278 research outputs found
Mode coupling of Schwarzschild perturbations: Ringdown frequencies
Within linearized perturbation theory, black holes decay to their final
stationary state through the well-known spectrum of quasinormal modes. Here we
numerically study whether nonlinearities change this picture. For that purpose
we study the ringdown frequencies of gauge-invariant second-order gravitational
perturbations induced by self-coupling of linearized perturbations of
Schwarzschild black holes. We do so through high-accuracy simulations in the
time domain of first and second-order Regge-Wheeler-Zerilli type equations, for
a variety of initial data sets. We consider first-order even-parity
perturbations and odd-parity ones, and all
the multipoles that they generate through self-coupling. For all of them and
all the initial data sets considered we find that ---in contrast to previous
predictions in the literature--- the numerical decay frequencies of
second-order perturbations are the same ones of linearized theory, and we
explain the observed behavior. This would indicate, in particular, that when
modeling or searching for ringdown gravitational waves, appropriately including
the standard quasinormal modes already takes into account nonlinear effects
Hamiltonian theory for the axial perturbations of a dynamical spherical background
We develop the Hamiltonian theory of axial perturbations around a general
time-dependent spherical background spacetime. Using the fact that the
linearized constraints are gauge generators, we isolate the physical and
unconstrained axial gravitational wave in a Hamiltonian pair of variables.
Then, switching to a more geometrical description of the system, we construct
the only scalar combination of them. We obtain the well-known Gerlach and
Sengupta scalar for axial perturbations, with no known equivalent for polar
perturbations. The strategy suggested and tested here will be applied to the
polar case in a separate article.Comment: 12 pages, accepted by Classical and Quantum Gravit
Solid lubricant behavior of MoS2 and WSe2-based nanocomposite coatings
Tribological coatings made of MoS2 and WSe2 phases and their corresponding combinations with tungsten carbide (WC) were prepared by non-reactive magnetron sputtering of individual targets of similar composition. A comparative tribological analysis of these multiphase coatings was done in both ambient air (30–40% relative humidity, RH) and dry nitrogen (RH<7%) environments using the same tribometer and testing conditions. A nanostructural study using advanced transmission electron microscopy of the initial coatings and examination of the counterfaces after the friction test using different analytical tools helped to elucidate what governs the tribological behavior for each type of environment. This allowed conclusions to be made about the influence of the coating microstructure and composition on the tribological response. The best performance obtained with a WSex film (specific wear rate of 2 × 10−8 mm3 N–1m–1 and a friction coefficient of 0.03–0.05) was compared with that of the well-established MoS2 lubricant material.The Spanish Ministry of Economy, Industry and Competitiveness [projects n° MAT2010-21597-C02-01, MAT2011-29074-C02-01; MAT2015-65539-P; MAT2015-69035-REDC], Junta de Andalucía [P10-TEP-67182] and Spanish National Research Council (CSIC) [201560E013] are acknowledged for their financial support
Numerical loop quantum cosmology: an overview
A brief review of various numerical techniques used in loop quantum cosmology
and results is presented. These include the way extensive numerical simulations
shed insights on the resolution of classical singularities, resulting in the
key prediction of the bounce at the Planck scale in different models, and the
numerical methods used to analyze the properties of the quantum difference
operator and the von Neumann stability issues. Using the quantization of a
massless scalar field in an isotropic spacetime as a template, an attempt is
made to highlight the complementarity of different methods to gain
understanding of the new physics emerging from the quantum theory. Open
directions which need to be explored with more refined numerical methods are
discussed.Comment: 33 Pages, 4 figures. Invited contribution to appear in Classical and
Quantum Gravity special issue on Non-Astrophysical Numerical Relativit
xPert: Computer algebra for metric perturbation theory
We present the tensor computer algebra package xPert for fast construction
and manipulation of the equations of metric perturbation theory, around
arbitrary backgrounds. It is based on the combination of explicit combinatorial
formulas for the n-th order perturbation of curvature tensors and their gauge
changes, and the use of highly efficient techniques of index canonicalization,
provided by the underlying tensor system xAct, for Mathematica. We give
examples of use and show the efficiency of the system with timings plots: it is
possible to handle orders n=4 or n=5 within seconds, or reach n=10 with timings
below 1 hour.Comment: 11 pages, 4 figures. Package can be downloaded from
http://metric.iem.csic.es/Martin-Garcia/xAct/xPert
The convergence method to calculate particles fluxes in x rays spectrometry techniques. Application in nuclear compounds
A method to calculate particle fluxes applicable in most of the spectroscopy
techniques is described. Flux intensities of backscattered or absorbed
electrons and emitted photons are calculated using a method of convergence to
solve the Invariant Embedding equations that are used to describe the particle
trajectories inside a solid sample. Our results are found to be helpful to
carry out a procedure for quantitative characterization using instrument such
as Electron Probe Microanalysis or other probes. Examples of application to
calculate the composition of ternary alloys are given and are compared with the
same calculations using another procedure.Comment: 15 page
Loop Quantum Cosmology: A Status Report
The goal of this article is to provide an overview of the current state of
the art in loop quantum cosmology for three sets of audiences: young
researchers interested in entering this area; the quantum gravity community in
general; and, cosmologists who wish to apply loop quantum cosmology to probe
modifications in the standard paradigm of the early universe. An effort has
been made to streamline the material so that, as described at the end of
section I, each of these communities can read only the sections they are most
interested in, without a loss of continuity.Comment: 138 pages, 15 figures. Invited Topical Review, To appear in Classical
and Quantum Gravity. Typos corrected, clarifications and references adde
SARS-CoV-2 positivity in offspring and timing of mother-to-child transmission: living systematic review and meta-analysis
OBJECTIVES: To assess the rates of SARS-CoV-2 positivity in babies born to mothers with SARS-CoV-2 infection, the timing of mother-to-child transmission and perinatal outcomes, and factors associated with SARS-CoV-2 status in offspring. DESIGN: Living systematic review and meta-analysis. DATA SOURCES: Major databases between 1 December 2019 and 3 August 2021. STUDY SELECTION: Cohort studies of pregnant and recently pregnant women (including after abortion or miscarriage) who sought hospital care for any reason and had a diagnosis of SARS-CoV-2 infection, and also provided data on offspring SARS-CoV-2 status and risk factors for positivity. Case series and case reports were also included to assess the timing and likelihood of mother-to-child transmission in SARS-CoV-2 positive babies. DATA EXTRACTION: Two reviewers independently extracted data and assessed study quality. A random effects model was used to synthesise data for rates, with associations reported using odds ratios and 95% confidence intervals. Narrative syntheses were performed when meta-analysis was inappropriate. The World Health Organization classification was used to categorise the timing of mother-to-child transmission (in utero, intrapartum, early postnatal). RESULTS: 472 studies (206 cohort studies, 266 case series and case reports; 28 952 mothers, 18 237 babies) were included. Overall, 1.8% (95% confidence interval 1.2% to 2.5%; 140 studies) of the 14 271 babies born to mothers with SARS-CoV-2 infection tested positive for the virus with reverse transcriptase polymerase chain reaction (RT-PCR). Of the 592 SARS-CoV-2 positive babies with data on the timing of exposure and type and timing of tests, 14 had confirmed mother-to-child transmission: seven in utero (448 assessed), two intrapartum (18 assessed), and five during the early postnatal period (70 assessed). Of the 800 SARS-CoV-2 positive babies with outcome data, 20 were stillbirths, 23 were neonatal deaths, and eight were early pregnancy losses; 749 babies were alive at the end of follow-up. Severe maternal covid-19 (odds ratio 2.4, 95% confidence interval 1.3 to 4.4), maternal death (14.1, 4.1 to 48.0), maternal admission to an intensive care unit (3.5, 1.7 to 6.9), and maternal postnatal infection (5.0, 1.2 to 20.1) were associated with SARS-CoV-2 positivity in offspring. Positivity rates using RT-PCR varied between regions, ranging from 0.1% (95% confidence interval 0.0% to 0.3%) in studies from North America to 5.7% (3.2% to 8.7%) in studies from Latin America and the Caribbean. CONCLUSION: SARS-CoV-2 positivity rates were found to be low in babies born to mothers with SARS-CoV-2 infection. Evidence suggests confirmed vertical transmission of SARS-CoV-2, although this is likely to be rare. Severity of maternal covid-19 appears to be associated with SARS-CoV-2 positivity in offspring. SYSTEMATIC REVIEW REGISTRATION: PROSPERO CRD42020178076. READERS' NOTE: This article is a living systematic review that will be updated to reflect emerging evidence. Updates may occur for up to two years from the date of original publication
Quantization of Midisuperspace Models
We give a comprehensive review of the quantization of midisuperspace models.
Though the main focus of the paper is on quantum aspects, we also provide an
introduction to several classical points related to the definition of these
models. We cover some important issues, in particular, the use of the principle
of symmetric criticality as a very useful tool to obtain the required
Hamiltonian formulations. Two main types of reductions are discussed: those
involving metrics with two Killing vector fields and spherically symmetric
models. We also review the more general models obtained by coupling matter
fields to these systems. Throughout the paper we give separate discussions for
standard quantizations using geometrodynamical variables and those relying on
loop quantum gravity inspired methods.Comment: To appear in Living Review in Relativit
Inflation from non-minimally coupled scalar field in loop quantum cosmology
The FRW model with non-minimally coupled massive scalar field has been
investigated in LQC framework. Considered form of the potential and coupling
allows applications to Higgs driven inflation. Out of two frames used in the
literature to describe such systems: Jordan and Einstein frame, the latter one
is applied. Specifically, we explore the idea of the Einstein frame being the
natural 'environment' for quantization and the Jordan picture having an
emergent nature. The resulting dynamics qualitatively modifies the standard
bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by
critical matter energy density, (ii) the Planck scale physics features the
'mexican hat' trajectory with two consecutive bounces and rapid expansion and
recollapse between them. Furthermore, for physically viable coupling strength
and initial data the subsequent inflation exceeds 60 e-foldings.Comment: Clarity improved. Replaced with revised version accepted in JCA
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