89 research outputs found
Acoustic scattering-extinction cross section and the acoustic force of electrostatic type
The analysis of the secondary Bjerknes force between two bubbles suggests
that this force is analogous to the electrostatic forces. The same analogy is
suggested by the existence of a scattering cross section of an acoustic wave on
a bubble. Our paper brings new arguments in support of this analogy. The study
which we perform is dedicated to the acoustic force and to the scattering cross
section at resonance in order to highlight their angular frequency independence
of the inductor wave. Also, our study reveals that the angular frequency and
the amplitude of the induction pressure wave are not related. Highlighting this
analogy will allow us a better understanding of the electrostatic interaction
if the electron is modeled as an oscillating bubble in the vacuum.Comment: 17 page
Solar system tests of brane world models
The classical tests of general relativity (perihelion precession, deflection
of light, and the radar echo delay) are considered for the Dadhich, Maartens,
Papadopoulos and Rezania (DMPR) solution of the spherically symmetric static
vacuum field equations in brane world models. For this solution the metric in
the vacuum exterior to a brane world star is similar to the Reissner-Nordstrom
form of classical general relativity, with the role of the charge played by the
tidal effects arising from projections of the fifth dimension. The existing
observational solar system data on the perihelion shift of Mercury, on the
light bending around the Sun (obtained using long-baseline radio
interferometry), and ranging to Mars using the Viking lander, constrain the
numerical values of the bulk tidal parameter and of the brane tension.Comment: 14 pages, to appear in Classical and Quantum Gravity. V2, minor
corrections and references adde
The Unseen Face of E-Business Project Development
The purpose of this paper is intent on identify and analyze the unseen factors of successful or failure of e-business project development. The IT managers must take into account both all costs involved in e-business development and all phases (analysis, design, testing, implementation, maintenance and operation) according to principle of project management for software/systems life cycle development. There are many solutions to exceed these factors of failure among could be counted outsourcing, a good project management, involvement of senior management, a real cost estimation etc.Zadanie pt. „Digitalizacja i udostępnienie w Cyfrowym Repozytorium Uniwersytetu Łódzkiego kolekcji czasopism naukowych wydawanych przez Uniwersytet Łódzki” nr 885/P-DUN/2014 zostało dofinansowane ze środków MNiSW w ramach działalności upowszechniającej nauk
North Atlantic 2001 - Part 4 Cruise No. 50, Leg 4 16 July – 12 August 2001, Reykjavik – Hamburg
Country-level determinants of the severity of the first global wave of the COVID-19 pandemic : an ecological study
Acknowledgements We would like to thank Dr Kathryn Martin, who provided valuable advice in study design. Funding This work was supported by the Aberdeen Clinical Academic Training Scheme.Peer reviewedPublisher PD
DELIVERABLE: D5.1 MONITORING AND VALIDATION STRATEGIES
This deliverable report will present the strategies developed for monitoring the case study demonstrations to be undertaken as part of WP4. The strategies presented will include both methods for quantitative validation, including data capture and relevant KPIs, and those catering for more qualitative evaluation using aspects such as contextual interviews, self-observations, and/or questionnaires.This work is part of the DR BOB Project. The DR-BOB Collaborative Project (Grant Agreement No. 696114) is co-funded by the European Commission, Information Society and Media Directorate-General, under the Horizon 2020 Programme (H2020)
f(R,L_m) gravity
We generalize the type gravity models by assuming that the
gravitational Lagrangian is given by an arbitrary function of the Ricci scalar
and of the matter Lagrangian . We obtain the gravitational field
equations in the metric formalism, as well as the equations of motion for test
particles, which follow from the covariant divergence of the energy-momentum
tensor. The equations of motion for test particles can also be derived from a
variational principle in the particular case in which the Lagrangian density of
the matter is an arbitrary function of the energy-density of the matter only.
Generally, the motion is non-geodesic, and takes place in the presence of an
extra force orthogonal to the four-velocity. The Newtonian limit of the
equation of motion is also considered, and a procedure for obtaining the
energy-momentum tensor of the matter is presented. The gravitational field
equations and the equations of motion for a particular model in which the
action of the gravitational field has an exponential dependence on the standard
general relativistic Hilbert--Einstein Lagrange density are also derived.Comment: 6 pages, no figures; minor modifications, references added; accepted
for publication in EPJ
Thin accretion disk signatures in dynamical Chern-Simons modified gravity
A promising extension of general relativity is Chern-Simons (CS) modified
gravity, in which the Einstein-Hilbert action is modified by adding a
parity-violating CS term, which couples to gravity via a scalar field. In this
work, we consider the interesting, yet relatively unexplored, dynamical
formulation of CS modified gravity, where the CS coupling field is treated as a
dynamical field, endowed with its own stress-energy tensor and evolution
equation. We consider the possibility of observationally testing dynamical CS
modified gravity by using the accretion disk properties around slowly-rotating
black holes. The energy flux, temperature distribution, the emission spectrum
as well as the energy conversion efficiency are obtained, and compared to the
standard general relativistic Kerr solution. It is shown that the Kerr black
hole provide a more efficient engine for the transformation of the energy of
the accreting mass into radiation than their slowly-rotating counterparts in CS
modified gravity. Specific signatures appear in the electromagnetic spectrum,
thus leading to the possibility of directly testing CS modified gravity by
using astrophysical observations of the emission spectra from accretion disks.Comment: 12 pages, 24 figures. V2: 10 pages, 13 figures, significant changes,
matches published versio
Thin accretion disk signatures of slowly rotating black holes in Ho\v{r}ava gravity
In the present work, we consider the possibility of observationally testing
Ho\v{r}ava gravity by using the accretion disk properties around slowly
rotating black holes of the Kehagias-Sfetsos solution in asymptotically flat
spacetimes. The energy flux, temperature distribution, the emission spectrum as
well as the energy conversion efficiency are obtained, and compared to the
standard slowly rotating general relativistic Kerr solution. Comparing the mass
accretion in a slowly rotating Kehagias-Sfetsos geometry in Ho\v{r}ava gravity
with the one of a slowly rotating Kerr black hole, we verify that the intensity
of the flux emerging from the disk surface is greater for the slowly rotating
Kehagias-Sfetsos solution than for rotating black holes with the same
geometrical mass and accretion rate. We also present the conversion efficiency
of the accreting mass into radiation, and show that the rotating
Kehagias-Sfetsos solution provides a much more efficient engine for the
transformation of the accreting mass into radiation than the Kerr black holes.
Thus, distinct signatures appear in the electromagnetic spectrum, leading to
the possibility of directly testing Ho\v{r}ava gravity models by using
astrophysical observations of the emission spectra from accretion disks.Comment: 12 pages, 15 figures. V2: 13 pages, clarifications and discussion
added; version accepted for publication in Classical and Quantum Gravit
Symmetry between partially polarised light and partial polarisers in the vectorial Pauli algebraic formalism
- …