331 research outputs found
Revisiting Rotational Perturbations and the Microwave Background
We consider general-relativistic rotational perturbations in homogeneous and
isotropic Friedman - Robertson - Walker (FRW) cosmologies. Taking linear
perturbations of FRW models, the general solution of the field equations
contains tensorial, vectorial and scalar functions. The vectorial terms are in
connection with rotations in the given model and due to the Sachs - Wolfe
effect they produce contributions to the temperature fluctuations of the cosmic
microwave background radiation (CMBR). In present paper we obtain the analytic
time dependence of these contributions in a spatially flat, FRW model with
pressureless ideal fluid, in the presence and the absence of a cosmological
constant. We find that the solution can be separated into an integrable and a
non-integrable part as is the situation in the case of scalar perturbations.
Analyzing the solutions and using the results of present observations we
estimate the order of magnitude of the angular velocity corresponding to the
rotation tensor at the time of decoupling and today.Comment: accepted for publication in Int. J. Mod. Phys.
Vector modes generated by primordial density fluctuations
While vector modes are usually ignored in cosmology since they are not
produced during inflation they are inevitably produced from the interaction of
density fluctuations of differing wavelengths. This effect may be calculated
via a second-order perturbative expansion. We investigate this effect during
the radiation era. We discuss the generation mechanism by investigating two
scalar modes interacting, and we calculate the power of vector modes generated
by a power-law spectrum of density perturbations on all scales.Comment: 10 pages, 2 figures, minor changes in main text and new appendix
added to match the accepted version for Physical Review D publicatio
Generalised relativistic Ohm's laws, extended gauge transformations and magnetic linking
Generalisations of the relativistic ideal Ohm's law are presented that
include specific dynamical features of the current carrying particles in a
plasma. Cases of interest for space and laboratory plasmas are identified where
these generalisations allow for the definition of generalised electromagnetic
fields that transform under a Lorentz boost in the same way as the real
electromagnetic fields and that obey the same set of homogeneous Maxwell's
equations
Shock waves in superconducting cosmic strings: growth of current
Intrinsic equations of motion of superconducting cosmic string may admit
solutions in the shock-wave form that implies discontinuity of the current term
\chi. The hypersurface of discontinuity propagates at finite velocity
determined by finite increment \Delta \chi =\chi_+ -\chi_-. The current
increases \chi_+>\chi_- in stable shocks but transition between spacelike (\chi
>0) and timelike (\chi<0) currents is impossible.Comment: 13 pages, 3 figure
New Relativistic Effects in the Dynamics of Nonlinear Hydrodynamical Waves
In Newtonian and relativistic hydrodynamics the Riemann problem consists of
calculating the evolution of a fluid which is initially characterized by two
states having different values of uniform rest-mass density, pressure and
velocity. When the fluid is allowed to relax, one of three possible
wave-patterns is produced, corresponding to the propagation in opposite
directions of two nonlinear hydrodynamical waves. New effects emerge in a
special relativistic Riemann problem when velocities tangential to the initial
discontinuity surface are present. We show that a smooth transition from one
wave-pattern to another can be produced by varying the initial tangential
velocities while otherwise maintaining the initial states unmodified. These
special relativistic effects are produced by the coupling through the
relativistic Lorentz factors and do not have a Newtonian counterpart.Comment: 4 pages, 5 figure
New Measure of the Dissipation Region in Collisionless Magnetic Reconnection
A new measure to identify a small-scale dissipation region in collisionless
magnetic reconnection is proposed. The energy transfer from the electromagnetic
field to plasmas in the electron's rest frame is formulated as a
Lorentz-invariant scalar quantity. The measure is tested by two-dimensional
particle-in-cell simulations in typical configurations: symmetric and
asymmetric reconnection, with and without the guide field. The innermost region
surrounding the reconnection site is accurately located in all cases. We
further discuss implications for nonideal MHD dissipation
Relativistic MHD Simulations of Jets with Toroidal Magnetic Fields
This paper presents an application of the recent relativistic HLLC
approximate Riemann solver by Mignone & Bodo to magnetized flows with vanishing
normal component of the magnetic field.
The numerical scheme is validated in two dimensions by investigating the
propagation of axisymmetric jets with toroidal magnetic fields.
The selected jet models show that the HLLC solver yields sharper resolution
of contact and shear waves and better convergence properties over the
traditional HLL approach.Comment: 12 pages, 5 figure
General Relativistic effects on the conversion of nuclear to two-flavour quark matter in compact stars
We investigate the General Relativistic (GR) effects on the conversion from
nuclear to two-flavour quark matter in compact stars, both static as well as
rotating. We find that GR effects lead to qualitative differences in rotating
stars, indicating the inadequacy of non-relativistic (NR) or even Special
Relativistic (SR) treatments for these cases.Comment: 4 pages, 4 figure
Relaxed States in Relativistic Multi-Fluid Plasmas
The evolution equations for a plasma comprising multiple species of charged
fluids with relativistic bulk and thermal motion are derived. It is shown that
a minimal fluid coupling model allows a natural casting of the evolution
equations in terms of generalized vorticity which treats the fluid motion and
electromagnetic fields equally. Equilibria can be found using a variational
principle based on minimizing the total enstrophy subject to energy and
helicity constraints. A subset of these equilibria correspond to minimum
energy. The equations for these states are presented with example solutions
showing the structure of the relaxed states.Comment: 8 pages, 2 figure
Memory and nonlocal effects in heat transport : from diffusive to ballistic regimes
The authors discuss a generalized transportmodel including memory and nonlocal effects, which aims to describe the transition of heat transport from the diffusive regime to the ballistic regime. By using an effective thermal conductivity depending on the Knudsen number, they describe in a single equation the behavior of conductivity in terms of the system size and a reduction in the limit flux through nanoscale devices
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