A Note on Gravitational Baryogenesis

The coupling between Ricci scalar curvature and the baryon number current dynamically breaks CPT in an expanding universe and leads to baryon asymmetry. We study the effect of time dependence of equation of state parameter of the FRW universe on this asymmetry.Comment: 10 pages, accepted for publication in Physical Review

Numerical Simulations of the Lagrangian Averaged Navier-Stokes (Lans-α) Equations for Forced Homogeneous Isotropic Turbulence

The modeling capabilities of the Lagrangian Averaged Navier-Stokes-α equations (LANS-α) is investigated in statistically stationary three-dimensional homogeneous and isotropic turbulence. The predictive abilities of the LANS-α equations are analyzed by comparison with DNS data. Two different forcing techniques were implemented to model the energetics of the energy containing scales. The resolved flow is examined by comparison of the energy spectra of the LANS-α and the DNS computations; furthermore, the correlation between the vorticity and the eigenvectors of the rate of the resolved strain tensor is studied. We find that the LANS-α equations captures the gross features of the flow while the wave activity below a given scale α is filtered by the non- linear dispersion

Dynamic modeling of α in the isotropic lagrangian averaged navier-stokes-α equations

A dynamic procedure for the Lagrangian Averaged Navier- Stokes-α (LANS-α) equations is developed where the variation in the parameter α in the direction of anisotropy is determined in a self-consistent way from data contained in the simulation itself. In order to derive this model, the incompressible Navier-Stokes equations are Helmholtz-filtered at the grid and a test filter levels. A Germano type identity is derived by comparing the filtered subgrid scale stress terms with those given in the LANS-α equations. Assuming constant α in homogenous directions of the flow and averaging in these directions, results in a nonlinear equation for the parameter α, which determines the variation of α in the non-homogeneous directions or in time. Consequently, the parameter α is calculated during the simulation instead of a pre-defined value. As an initial test, the dynamic LANS-α model is used to compute isotropic homogenous forced and decaying turbulence, where α is constant over the computational domain, but is allowed to vary in time. The resulting simulations are compared with direct numerical simulations and with the LANS-α simulations using fixed value of α. As expected, α is found to change rapidly during the first eddy turn-over time during the simulations. It is also observed that by using the dynamic LANS-α procedure a more accurate simulation of the isotropic homogeneous turbulence is achieved. The energy spectra and the total kinetic energy decay are captured more accurately as compared with the LANS-α simulations using a fixed α. The current results suggest some promising applications of this dynamic LANS-α model, such as to a spatially varying turbulent flow, which we hope to undertake in future research

Spin-orbit-torque driven magnetoimpedance in Pt-layer/magnetic-ribbon heterostructures

When a flow of electron passes through a paramagnetic layer with strong spin-orbit-coupling such as platinum (Pt), a net spin current is produced via spin Hall effect (SHE). This spin current can exert a torque on the magnetization of an adjacent ferromagnetic layer which can be probed via magnetization dynamic response, e.g. spin-torque ferromagnetic resonance (ST-FMR). Nevertheless, that effect in lower frequency magnetization dynamic regime (MHz) where skin effect occurs in high permeability ferromagnetic conductors namely the magneto-impedance (MI) effect can be fundamentally important which has not been studied so far. Here, by utilizing the MI effect in magnetic-ribbon/Pt heterostructure with high magnetic permeability that allows the ac current effectively confined at the skin depth of ~100 nm thickness, the effect of spin-orbit-torque (SOT) induced by the SHE probed via MI measurement is investigated. We observed a systematic MI frequency shift that increases by increasing the applied current amplitude and thickness of the Pt layer (varying from 0 nm to 20 nm). In addition, the role of Pt layer in ribbon/Pt heterostructure is evaluated with ferromagnetic resonance (FMR) effect representing standard Gilbert damping increase as the result of presence of the SHE. Our results unveil the role of SOT in dynamic control of the transverse magnetic permeability probed with impedance spectroscopy as useful and valuable technique for detection of future SHE devices.Comment: 15 pages,6 figure

Attractor solutions for general hessence dark energy

As a candidate for the dark energy, the hessence model has been recently introduced. We discuss the critical points of this model in almost general case, that is for arbitrary hessence potential and almost arbitrary hessence-background matter interaction. It is shown that in all models, there always exist some stable late-time attractors. It is shown that our general results coincide with those solutions obtained earlier for special cases, but some of them are new. These new solutions have two unique characteristics. First the hessence field has finite value in these solutions and second, their stabilities depend on the second derivative of the hessence potential.Comment: 11 pages. Add some explanations about the autonomousity of the equations, and also a conclusion section was added. To appear in Phys. Rev. D (2006

Multicomponent solution in modified theory of gravity in the early universe

We study the modified theory of gravity in Friedmann Robertson Walker universe composed of several perfect fluids. We consider the power law inflation and determine the equation of state parameters in terms of the parameters of modified gravity's Lagrangian in the early universe. We also discuss briefly the gravitational baryogenesis in this model.Comment: 9 pages, accepted for publication in Physical Review

Electrostatic self-energy and Bekenstein entropy bound in the massive Schwinger model

We obtain the electrostatic energy of two opposite charges near the horizon of stationary black-holes in the massive Schwinger model. Besides the confining aspects of the model, we discuss the Bekenstein entropy upper bound of a charged object using the generalized second law. We show that despite the massless case, in the massive Schwinger model the entropy of the black hole and consequently the Bekenstein bound are altered by the vacuum polarization.Comment: 14 pages, accepted for publication in "Gen. Rel. Grav. (2005)