2,509 research outputs found
Alternative mechanism of avoiding the big rip or little rip for a scalar phantom field
Depending on the choice of its potential, the scalar phantom field
(the equation of state parameter ) leads to various catastrophic fates of
the universe including big rip, little rip and other future singularity. For
example, big rip results from the evolution of the phantom field with an
exponential potential and little rip stems from a quadratic potential in
general relativity (GR). By choosing the same potential as in GR, we suggest a
new mechanism to avoid these unexpected fates (big and little rip) in the
inverse-\textit{R} gravity. As a pedagogical illustration, we give an exact
solution where phantom field leads to a power-law evolution of the scale factor
in an exponential type potential. We also find the sufficient condition for a
universe in which the equation of state parameter crosses divide. The
phantom field with different potentials, including quadratic, cubic, quantic,
exponential and logarithmic potentials are studied via numerical calculation in
the inverse-\textit{R} gravity with correction. The singularity is
avoidable under all these potentials. Hence, we conclude that the avoidance of
big or little rip is hardly dependent on special potential.Comment: 9 pages,6 figure
Three-Dimensional MHD Simulation of Caltech Plasma Jet Experiment: First Results
Magnetic fields are believed to play an essential role in astrophysical jets
with observations suggesting the presence of helical magnetic fields. Here, we
present three-dimensional (3D) ideal MHD simulationsof the Caltech plasma jet
experiment using a magnetic tower scenario as the baseline model. Magnetic
fields consist of an initially localized dipole-like poloidal component and a
toroidal component that is continuously being injected into the domain. This
flux injection mimics the poloidal currents driven by the anode-cathode voltage
drop in the experiment. The injected toroidal field stretches the poloidal
fields to large distances, while forming a collimated jet along with several
other key features. Detailed comparisons between 3D MHD simulations and
experimental measurements provide a comprehensive description of the interplay
among magnetic force, pressure and flow effects. In particular, we delineate
both the jet structure and the transition process that converts the injected
magnetic energy to other forms. With suitably chosen parameters that are
derived from experiments, the jet in the simulation agrees quantitatively with
the experimental jet in terms of magnetic/kinetic/inertial energy, total
poloidal current, voltage, jet radius, and jet propagation velocity.
Specifically, the jet velocity in the simulation is proportional to the
poloidal current divided by the square root of the jet density, in agreement
with both the experiment and analytical theory. This work provides a new and
quantitative method for relating experiments, numerical simulations and
astrophysical observation, and demonstrates the possibility of using
terrestrial laboratory experiments to study astrophysical jets.Comment: accepted by ApJ 37 pages, 15 figures, 2 table
Solar system tests for realistic models with nonminimal torsion-matter coupling
In the previous paper, we have constructed two models with nonminimal
torsion-matter coupling extension, which are successful in describing the
evolution history of the Universe including the radiation-dominated era, the
matter-dominated era, and the present accelerating expansion. Meantime, the
significant advantage of these models is that they could avoid the cosmological
constant problem of CDM. However, the nonminimal coupling between
matter and torsion will affect the tests of Solar system. In this paper, we
study the effects of Solar system in these models, including the gravitation
redshift, geodetic effect and perihelion preccesion. We find that Model I can
pass all three of the Solar system tests. For Model II, the parameter is
constrained by the measure of the perihelion precession of Mercury.Comment: 10 page
Casimir pistons with hybrid boundary conditions
The Casimir effect giving rise to an attractive or repulsive force between
the configuration boundaries that confine the massless scalar field is
reexamined for one to three-dimensional pistons in this paper. Especially, we
consider Casimir pistons with hybrid boundary conditions, where the boundary
condition on the piston is Neumann and those on other surfaces are Dirichlet.
We show that the Casimir force on the piston is always repulsive, in contrast
with the same problem where the boundary conditions are Dirichlet on all
surfaces.Comment: 8 pages, 4 figures,references added, minor typos correcte
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