Search results for "classical"
showing 10 items of 2294 documents
Revisiting a vector-tensor theory of gravitation
2011
A certain vector-tensor theory of gravitation has been recently studied. In this theory, the zero-order energy density of the vector field could play the role of dark energy. In such a case, the question is: could the theory explain current cosmological observations as well as the so-called concordance model? Previous papers on the subject only consider a reduced number of current observations. We consider a wider set of observations including supernovae of type Ia, cosmic microwave background anisotropies, and the power spectrum of the energy density fluctuations. Results imply that, for negligible scalar perturbations of the vector field, the theory does not work.
Time-resolved photoabsorption in finite systems: A first-principles NEGF approach
2016
We describe a first-principles NonEquilibrium Green’s Function (NEGF) approach to time-resolved photoabsortion spectroscopy in atomic and nanoscale systems. The method is used to highlight a recently discovered dynamical correlation effect in the spectrum of a Krypton gas subject to a strong ionizing pump pulse. We propose a minimal model that captures the effect, and study the performance of time-local approximations versus time-nonlocal ones. In particular we implement the time-local Hartree-Fock and Markovian second Born (2B) approximation as well as the exact adiabatic approximation within the Time-Dependent Density Functional Theory framework. For the time-nonlocal approximation we ins…
Creatable universes: A new approach
2010
We are interested in the non asymptotically flat space-times for which all the momenta (energy, 3-momentum and angular 4-momenta) are conserved in time. We call universes such space-times. Starting from the Weinberg definition of the momenta associated to a spacelike 3-surface, we give a coordinate prescription to properly define the energy of a universe. The prescription includes the vanishing of linear and angular 3-momenta. This result allows us to consider the case of universes with vanishing 4-momenta (creatable universes) in a consistent way.
Transition From Ideal To Viscous Mach Cones In A Partonic Transport Model
2013
Using a partonic transport model we investigate the evolution of conical structures in ultrarelativistic matter. Using two different source terms and varying the transport properties of the matter we study the formation of Mach Cones. Furthermore, in an additional study we extract the two-particle correlations from the numerical calculations and compare them to an analytical approximation. The influence of the viscosity to the shape of Mach Cones and the corresponding two-particle correlations is studied by adjusting the cross section of the medium. peerReviewed
Dynamics of oscillating magnetized relativistic tori around a Schwarzschild black hole
2007
We present a comprehensive numerical study of the dynamics of magnetized relativistic axisymmetric tori orbiting in the background spacetime of a Schwarzschild black hole. The tori are modeled as having a purely toroidal magnetic field and a constant distribution of the specific angular momentum. Following previous investigations of tori in a purely hydrodynamical context, the dynamics of these objects has been studied upon the introduction of a perturbation which, for the values of the magnetic field considered here, triggers quasi-periodic oscillations (QPOs) lasting tens of orbital periods. As in the hydrodynamical case, the spectral distribution of the eigenfrequencies shows the presenc…
Emergent pattern formation of active magnetic suspensions in an external field
2020
We study collective self-organization of weakly magnetic active suspensions in a uniform external field by analyzing a mesoscopic continuum model that we have recently developed. Our model is based on a Smoluchowski equation for a particle probability density function in an alignment field coupled to a mean-field description of the flow arising from the activity and the alignment torque. Performing linear stability analysis of the Smoluchowski equation and the resulting orientational moment equations combined with non-linear 3D simulations, we provide a comprehensive picture of instability patterns as a function of strengths of activity and magnetic field. For sufficiently high activity and…
Generation of Schrödinger Cats in Trapped Ions
2004
A quantum system in interaction with a repeatedly measured one is subjected to a non-unitary time evolution provoking the decay of some states in favor of the remaining ones. Under appropriate hypotheses the system may be addressed exactly toward a quantum state or pushed into a pre-selected finite-dimensional subspace. On the basis of such a general strategy, we propose to exploit suitable vibronic couplings in order to ‘extract’ trapped ion center of mass states of motion characterized by well defined absolute value of an angular momentum projection. In particular, since it implies the simultaneous presence of opposite angular momentum projections, we show the realizability of Schrödinger…
Mode coupling theory for molecular liquids: What can we learn from a system of hard ellipsoids?
1999
Molecular fluids show rich and complicated dynamics close to the glass transition. Some of these observations are related to the fact that translational and orientational degrees of freedom couple in nontrivial ways. A model system which can serve as a paradigm to understand these couplings is a system of hard ellipsoids of revolution. To test this we compare at the ideal glass transition the static molecular correlators of a linear A-B Lennard-Jones molecule obtained from a molecular dynamics simulation with a selected fluid of hard ellipsoids for which the static correlators have been obtained using Percus-Yevick theory. We also demonstrate that the critical non-ergodicity parameters obta…
Ideal bulk pressure of active Brownian particles
2016
The extent to which active matter might be described by effective equilibrium concepts like temperature and pressure is currently being discussed intensely. Here, we study the simplest model, an ideal gas of noninteracting active Brownian particles. While the mechanical pressure exerted onto confining walls has been linked to correlations between particles' positions and their orientations, we show that these correlations are entirely controlled by boundary effects. We also consider a definition of local pressure, which describes interparticle forces in terms of momentum exchange between different regions of the system. We present three pieces of analytical evidence which indicate that such…
The relaxation-time limit in the quantum hydrodynamic equations for semiconductors
2006
Abstract The relaxation-time limit from the quantum hydrodynamic model to the quantum drift–diffusion equations in R 3 is shown for solutions which are small perturbations of the steady state. The quantum hydrodynamic equations consist of the isentropic Euler equations for the particle density and current density including the quantum Bohm potential and a momentum relaxation term. The momentum equation is highly nonlinear and contains a dispersive term with third-order derivatives. The equations are self-consistently coupled to the Poisson equation for the electrostatic potential. The relaxation-time limit is performed both in the stationary and the transient model. The main assumptions are…