2,759 research outputs found
Cosmological model with non-minimally coupled fermionic field
A model for the Universe is proposed whose constituents are: (a) a dark
energy field modeled by a fermionic field non-minimally coupled with the
gravitational field, (b) a matter field which consists of pressureless baryonic
and dark matter fields and (c) a field which represents the radiation and the
neutrinos. The coupled system of Dirac's equations and Einstein field equations
is solved numerically by considering a spatially flat homogeneous and isotropic
Universe. It is shown that the proposed model can reproduce the expected
red-shift behaviors of the deceleration parameter, of the density parameters of
each constituent and of the luminosity distance. Furthermore, for small values
of the red-shift the constant which couples the fermionic and gravitational
fields has a remarkable influence on the density and deceleration parameters.Comment: Accepted for publication in Europhysics Letter
Non-Markovian entanglement dynamics of quantum continuous variable systems in thermal environments
We study two continuous variable systems (or two harmonic oscillators) and
investigate their entanglement evolution under the influence of non-Markovian
thermal environments. The continuous variable systems could be two modes of
electromagnetic fields or two nanomechanical oscillators in the quantum domain.
We use quantum open system method to derive the non-Markovian master equations
of the reduced density matrix for two different but related models of the
continuous variable systems. The two models both consist of two interacting
harmonic oscillators. In model A, each of the two oscillators is coupled to its
own independent thermal reservoir, while in model B the two oscillators are
coupled to a common reservoir. To quantify the degrees of entanglement for the
bipartite continuous variable systems in Gaussian states, logarithmic
negativity is used. We find that the dynamics of the quantum entanglement is
sensitive to the initial states, the oscillator-oscillator interaction, the
oscillator-environment interaction and the coupling to a common bath or to
different, independent baths.Comment: 10 two-column pages, 8 figures, to appear in Phys. Rev.
Non-Markovian dynamics of a nanomechanical resonator measured by a quantum point contact
We study the dynamics of a nanomechanical resonator (NMR) subject to a
measurement by a low transparency quantum point contact (QPC) or tunnel
junction in the non-Markovian domain. We derive the non-Markovian
number-resolved (conditional) and unconditional master equations valid to
second order in the tunneling Hamiltonian without making the rotating-wave
approximation and the Markovian approximation, generally made for systems in
quantum optics. Our non-Markovian master equation reduces, in appropriate
limits, to various Markovian versions of master equations in the literature. We
find considerable difference in dynamics between the non-Markovian cases and
its Markovian counterparts. We also calculate the time-dependent transport
current through the QPC which contains information about the measured NMR
system. We find an extra transient current term proportional to the expectation
value of the symmetrized product of the position and momentum operators of the
NMR. This extra current term, with a coefficient coming from the combination of
the imaginary parts of the QPC reservoir correlation functions, has a
substantial contribution to the total transient current in the non-Markovian
case, but was generally ignored in the studies of the same problem in the
literature. Considering the contribution of this extra term, we show that a
significantly qualitative and quantitative difference in the total transient
current between the non-Markovian and the Markovian wide-band-limit cases can
be observed. Thus, it may serve as a witness or signature of the non-Markovian
features in the coupled NMR-QPC system.Comment: Accepted for publication in Physical Review B (20 pages, 13 figures
Transport coefficients of multi-particle collision algorithms with velocity-dependent collision rules
Detailed calculations of the transport coefficients of a recently introduced
particle-based model for fluid dynamics with a non-ideal equation of state are
presented. Excluded volume interactions are modeled by means of biased
stochastic multiparticle collisions which depend on the local velocities and
densities. Momentum and energy are exactly conserved locally. A general scheme
to derive transport coefficients for such biased, velocity dependent collision
rules is developed. Analytic expressions for the self-diffusion coefficient and
the shear viscosity are obtained, and very good agreement is found with
numerical results at small and large mean free paths. The viscosity turns out
to be proportional to the square root of temperature, as in a real gas. In
addition, the theoretical framework is applied to a two-component version of
the model, and expressions for the viscosity and the difference in diffusion of
the two species are given.Comment: 31 pages, 8 figures, accepted by J. Phys. Cond. Matte
Lagrangian dust model simulations for a case of moist convective dust emission and transport in the western Sahara region during Fennec/LADUNEX
Do the ends justify the means? Problematizing social acceptance and instrumentally-driven community engagement in proposed energy projects
Proposed energy projects across rural working landscapes play an important role in energy transitions. While community engagement has been increasingly a part of these projects, instrumental motivations for engagement and the emphasis placed on achieving social acceptance has remained uncritically examined. Here, we aim to highlight relationships between actor rationale, the structuring of engagement processes, and how communities perceive the driving forces behind engagement practices. To do so, we draw on lived experiences of communities facing proposed shale gas and wind energy projects across rural working landscapes in the UK and Canada, respectively. We find that engagement is often perceived by community members as insincere, insufficient, ineffective and instrumentally-driven. We suggest that a more community-centered approach to engagement is necessary and will require a move beyond existing engagement and acceptance practice and frameworks. This can include creating more inclusive decision-making processes where powers are balanced and designing community engagement to incorporate multiple rationales beyond achieving social acceptance of energy projects.info:eu-repo/semantics/publishedVersio
Super star clusters and Supernovae in interacting LIRGs unmasked by NIR adaptive optics
We report on an on-going near-IR adaptive optics survey targeting interacting
luminous IR galaxies. High-spatial resolution NIR data are crucial to enable
interpretation of kinematic, dynamical and star formation (SF) properties of
these very dusty objects. Whole progenitor nuclei in the interactions can be
missed if only optical HST imaging is used. Here we specifically present the
latest results regarding core-collapse supernovae found within the highly
extincted nuclear regions of these galaxies. Direct detection and study of such
highly obscured CCSNe is crucial for revising the optically-derived SN rates
used for providing an independent measurement of the SF history of the
Universe. We also present thus-far the first NIR luminosity functions of super
star cluster (SSC) candidates. The LFs can then be used to constrain the
formation and evolution of SSCs via constraints based on initial mass functions
and cluster disruption models.Comment: 6 pages. To appear in proceedings of 'Galaxies and their Masks'
(Namibia, April 2010), published by Springer, New York, eds. D.L. Block, K.C.
Freeman, I. Puerar
HIPASS Detection of an Intergalactic Gas Cloud in the NGC 2442 Group
We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of a gas
cloud associated with the asymmetric spiral galaxy NGC 2442. This object,
designated HIPASS J0731-69, contains ~10^9 M_sun of HI, or nearly one-third as
much atomic gas as NGC 2442 itself. No optical counterpart to any part of
HIPASS J0731-69 has yet been identified, consistent with the gas being diffuse,
and with its stream-like kinematics. If the gas in HIPASS J0731-69 was once
part of NGC 2442, then it was most likely a fairly recent tidal encounter with
a moderately massive companion which tore it loose, although the possibility of
ram-pressure stripping cannot be ruled out. This discovery highlights the
potential of the HIPASS data for yielding new clues to the nature of some of
the best-known galaxies in the local universe.Comment: 8 pages, 4 figures, uses "emulateapj5.sty". Accepted for publication
in ApJ, Vol. 555, 1 July 2001. Figs 1 and 2 included as JPE
Accounting students' expectations and transition experiences of supervised work experience
Political and economic discourses position employability as a responsibility of higher education, which utilise mechanisms such as supervised work experience (SWE) to embed employability into the undergraduate curriculum. However, sparse investigation of students' contextualised experiences of SWE results in little being known about the mechanisms through which students derive employability benefits from SWE. The aim of this study is to examine the impact of students' expectation and conception of workplace learning on their transition into SWE. Analysis of accounting students' experiences reveal two broad conceptions of workplace learning, the differing impacts of which on transition experience are explored using existing learning transfer perspectives. Students displaying the more common 'technical' conception construct SWE as an opportunity to develop technical, knowledge-based expertise and abilities that prioritize product-based or cognitive learning transfer. Students with an 'experiential' conception were found to construct SWE primarily as an experience through which the development of personal skills and abilities beyond technical expertise are prioritized using process-based or socio-cultural learning transfer. Further data analysis suggests that these two learning transfer approaches have differing impacts on students' employability development which may indicate a need for universities to consider how to develop appropriate student expectations of and approaches to SWE and meaningful support for students' SWE transition
Application of Renormalization to Potential Scattering
A recently proposed renormalization scheme can be used to deal with
nonrelativistic potential scattering exhibiting ultraviolet divergence in
momentum space. A numerical application of this scheme is made in the case of
potential scattering with divergence for small r, common in molecular
and nuclear physics, by the use of cut-offs in momentum and configuration
spaces. The cut-off is finally removed in terms of a physical observable and
model-independent result is obtained at low energies. The expected variation of
the off-shell behavior of the t matrix arising from the renormalization scheme
is also discussed.Comment: 15 pages plus 5 figure
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