470 research outputs found
Cosmological constant influence on cosmic string spacetime
We investigate the line element of spacetime around a linear cosmic string in
the presence of a cosmological constant. We obtain the metric and argue that it
should be discarded because of asymptotic considerations. Then a time dependent
and consistent form of the metric is obtained and its properties are discussed.Comment: 3 page
Self-similar structure of a hot magnetized flow with thermal conduction
We have explored the structure of hot magnetized accretion flow with thermal
conduction. The importance of thermal conduction in hot accretion flows has
been confirmed by observations of the hot gas surrounding Sgr and a few
other nearby galactic nuclei. For a steady state structure of such accretion
flows a set of self similar solutions are presented. In this paper, we have
actually tried to re-check the solution presented by Abbassi et al. (2008)
using a physical constrain. In this study we find that Eq 29 places a new
constrain that limits answers presented by Abbassi et al. 2008. In that paper
the parameter space in which it is established in the new constrain was
plotted. However, the new requirement makes up only a small parameter space
with physically acceptable solutions. And now in this manuscript we have
followed the idea with more effort, and tried to find out how thermal
conduction influences the structur of the disks in a physical parameter space.
We have found out that the existence of thermal conduction will lead to
reduction of accretion and radial and azimuthal velocities as well as the
vertical thickness of the disk, which is slightly reduced. Moreover, the
surface density of the disk will increase when the thermal conduction becomes
important in the hot magnetized flow.Comment: Accepted for publication, AP
Gravitational Waves in a Spatially Closed deSitter Spacetime
Perturbation of gravitational fields may be decomposed into scalar,vector and
tensor components.In this paper we concern with the evolution of tensor mode
perturbations in a spatially closed deSitter background of RW form. It may be
thought as gravitional waves in a classical description. The chosen background
has the advantage of to be maximally extended and symmetric. The spatially flat
models commonly emerge from inflationary scenarios are not completely
extended.We first derive the general weak field equations.Then the form of the
field equations in two special cases, planar and spherical waves are obtained
and their solutions are presented. We conclued with discussing the significance
of the results and their implications.Comment: 16 pages,no figure
Trans-Planckian Effect in Cosmology
Apart from the assumption that the inflation started at an infinite time in
the past, the more realistic initial state of the quantum fluctuations is
described by a mixed quantum state imposed at a finite value of the initial
time. One of the most important non-trivial vacua is the -vacuum, which
is specified by a momentum cutoff \cite{Danielsson:2002kx}. As a
consequence, the initial condition is imposed at different initial times for
the different -modes. This modifies the amplitude of the quantum
fluctuations, and thus the corresponding power spectra. In this paper, we
consider the imprint of the -vacuum state on the power spectrum of
scalar perturbations in a generic gravity by assuming an ultraviolet
cutoff . As a specific model, we consider the Starobinsky model and
find the trans-Planckian power spectrum. We find that the leading order
corrections to the scalar power spectra in gravity have an oscillatory
behavior as in general relativity \cite{Lim}, and furthermore, the results are
in sufficient agreement with the CDM model.Comment: 21 pages, 5 figures, 1 table
Vertically Self-Gravitating ADAFs in the Presence of Toroidal Magnetic Field
Force due to the self-gravity of the disc in the vertical direction is
considered to study its possible effects on the structure of a magnetized
advection-dominated accretion disc. We present steady-sate self similar
solutions for the dynamical structure of such a type of the accretion flows.
Our solutions imply reduced thickness of the disc because of the self-gravity.
It also imply that the thickness of the disc will increase by adding the
magnetic field strength.Comment: Accepted for publication in Astrophysics and Space Science
Energy-momentum Density of Gravitational Waves
In this paper, we elaborate the problem of energy-momentum in general
relativity by energy-momentum prescriptions theory. Our aim is to calculate
energy and momentum densities for the general form of gravitational waves. In
this connection, we have extended the previous works by using the prescriptions
of Bergmann and Tolman. It is shown that they are finite and reasonable. In
addition, using Tolman prescription, exactly, leads to same results that have
been obtained by Einstein and Papapetrou prescriptions.Comment: LaTeX, 9 pages, 1 table: added reference
Electrolyte gate dependent high-frequency measurement of graphene field-effect transistor for sensing applications
We performed radiofrequency (RF) reflectometry measurements at 2.4 GHz on
electrolyte-gated graphene field-effect transistors (GFETs) utilizing a tunable
stub-matching circuit for impedance matching. We demonstrate that the gate
voltage dependent RF resistivity of graphene can be deduced even in the
presence of the electrolyte which is in direct contact with the graphene layer.
The RF resistivity is found to be consistent with its DC counterpart in the
full gate voltage range. Furthermore, in order to access the potential of
high-frequency sensing for applications, we demonstrate time-dependent gating
in solution with nanosecond time resolution.Comment: 14 pages, 4 figure
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