912 research outputs found
Single atom quantum walk with 1D optical superlattices
A proposal for the implementation of quantum walks using cold atom technology
is presented. It consists of one atom trapped in time varying optical
superlattices. The required elements are presented in detail including the
preparation procedure, the manipulation required for the quantum walk evolution
and the final measurement. These procedures can be, in principle, implemented
with present technology.Comment: 6 pages, 7 figure
Fully relativistic three-dimensional Cauchy-characteristic matching
A fully relativistic three-dimensional Cauchy-characteristic matching (CCM)
algorithm is implemented in a numerical relativity code SpECTRE. The method is
free of approximations and can be applied to any physical system. We test the
algorithm with various scenarios, including the propagation of Teukolsky waves
within a flat background, the perturbation of a Kerr black hole with a
Teukolsky wave, and the injection of a gravitational-wave pulse from the
characteristic grid. Our investigations reveal no numerical instabilities in
the simulations. In addition, the tests indicate that the CCM algorithm
effectively directs characteristic information into the inner Cauchy system,
yielding higher precision in waveforms and smaller violations of Bondi-gauge
constraints, especially when the outer boundary of the Cauchy evolution is at a
smaller radius
Relativistic hydrodynamics on spacelike and null surfaces: Formalism and computations of spherically symmetric spacetimes
We introduce a formulation of Eulerian general relativistic hydrodynamics
which is applicable for (perfect) fluid data prescribed on either spacelike or
null hypersurfaces. Simple explicit expressions for the characteristic speeds
and fields are derived in the general case. A complete implementation of the
formalism is developed in the case of spherical symmetry. The algorithm is
tested in a number of different situations, predisposing for a range of
possible applications. We consider the Riemann problem for a polytropic gas,
with initial data given on a retarded/advanced time slice of Minkowski
spacetime. We compute perfect fluid accretion onto a Schwarzschild black hole
spacetime using ingoing null Eddington-Finkelstein coordinates. Tests of fluid
evolution on dynamic background include constant density and TOV stars sliced
along the radial null cones. Finally, we consider the accretion of
self-gravitating matter onto a central black hole and the ensuing increase in
the mass of the black hole horizon.Comment: 23 pages, 13 figures, submitted to Phys. Rev.
Conformally rescaled spacetimes and Hawking radiation
We study various derivations of Hawking radiation in conformally rescaled
metrics. We focus on two important properties, the location of the horizon
under a conformal transformation and its associated temperature. We find that
the production of Hawking radiation cannot be associated in all cases to the
trapping horizon because its location is not invariant under a conformal
transformation. We also find evidence that the temperature of the Hawking
radiation should transform simply under a conformal transformation, being
invariant for asymptotic observers in the limit that the conformal
transformation factor is unity at their location.Comment: 22 pages, version submitted to journa
Nonlinearities in black hole ringdowns
The gravitational wave strain emitted by a perturbed black hole (BH) ringing down is typically modeled analytically using first-order BH perturbation theory. In this Letter we show that second-order effects are necessary for modeling ringdowns from BH merger simulations. Focusing on the strain's angular harmonic, we show the presence of a quadratic effect across a range of binary BH mass ratios that agrees with theoretical expectations. We find that the quadratic mode amplitude exhibits quadratic scaling with the fundamental mode -- its parent mode. The nonlinear mode's amplitude is comparable to or even larger than that of the linear modes. Therefore correctly modeling ringdown -- improving mismatches by an order of magnitude -- requires the inclusion of nonlinear effects
Formative evaluation of electricity distribution utilities using data envelopment analysis
The use of Data Envelopment Analysis (DEA) in the electricity distribution sector has been prolific in the number of papers published in research journals. However, while numerous studies have been documented, they have mostly been summative. Their aim has been predominantly descriptive and
classificatory. This paper argues that evaluations of a formative nature are more effective than summative studies in promoting a better understanding of the structures and processes of electricity
distribution utilities and, consequently, are more appropriate to contribute to performance improvement.
To illustrate the use of DEA for formative evaluation, and highlight some of the difficulties of using DEA in practice, this paper compares the cost-efficiency of the Portuguese electricity distribution companies from 2002 to 2006. A dynamic analysis using Malmquist Indices is also conducted in order to evaluate the changes in productivity over this period. Our analysis shows that the application of DEA
for formative purposes meets some difficulties. In particular it shows that while the modelling of productivity/efficiency scores using DEA is relatively straightforward, it is comparatively more difficult to develop models that are economically valid and that produce results with face validity. On the basis of the insights derived from this analysis, the paper provides some recommendations regarding the successful application of DEA for performance improvement
Lack of correlation of stem cell markers in breast cancer stem cells
BACKGROUND: Various markers are used to identify the unique sub-population of breast cancer cells with stem cell properties. Whether these markers are expressed in all breast cancers, identify the same population of cells, or equate to therapeutic response is controversial. METHODS: We investigated the expression of multiple cancer stem cell markers in human breast cancer samples and cell lines in vitro and in vivo, comparing across and within samples and relating expression with growth and therapeutic response to doxorubicin, docetaxol and radiotherapy. RESULTS: CD24, CD44, ALDH and SOX2 expression, the ability to form mammospheres and side-population cells are variably present in human cancers and cell lines. Each marker identifies a unique rather than common population of cancer cells. In vivo, cells expressing these markers are not specifically localized to the presumptive stem cell niche at the tumour/stroma interface. Repeated therapy does not consistently enrich cells expressing these markers, although ER-negative cells accumulate. CONCLUSIONS: Commonly employed methods identify different cancer cell sub-populations with no consistent therapeutic implications, rather than a single population of cells. The relationships of breast cancer stem cells to clinical parameters will require identification of specific markers or panels for the individual cancer
Nonlinear Effects In Black Hole Ringdown From Scattering Experiments I: spin and initial data dependence of quadratic mode coupling
We investigate quadratic quasinormal mode coupling in black hole spacetime
through numerical simulations of single perturbed black holes using both
numerical relativity and second-order black hole perturbation theory. Focusing
on the dominant quadrupolar modes, we find good agreement (within
) between these approaches, with discrepancies attributed to
truncation error and uncertainties from mode fitting. Our results align with
earlier studies extracting the coupling coefficients from select binary black
hole merger simulations, showing consistency for the same remnant spins.
Notably, the coupling coefficient is insensitive to a diverse range of initial
data, including configurations that led to a significant (up to ) increase
in the remnant black hole mass. These findings present opportunities for
testing the nonlinear dynamics of general relativity with ground-based
gravitational wave observatories. Lastly, we provide evidence of a bifurcation
in coupling coefficients between counter-rotating and co-rotating quasinormal
modes as black hole spin increases
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