743 research outputs found
Accurate gravitational waveforms for binary-black-hole mergers with nearly extremal spins
Motivated by the possibility of observing gravitational waves from merging
black holes whose spins are nearly extremal (i.e., 1 in dimensionless units),
we present numerical waveforms from simulations of merging black holes with the
highest spins simulated to date: (1) a 25.5-orbit inspiral, merger, and
ringdown of two holes with equal masses and spins of magnitude 0.97 aligned
with the orbital angular momentum; and (2) a previously reported 12.5-orbit
inspiral, merger, and ringdown of two holes with equal masses and spins of
magnitude 0.95 anti-aligned with the orbital angular momentum. First, we
consider the horizon mass and spin evolution of the new aligned-spin
simulation. During the inspiral, the horizon area and spin evolve in remarkably
close agreement with Alvi's analytic predictions, and the remnant hole's final
spin agrees reasonably well with several analytic predictions. We also find
that the total energy emitted by a real astrophysical system with these
parameters---almost all of which is radiated during the time included in this
simulation---would be 10.952% of the initial mass at infinite separation.
Second, we consider the gravitational waveforms for both simulations. After
estimating their uncertainties, we compare the waveforms to several
post-Newtonian approximants, finding significant disagreement well before
merger, although the phase of the TaylorT4 approximant happens to agree
remarkably well with the numerical prediction in the aligned-spin case. We find
that the post-Newtonian waveforms have sufficient uncertainty that hybridized
waveforms will require far longer numerical simulations (in the absence of
improved post-Newtonian waveforms) for accurate parameter estimation of
low-mass binary systems.Comment: 17 pages, 7 figures, submitted to Classical and Quantum Gravit
Distributed enterprise search using software agents
In this paper we introduce a distributed information retrieval
system using agent-based technology. In this multiagent
system, each agent has its own specific task and can
be used to handle a specific document repository. The system
is designed to automatically comply with access restriction
rules that are normally enforced in companies. It is
used in the administration offices of the German capital city
Berlin where it serves as a testbed for further research on
aggregated search in an enterprise environment with roughly
50,000 employees
On the temperature dependence of the interaction-induced entanglement
Both direct and indirect weak nonresonant interactions are shown to produce
entanglement between two initially disentangled systems prepared as a tensor
product of thermal states, provided the initial temperature is sufficiently
low. Entanglement is determined by the Peres-Horodeckii criterion, which
establishes that a composite state is entangled if its partial transpose is not
positive. If the initial temperature of the thermal states is higher than an
upper critical value the minimal eigenvalue of the partially
transposed density matrix of the composite state remains positive in the course
of the evolution. If the initial temperature of the thermal states is lower
than a lower critical value the minimal eigenvalue of the
partially transposed density matrix of the composite state becomes negative
which means that entanglement develops. We calculate the lower bound
for and show that the negativity of the composite state is negligibly
small in the interval . Therefore the lower bound temperature
can be considered as \textit{the} critical temperature for the
generation of entanglement.Comment: 27 pages and 7 figure
On Toroidal Horizons in Binary Black Hole Inspirals
We examine the structure of the event horizon for numerical simulations of
two black holes that begin in a quasicircular orbit, inspiral, and finally
merge. We find that the spatial cross section of the merged event horizon has
spherical topology (to the limit of our resolution), despite the expectation
that generic binary black hole mergers in the absence of symmetries should
result in an event horizon that briefly has a toroidal cross section. Using
insight gained from our numerical simulations, we investigate how the choice of
time slicing affects both the spatial cross section of the event horizon and
the locus of points at which generators of the event horizon cross. To ensure
the robustness of our conclusions, our results are checked at multiple
numerical resolutions. 3D visualization data for these resolutions are
available for public access online. We find that the structure of the horizon
generators in our simulations is consistent with expectations, and the lack of
toroidal horizons in our simulations is due to our choice of time slicing.Comment: Submitted to Phys. Rev.
Comparing Post-Newtonian and Numerical-Relativity Precession Dynamics
Binary black-hole systems are expected to be important sources of
gravitational waves for upcoming gravitational-wave detectors. If the spins are
not colinear with each other or with the orbital angular momentum, these
systems exhibit complicated precession dynamics that are imprinted on the
gravitational waveform. We develop a new procedure to match the precession
dynamics computed by post-Newtonian (PN) theory to those of numerical binary
black-hole simulations in full general relativity. For numerical relativity NR)
simulations lasting approximately two precession cycles, we find that the PN
and NR predictions for the directions of the orbital angular momentum and the
spins agree to better than with NR during the inspiral,
increasing to near merger. Nutation of the orbital plane on the
orbital time-scale agrees well between NR and PN, whereas nutation of the spin
direction shows qualitatively different behavior in PN and NR. We also examine
how the PN equations for precession and orbital-phase evolution converge with
PN order, and we quantify the impact of various choices for handling partially
known PN terms
Comparing Gravitational Waveform Extrapolation to Cauchy-Characteristic Extraction in Binary Black Hole Simulations
We extract gravitational waveforms from numerical simulations of black hole
binaries computed using the Spectral Einstein Code. We compare two extraction
methods: direct construction of the Newman-Penrose (NP) scalar at a
finite distance from the source and Cauchy-characteristic extraction (CCE). The
direct NP approach is simpler than CCE, but NP waveforms can be contaminated by
near-zone effects---unless the waves are extracted at several distances from
the source and extrapolated to infinity. Even then, the resulting waveforms can
in principle be contaminated by gauge effects. In contrast, CCE directly
provides, by construction, gauge-invariant waveforms at future null infinity.
We verify the gauge invariance of CCE by running the same physical simulation
using two different gauge conditions. We find that these two gauge conditions
produce the same CCE waveforms but show differences in extrapolated-
waveforms. We examine data from several different binary configurations and
measure the dominant sources of error in the extrapolated- and CCE
waveforms. In some cases, we find that NP waveforms extrapolated to infinity
agree with the corresponding CCE waveforms to within the estimated error bars.
However, we find that in other cases extrapolated and CCE waveforms disagree,
most notably for "memory" modes.Comment: 26 pages, 20 figure
The Enculturation Experience of Three Chinese American Adolescents: A Multiple Case Study
The authors designed a qualitative, multiple case study that employed the photovoice method to explore how enculturation is experienced by three Chinese adolescents living with their families in a nonethnically dense cultural community. A total of 18 one-on-one interviews were conducted with three youth and their parents. Photos were also used as elicitation tools to understand the meaning of enculturation for each individual. Case descriptions of each adolescent are presented, followed by five cross-case themes: (a) Self- Identifying as Chinese, (b) Parental Strictness, (c) Multiple Groups of Comparison, (d) (Not) Having a Chinese Community, and (e) Messages to Excel. The findings provide a descriptive understanding of how adolescent enculturation is shaped by the family, community, and their intersections. Implications for research and practice, such as the continued need to understand enculturation as a dynamic phenomenon and process, are presented
The Enculturation Experience of Three Chinese American Adolescents: A Multiple Case Study
The authors designed a qualitative, multiple case study that employed the photovoice method to explore how enculturation is experienced by three Chinese adolescents living with their families in a nonethnically dense cultural community. A total of 18 one-on-one interviews were conducted with three youth and their parents. Photos were also used as elicitation tools to understand the meaning of enculturation for each individual. Case descriptions of each adolescent are presented, followed by five cross-case themes: (a) Self- Identifying as Chinese, (b) Parental Strictness, (c) Multiple Groups of Comparison, (d) (Not) Having a Chinese Community, and (e) Messages to Excel. The findings provide a descriptive understanding of how adolescent enculturation is shaped by the family, community, and their intersections. Implications for research and practice, such as the continued need to understand enculturation as a dynamic phenomenon and process, are presented
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