共查询到20条相似文献，搜索用时 31 毫秒
1.
Mahesh Kumar Yadav Anirudh Pradhan Sheel Kumar Singh 《Astrophysics and Space Science》2007,311(4):423-429
Some Bianchi type-I viscous fluid string cosmological models with magnetic field are investigated. The viscosity coefficient
of bulk viscous fluid is assumed to be a power function of mass density ξ(t)=ξ
0
ρ
m
, where ξ
0 and m are constants. To get a determinate model, we assume conditions ρ=(1+ω)λ, where ρ is rest energy density, ω a positive constant and λ the string tension density and expansion θ is proportional to eigen value σ
11 of the shear tensor σ
j
i
. The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field is discussed.
相似文献
2.
S. K. Tripathy S. K. Nayak S. K. Sahu T. R. Routray 《Astrophysics and Space Science》2009,323(1):91-96
Bianchi type-I string cosmological models are studied in Saez-Ballester theory of gravitation when the source for the energy
momentum tensor is a viscous string cloud coupled to gravitational field. The bulk viscosity is assumed to vary with time
and is related to the scalar expansion. The relationship between the proper energy density ρ and string tension density λ are investigated from two different cosmological models. 相似文献
3.
The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory
is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously
as the Universe expands, are obtained by using the power-law relation φ = bR
n
, which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation
dominated era of the universe. The physical properties of cosmological solutions are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
The propagation of spiral density waves in a differentially rotating, self-gravitating, magnetoactive and highly flattened disk is investigated by using the asymptotic theory for tightly wound spirals developed by Lin and his collaborators. We adopt the continuum fluid model as the primary basis, and our treatment will be largely analytical. The disk plasma is studied in the frozen field approximation and inhomogenceous magnetic fields in the plane of the disk are considered in detail.In a differentially rotating disk with strong magnetic fields, the field lines will be considerably distorted and the mutual influence of magnetic fields and differential rotation is by no means obvious.In this paper we present a new asymptotic dispersion relation for tightly wound spiral density waves with magnetic fields along the spiral armsB
(r). The effects of the magnetic fields lead to such terms likek
2(a
2 +V
A
2
), wherek is the wave number,a represents the speed of sound,V
A = (B
2/4)1/2 is the Alfvén speed,B denotes the field strength, and is the plasma density. These terms depict the well-known magnetoacoustic waves and could have been anticipated without a detailed computation. However the interaction of magnetic fields and differential rotation may give rise to other significant terms which are not so easy to foresee.We also present a more exact local dispersion relation by using the WKB approximation and study the effects of magnetic fields on the growth rate through the parametersQ andJ defined in the literature.Although the effects of the magnetic fields are rather insignificant for applications to Galactic dynamics, the effects of the magnetic fields are important for applications to star formation and problems related to the solar nebula. 相似文献
5.
L.R.S. Bianchi Type I string dust cosmological models with and without magnetic field following the techniques used by Letelier
and Stachel, is investigated. To get a determinate solution, we assume a conditionσ is proportional to scalar of expansion
θ where σ is shear and θ is scalar of expansion and which leads to A=ℓ B
nwhere n is a constant and ℓ is proportionality constant. Some special models are also investigated by introducing the transformation,
, which leads to Riccati type differential equation. The physical and geometrical aspects of the models are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Some aspects of viscous cosmological models, mainly of Bianchi type-I, are studied, in particular with the purpose of trying to obtain a natural explanation of why the entropy per baryon in the universe, ~ 109, is so large. Using the FRW metric it is first shown, in agreement with previous workers, that the expressions for the bulk viscosity as derived from kinetic theory in the plasma era is incapable of explaining the large value of. However it is possible to imagine the viscosity to be an impulse viscosity operative in one or several phase transitions in the early universe. This is the main idea elaborated on in the present paper. It is shown that in thek = 0 FRW space, an impulse bulk viscosity infl ~ 1060 g cm–1 s–1) acting at the phase transition at the end of the inflationary epoch corresponds to the correct entropy. If the space is anisotropic, it is natural to exploit the analogy with classical fluid dynamics to introduce the turbulent viscosity concept. This is finally discussed, in relation to an anisotropy introduced in the universe via the Kasner metric. 相似文献
7.
String cosmological models with bulk viscosity are investigated in Kantowski-Sachs space-time. To obtain a determinate solution,
it is assumed that the coefficient of bulk viscosity is a power function of the scalar of expansion ζ = kθm and the scalar of expansion is proportional to the shear scalar θ ∝ σ, which leads to a relation between metric potentials
R = AS
n
. The physical and geometrical aspects of the model are also discussed. It is shown that the bulk viscosity has significant
influence on the evolution of the universe. There is a ‘big bang’ start in the model when m ≤ 1 but there is no ‘big bang’
start when m > 1. 相似文献
8.
Bianchi Type I magnetized string cosmological model following the techniques used by Letelier and Stachel, is investigated.
To get a determinate model, we assume a condition ∊ = λ (geometric string) where ∊ is the rest energy density, λ the string
tension density and expansion (θ) is proportional to eigen value σ1
1 of shear tensor (σ
j
i
), which leads to A = ℓ (BC)
n
where A, B, C are metric potentials and ℓ and n are constants. The behaviour of the model in presence and absence of magnetic field is discussed. The physical and geometrical
aspects of the model are also discussed. 相似文献
9.
Bianchi Type I string dust cosmological models in presence and absence of magnetic field following the techniques used by
Letelier and Stachel, are investigated. To get the deterministic solution, we have assumed that σ
11 is proportional to the expansion (θ) where σ
11 is the eigen value of shear tensor (σ
i
j
) and which leads to A=N(BC)−n
, n>0 where A,B,C are metric potentials and
, N and ℓ are constants. The behaviour of the models in presence and absence of magnetic field are discussed. The other physical and
geometrical aspects of the model are also discussed. 相似文献
10.
A new class of exact solutions of Einstein’s field equations with a bulk viscous fluid for an LRS Bianchi type-Ia obtained
by using a time dependent deceleration parameter and cosmological term Λ. The coefficient of bulk viscosity is assumed to
be a power function of mass density (ξ=ξ
0
ρ
n
). We have obtained a general solution of the field equations from which six models of the universe are derived: exponential,
polynomial and sinusoidal form respectively. The behaviour of these models of the universe are also discussed in the frame
of reference of recent supernovae Ia observations.
相似文献
11.
I. N. Toptygin 《Astrophysics and Space Science》1973,20(2):351-371
The acceleration of fast particles by Alfvén and magnetic sound waves of small amplitude is considered. The waves exist against the background of a strong, uniform magnetic field. We take into accunnt the contributions to acceleration from a large scale random field (harmonics withk<R
–1, whereR is the Larmor radius), as well as from a small scale field (k>R
–1). The small scale field was considered by perturbation theory, while large scale random field-in an adiabatic approximation. The energy dependence of the diffusion coefficient in momentum space, and the time of acceleration are estimated. The possible anisotropy of angular distribution is taken into account.If the Alfvén waves have spectral power index >2 and wave amplitude is small enough, then the energy dependance of the diffusion coefficient is stronger than in the case of the Fermi acceleration. For magnetic sound waves with 2 the energy dependance of the diffusion coefficient is the same as for the Fermi acceleration, but for <2 this dependance is less.The space diffusion coefficient of particles across the regular magnetic field is estimated. It is shown that this diffusion is due mainly to the large scale random field. 相似文献
12.
Some Bianchi Type IX string cosmological models with bulk viscous fluid for massive string is investigated. To get a determinate
solution, a supplementary condition a=b
n, between metric potentials, is used wherea and b are function of time alone. A particular solution for n=0 is also discussed. The behaviour of the model in presence and absence
of bulk viscosity, is discussed. The physical and implications of the model is also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
A. V. Hoperskov Yu. V. Mustsevaya V. V. Mustsevoy 《Astrophysics and Space Science》1992,195(2):373-387
In a slab jet model the influence of strong magnetic fields and density contrasts on the development of instabilities caused by velocity contrasts is studied and applied to disc accretion onto magnetized compact object.The perturbations propagating transverse to a magnetic field in external regions are shown not to be stabilized. A strong density contrast at the jet boundary (R=ex/in 1) does not stabilize the instability of acoustic resonance type (ARTI), the fundamental symmetric and antisymmetric modes being still unstable for any finite value ofR. At the same time a criticalR-value exists (R1/M
2,M is the Mach number) at which the higher reflection harmonics are stabilized.A comparative analysis is made of ARTI and Kelvin-Helmholtz instability that is developed by surface modes of the interfaces between the disc material and magnetic field (magnetosphere) is performed. ARTI may be responsible for the accreting material penetration into the magnetosphere as well as other mechanisms.We have to note that the difference in names is rather traditional here and is to emphasize the difference in the models. 相似文献
14.
In this paper, it is shown that five dimensional LRS Bianchi type-I string cosmological models do not survive for Geometric
and Takabayasi string whereas Barotropic string i.e. ρ=ρ(λ) survives and degenerates string with ρ+λ=0 in scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986). Further we studied some physical and geometrical properties of the model. 相似文献
15.
I. Stellmacher 《Celestial Mechanics and Dynamical Astronomy》1999,75(3):185-200
The motion of Hyperion is an almost perfect application of second kind and second genius orbit, according to Poincaré’s classification.
In order to construct such an orbit, we suppose that Titan’s motion is an elliptical one and that the observed frequencies
are such that 4n
H−3n
T+3n
ω=0, where n
H, n
T are the mean motions of Hyperion and Titan, n
ω is the rate of rotation of Hyperion’s pericenter. We admit that the observed motion of Hyperion is a
periodic motion
such as
. Then,
.N
H, N
T, k∈ N
+. With that hypothesis we show that Hyperion’s orbit tends to a particular periodic solution among the periodic solutions
of the Keplerian problem, when Titan’s mass tends to zero. The condition of periodicity allows us to construct this orbit
which represents the real motion with a very good approximation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
John P. Vinti 《Celestial Mechanics and Dynamical Astronomy》1977,16(4):391-406
Newtonian cosmology is developed with the assumption that the gravitational constantG diminishes with time. The functional form adopted forG(t), a modification of a suggestion of Dirac, isG=A(k+t)
–1, wheret is the age of the Universe and a small constantk is inserted to avoid a singularity in the two-body problem. IfR is the scale factor, normalized to unity at an epoch time , the differential equation is then
. Here 0 is the mean density at the epoch time. With the above form forG(t), the solution is reducible to quadratures.The scale factorR either increases indefinitely or has one and only one maximum. LetH
0 be the present value of Hubble's constant
/R and 0c
the minimum density for a maximum ofR, i.e., for closure of the Universe. The conditions for a maximum lead to a boundary curve of 0c
versusH
0 and the numbers indicate strongly that thisG-variable Newtonian model corresponds to an open universe. An upward estimate of the age of the Universe from 1010 yr to five times such a value would still lead to the same conclusion.The present Newtonian cosmology appears to refute the statement, sometimes made, that the Dirac model forG necessarily leads to the conclusion that the age of the Universe is one-third the Hubble time. Appendix B treats this point, explaining that this incorrect conclusion arises from using all the assumptions in Dirac (1938). The present paper uses only Dirac's final result, viz,G(k+t)–1, superposing it on the differential equation
. 相似文献
17.
The evolution of the current sheet in the electric current direction (in the guiding magnetic field direction) is studied
numerically in the 3-D particle-in-cell model with two current sheets and periodic boundary conditions. In the regime with
(where v
D and
are the electric current drift and electron thermal velocities, respectively) the current sheets are unstable owing to the
Buneman and kink instabilities and become strongly fragmented. During their evolution, in addition to an increase of the energy
of the electric field component in the guiding magnetic field direction, the energies of the electric field components in
the perpendicular direction are even more enhanced. In the current sheet the anomalous resistivity (η
anom/η
C∼7×105, where η
C is the classical resistivity) is generated and thus the magnetic field dissipates. Most of the dissipated magnetic energy
is transformed into the electron kinetic energy in the direction of the electric current. The associated electric field accelerates
the electrons from the tail of the distribution function. 相似文献
18.
Peter Goldreich 《Astrophysics and Space Science》2001,278(1-2):17-23
The inertial range of incompressible MHD turbulence is most conveniently described in terms of counter propagating waves.
Shear Alfvén waves control the cascade dynamics. Slow waves play a passive role and adopt the spectrum set by the shear Alfvén
waves. Cascades composed entirely of shear Alfvén waves do not generate a significant measure of slow waves. MHD turbulence
is anisotropic with energy cascading more rapidly along k
⊥ than along k
∥. Anisotropy increases with k
⊥ such that the excited modes are confined inside a cone bounded by k
∥∝ k
perp
2/3. The opening angle of the cone, θ(k
⊥)∝ k
⊥
-1/3, defines the scale dependent anisotropy. MHD turbulence is generically strong in the sense that the waves which comprise
it are critically damped. Nevertheless, deep inside the inertial range, turbulent fluctuations are small. Their energy density
is less than that of the background field by a factor θ2(k
⊥)≪. MHD cascades are best understood geometrically. Wave packets suffer distortions as they move along magnetic field lines
perturbed by counter propagating wave packets. Field lines perturbed by unidirectional waves map planes perpendicular to the
local field into each other. Shear Alfvén waves are responsible for the mapping's shear and slow waves for its dilatation.
The former exceeds the latter by θ-1(k
⊥)≫ 1 which accounts for dominance of the shear Alfvén waves in controlling the cascade dynamics.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
From high-quality direct frames taken by the Soviet Stratospheric Solar Observatory, using coherent optical methods, the two-dimensional spatial spectrum of the photospheric brightness field was obtained (Figure 1). This spectrum is isotropic and continuous. Spectral densities P(k) and A(k) = 2k
P(k), where k is the radial wavenumber, were estimated for the solar disc centre, and their statistical uncertainty calculated. P(k) has a maximum near k = 10–3 km–1 and then it tends to fall after that to zero frequency. The k dependence of A(k) cannot be satisfactorily approximated by a power law. For the highest frequencies studied, the spectrum falls as k
–9. The measured statistical uncertainty of the spectra of individual domains for k 125 × 10–4 km–1 is in agreement with that calculated for a gaussian homogeneous field. But for a higher k the uncertainty may essentially exceed that of a gaussian homogeneous field. The true rms value for 4650 is equal to about 29%. 相似文献
20.
An LRS Bianchi type I string dust cosmological model with and without bulk viscosity following a method used by Letelier and
Stachel, is investigated. To get a determine solution, it is assumed that σ∝θ whereσ is shear and θ is scalar of expansion
and which leads to A =αBn were n is a constant. The physical and geometrical aspects of the model are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献