1,759 research outputs found
Space science/space station attached payload pointing accommodation study: Technology assessment white paper
Technology assessment is performed for pointing systems that accommodate payloads of large mass and large dimensions. Related technology areas are also examined. These related areas include active thermal lines or power cables across gimbals, new materials for increased passive damping, tethered pointing, and inertially reacting pointing systems. Conclusions, issues and concerns, and recommendations regarding the status and development of large pointing systems for space applications are made based on the performed assessments
Mass Inflation in the Loop Black Hole
In classical general relativity the Cauchy horizon within a two-horizon black
hole is unstable via a phenomenon known as mass inflation, in which the mass
parameter (and the spacetime curvature) of the black hole diverges at the
Cauchy horizon. Here we study this effect for loop black holes -- quantum
gravitationally corrected black holes from loop quantum gravity -- whose
construction alleviates the singularity present in their classical
counterparts. We use a simplified model of mass inflation, which makes use of
the generalized DTR relation, to conclude that the Cauchy horizon of loop black
holes indeed results in a curvature singularity similar to that found in
classical black holes. The DTR relation is of particular utility in the loop
black hole because it does not directly rely upon Einstein's field equations.
We elucidate some of the interesting and counterintuitive properties of the
loop black hole, and corroborate our results using an alternate model of mass
inflation due to Ori.Comment: Latex 20 pages, 7 figure
Nonabelian solutions in AdS_4 and d=11 supergravity
We consider solutions of the four dimensional Einstein-Yang-Mills system with
a negative cosmological constant , where is the nonabelian
gauge coupling constant. This theory corresponds to a consistent truncation of
gauged supergravity and may be uplifted to supergravity. A
systematic study of all known solutions is presented as well as new
configurations corresponding to rotating regular dyons and rotating nonabelian
black holes. The thermodynamics of the static black hole solutions is also
discussed. The generic EYM solutions present a nonvanishing magnetic flux at
infinity and should give us information about the structure of a CFT in a
background SU(2) field. We argue that the existence of these configurations
violating the no hair conjecture is puzzling from the AdS/CFT point of view.Comment: 52 pages; 24 figures; v2: minor changes, references added, version to
be published in PR
Gravitationally Collapsing Shells in (2+1) Dimensions
We study gravitationally collapsing models of pressureless dust, fluids with
pressure, and the generalized Chaplygin gas (GCG) shell in (2+1)-dimensional
spacetimes. Various collapse scenarios are investigated under a variety of the
background configurations such as anti-de Sitter(AdS) black hole, de Sitter
(dS) space, flat and AdS space with a conical deficit. As with the case of a
disk of dust, we find that the collapse of a dust shell coincides with the
Oppenheimer-Snyder type collapse to a black hole provided the initial density
is sufficiently large. We also find -- for all types of shell -- that collapse
to a naked singularity is possible under a broad variety of initial conditions.
For shells with pressure this singularity can occur for a finite radius of the
shell. We also find that GCG shells exhibit diverse collapse scenarios, which
can be easily demonstrated by an effective potential analysis.Comment: 27 pages, Latex, 11 figures, typos corrected, references added, minor
amendments in introduction and conclusion introd
Report on the State of Available Data for the Study of International Trade and Foreign Direct Investment
This report, prepared for the Committee on Economic Statistics of the American Economic Association, examines the state of available data for the study of international trade and foreign direct investment. Data on values of imports and exports of goods are of high quality and coverage, but price data suffer from insufficient detail. It would be desirable to have more data measuring value-added in trade as well as prices of comparable domestic and imported inputs. Value data for imports and exports of services are too aggregated and valuations are questionable, while price data for service exports and imports are almost non-existent. Foreign direct investment data are of high quality but quality has suffered from budget cuts. Data on trade in intellectual property are fragmentary. The intangibility of the trade makes measurement difficult, but budget cuts have added to the difficulties. Modest funding increases would result in data more useful for research and policy analysis.
Generation of Cosmological Seed Magnetic Fields from Inflation with Cutoff
Inflation has the potential to seed the galactic magnetic fields observed
today. However, there is an obstacle to the amplification of the quantum
fluctuations of the electromagnetic field during inflation: namely the
conformal invariance of electromagnetic theory on a conformally flat underlying
geometry. As the existence of a preferred minimal length breaks the conformal
invariance of the background geometry, it is plausible that this effect could
generate some electromagnetic field amplification. We show that this scenario
is equivalent to endowing the photon with a large negative mass during
inflation. This effective mass is negligibly small in a radiation and matter
dominated universe. Depending on the value of the free parameter of the theory,
we show that the seed required by the dynamo mechanism can be generated. We
also show that this mechanism can produce the requisite galactic magnetic field
without resorting to a dynamo mechanism.Comment: Latex, 16 pages, 2 figures, 4 references added, minor corrections;
v4: more references added, boundary term written in a covariant form,
discussion regarding other gauge fields added, submitted to PRD; v5: matched
with the published versio
Nuttier Bubbles
We construct new explicit solutions of general relativity from double
analytic continuations of Taub-NUT spacetimes. This generalizes previous
studies of 4-dimensional nutty bubbles. One 5-dimensional locally
asymptotically AdS solution in particular has a special conformal boundary
structure of . We compute its boundary stress tensor and
relate it to the properties of the dual field theory. Interestingly enough, we
also find consistent 6-dimensional bubble solutions that have only one timelike
direction. The existence of such spacetimes with non-trivial topology is
closely related to the existence of the Taub-NUT(-AdS) solutions with more than
one NUT charge. Finally, we begin an investigation of generating new solutions
from Taub-NUT spacetimes and nuttier bubbles. Using the so-called Hopf duality,
we provide new explicit time-dependent backgrounds in six dimensions.Comment: 32 pages, 1 figure; v.3. typos corrected. Matches the published
versio
A note on spherically symmetric naked singularities in general dimension
We discuss generalizations of the recent theorem by Dafermos (hep-th/0403033)
forbidding a certain class of naked singularities in the spherical collapse of
a scalar field. Employing techniques similar to the ones Dafermos used, we
consider extending the theorem (1) to higher dimensions, (2) by including more
general matter represented by a stress-energy tensor satisfying certain
assumptions, and (3) by replacing the spherical geometry by a toroidal or
higher genus (locally hyperbolic) one. We show that the extension to higher
dimensions and a more general topology is straightforward; on the other hand,
replacing the scalar field by a more general matter content forces us to shrink
the class of naked singularities we are able to exclude. We then show that the
most common matter theories (scalar field interacting with a non-abelian gauge
field and a perfect fluid satisfying certain conditions) obey the assumptions
of our weaker theorem, and we end by commenting on the applicability of our
results to the five-dimensional AdS scenarii considered recently in the
literature.Comment: 16 pages, no figures, typos fixe
Palatini Variational Principle for an Extended Einstein-Hilbert Action
We consider a Palatini variation on a generalized Einstein-Hilbert action. We
find that the Hilbert constraint, that the connection equals the Christoffel
symbol, arises only as a special case of this general action, while for
particular values of the coefficients of this generalized action, the
connection is completely unconstrained. We discuss the relationship between
this situation and that usually encountered in the Palatini formulation.Comment: 14 pages, LaTe
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