9,037 research outputs found
Formation and decay of Einstein-Yang-Mills black holes
We study various aspects of black holes and gravitational collapse in
Einstein-Yang-Mills theory under the assumption of spherical symmetry.
Numerical evolution on hyperboloidal surfaces extending to future null infinity
is used. We begin by constructing colored and Reissner-Nordstrom black holes on
surfaces of constant mean curvature and analyze their perturbations. These
linearly perturbed black holes are then evolved into the nonlinear regime and
the masses of the final Schwarzschild black holes are computed as a function of
the initial horizon radius. We compare with an information-theoretic bound on
the lifetime of unstable hairy black holes derived by Hod. Finally we study
critical phenomena in gravitational collapse at the threshold between different
Yang-Mills vacuum states of the final Schwarzschild black holes, where the n=1
colored black hole forms the critical solution. The work of Choptuik et al.
(1999) is extended by using a family of initial data that includes another
region in parameter space where the colored black hole with the opposite sign
of the Yang-Mills potential forms the critical solution. We investigate the
boundary between the two regions and discover that the Reissner-Nordstrom
solution appears as a new approximate codimension-two attractor.Comment: 13 pages, 12 figures. Minor changes to agree with published versio
Is Authority Always Constructed and Contextual? A Classical Challenge to the Framework for Information Literacy
The 2015 Framework for Information Literacy for Higher Education (or Framework) is the latest effort of academic librarians to provide relevant guidance for the teaching of information literacy. One claim made within this “living document,” in line with current academic trends of constructivism and social constructivism, is that “Authority is Constructed and Contextual.” Questions are raised concerning authority’s relationship to the idea of truth, and an effort is made, largely through a Socratic method of inquiry, to delve into the meaning of the Framework’s statement on authority using the further explanations provided concerning this particular “frame,” as well as the context of the entire document. Connections between the nature of authority, responsibility, and the ethical direction of the Framework are considered, and the relevance of the matter of truth is brought to bear here as well. Finally, the conclusion is reached that in light of the investigation’s findings, the current statement that “Authority is Constructed and Contextual” is fraught with significant difficulties, and a statement akin to “Issues of Authority are Contextual and Nuanced” is warranted instead
The Evaluation of Immigration Policies
This chapter summarizes the literature on the evaluation of immigration policies. It brings together two strands of the literature dealing with the evaluation of labor market programs and with the economic integration of immigrants. Next to immigrant selection and settlement policies, there are four types of interventions that aim at improving the economic and social outcomes of immigrants: a) introduction programs, b) language training, c) labor market programs, and d) anti-discrimination policies. The chapter discusses problems associated with the evaluation of such programs, presents methodological approaches to circumvent these problems, and surveys empirical results and findings. It concludes with lessons from previous research and identifies avenues for future research.migration, program evaluation, immigrant selection, settlement policy, introduction programs, discrimination, active labor market policy, language training, integration
Numerical Brill-Lindquist initial data with a Schwarzschildean end at spatial infinity
We construct numerically time-symmetric initial data that are
Schwarzschildean at spatial infinity and Brill-Lindquist in the interior. The
transition between these two data sets takes place along a finite gluing region
equipped with an axisymmetric Brill wave metric. The construction is based on
an application of Corvino's gluing method using Brill waves due to Giulini and
Holzegel. Here, we use a gluing function that includes a simple angular
dependence. We also investigate the dependence of the ADM mass of our
construction on the details of the gluing procedure.Comment: 6 pages, 1 figure, 1 table. Conference proceedings for the Spanish
Relativity Meeting, Valencia 201
Critical phenomena in the general spherically symmetric Einstein-Yang-Mills system
We study critical behavior in gravitational collapse of a general spherically
symmetric Yang-Mills field coupled to the Einstein equations. Unlike the
magnetic ansatz used in previous numerical work, the general Yang-Mills
connection has two degrees of freedom in spherical symmetry. This fact changes
the phenomenology of critical collapse dramatically. The magnetic sector
features both type I and type II critical collapse, with universal critical
solutions. In contrast, in the general system type I disappears and the
critical behavior at the threshold between dispersal and black hole formation
is always type II. We obtain values of the mass scaling and echoing exponents
close to those observed in the magnetic sector, however we find some
indications that the critical solution differs from the purely magnetic
discretely self-similar attractor and exact self-similarity and universality
might be lost. The additional "type III" critical phenomenon in the magnetic
sector, where black holes form on both sides of the threshold but the
Yang-Mills potential is in different vacuum states and there is a mass gap,
also disappears in the general system. We support our dynamical numerical
simulations with calculations in linear perturbation theory; for instance, we
compute quasi-normal modes of the unstable attractor (the Bartnik-McKinnon
soliton) in type I collapse in the magnetic sector.Comment: 15 pages, 15 figures; v2: matches published versio
Spectral approach to axisymmetric evolution of Einstein's equations
We present a new formulation of Einstein's equations for an axisymmetric
spacetime with vanishing twist in vacuum. We propose a fully constrained scheme
and use spherical polar coordinates. A general problem for this choice is the
occurrence of coordinate singularities on the axis of symmetry and at the
origin. Spherical harmonics are manifestly regular on the axis and hence take
care of that issue automatically. In addition a spectral approach has
computational advantages when the equations are implemented. Therefore we
spectrally decompose all the variables in the appropriate harmonics. A central
point in the formulation is the gauge choice. One of our results is that the
commonly used maximal-isothermal gauge turns out to be incompatible with tensor
harmonic expansions, and we introduce a new gauge that is better suited. We
also address the regularisation of the coordinate singularity at the origin.Comment: 6 pages, based on a talk given by one of the authors at the Spanish
Relativity Meeting ERE14 in Valencia, published versio
Superradiance of a charged scalar field coupled to the Einstein-Maxwell equations
We consider the Einstein-Maxwell-Klein-Gordon equations for a spherically
symmetric scalar field scattering off a Reissner-Nordstr\"om black hole in
asymptotically flat spacetime. The equations are solved numerically using a
hyperboloidal evolution scheme. For suitable frequencies of the initial data,
superradiance is observed, leading to a substantial decrease of mass and charge
of the black hole. We also derive a Bondi mass loss formula using the Kodama
vector field and investigate the late-time decay of the scalar field.Comment: 12 pages, 9 figures; 3 references adde
Report No. 8: Evaluation of the Active Labor Market Program "Beautiful Serbia"
Final report on behalf of the United Nations Development Program (UNDP) Serbia and Montenegro, Bonn 2006 (84 pages)
Outer boundary conditions for Einstein's field equations in harmonic coordinates
We analyze Einstein's vacuum field equations in generalized harmonic coordinates on a compact spatial domain with boundaries. We specify a class of boundary conditions, which is constraint-preserving and sufficiently general to include recent proposals for reducing the amount of spurious reflections of gravitational radiation. In particular, our class comprises the boundary conditions recently proposed by Kreiss and Winicour, a geometric modification thereof, the freezing-Ψ0 boundary condition and the hierarchy of absorbing boundary conditions introduced by Buchman and Sarbach. Using the recent technique developed by Kreiss and Winicour based on an appropriate reduction to a pseudo-differential first-order system, we prove well posedness of the resulting initial-boundary value problem in the frozen coefficient approximation. In view of the theory of pseudo-differential operators, it is expected that the full nonlinear problem is also well posed. Furthermore, we implement some of our boundary conditions numerically and study their effectiveness in a test problem consisting of a perturbed Schwarzschild black hole
A flow approach to Bartnik's static metric extension conjecture in axisymmetry
We investigate Bartnik's static metric extension conjecture under the
additional assumption of axisymmetry of both the given Bartnik data and the
desired static extensions. To do so, we suggest a geometric flow approach,
coupled to the Weyl-Papapetrou formalism for axisymmetric static solutions to
the Einstein vacuum equations. The elliptic Weyl-Papapetrou system becomes a
free boundary value problem in our approach. We study this new flow and the
coupled flow--free boundary value problem numerically and find axisymmetric
static extensions for axisymmetric Bartnik data in many situations, including
near round spheres in spatial Schwarzschild of positive mass.Comment: 60 pages, 13 figures. Expanded Section 3.3 to address longtime
existence and uniqueness of solutions to the linearised flow equations. To
appear in Pure and Applied Mathematics Quarterly, special issue in honour of
Robert Bartni
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