a runaway shrink of the horizon (a nonperturbative
effect forbidden in General Relativity) which eventually unveils a
high-curvature elliptic region. While this might hint to the formation of a
naked singularity (and hence to a violation of the weak cosmic censorship) or
of a pathological spacetime region, a different numerical formulation of the
initial-value problem in this theory might be required to rule out other
possibilities, including the transition from the critical black hole to a
stable horizonless remnant. Our study is relevant in the context of the
information-loss paradox, dark-matter remnants, and for constraints on
microscopic primordial black holes.Comment: 6 pages, 3 figures. v2: extended discussion and clarification.
Results and conclusions unchanged. v3: minor changes to match version to
appear in PRL. Companion paper available at arXiv:2205.1300
Nonperturbative gedanken experiments in Einstein-dilaton-Gauss-Bonnet gravity: nonlinear transitions and tests of the cosmic censorship beyond General Relativity
Full text linkAs the only gravity theory with quadratic curvature terms and second-order
field equations, Einstein-dilaton-Gauss-Bonnet gravity is a natural testbed to
probe the high-curvature regime beyond General Relativity in a fully
nonperturbative way. Due to nonperturbative effects of the dilatonic coupling,
black holes in this theory have a minimum mass which separates a stable branch
from an unstable one. The minimum mass solution is a double point in the phase
diagram of the theory, wherein the critical black hole and a wormhole solution
coexist. We perform extensive nonlinear simulations of the spherical collapse
onto black holes with scalar hair in this theory, especially focusing on the
region near the minimum mass. We study the nonlinear transition from the
unstable to the stable branch and assess the nonlinear stability of the latter.
Furthermore, motivated by modeling the mass loss induced by Hawking radiation
near the minimum mass at the classical level, we study the collapse of a
phantom field onto the black hole. When the black-hole mass decreases past the
critical value, the apparent horizon shrinks significantly, eventually
unveiling a high-curvature elliptic region. We argue that evaporation in this
theory is bound to either violate the weak cosmic censorship or produce
horizonless remnants. Addressing the end-state might require a different
evolution scheme.Comment: 27 pages, 23 figures, companion paper of arXiv:2205.13006 [gr-qc] .
v2: added references and comparison between event horizon and apparent
horizon. v3: extended discussion with new simulations of a pair of positive-
and negative- energy wave packets to mimic the Hawking quantum pair. Results
and conclusions unchanged. v4: minor changes to match version to appear in
PR
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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