共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
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
3.
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
4.
Cylindrically symmetric inhomogeneous string cosmological model in presence of electromagnetic field is investigated. We have
assumed that F
23 is the only non-vanishing component of F
ij
. To get the deterministic solution, it has been assumed that the expansion (θ) in the model is proportional to the eigen value σ
1
1 of the shear tensor σ
i
j
. The physical and geometric aspects of the model are also discussed.
相似文献
5.
Anil Kumar Yadav Anirudh Pradhan Ajay Kumar Singh 《Astrophysics and Space Science》2012,337(1):379-385
The present study deals with spatially homogeneous and totally anisotropic locally rotationally symmetric (LRS) Bianchi type
I cosmological model with variable G and Λ in presence of imperfect fluid. To get the deterministic model of Universe, we assume that the expansion (θ) in the model is proportional to shear (σ). This condition leads to A=ℓB
n
, where A, B are metric potential. The cosmological constant Λ is found to be decreasing function of time and it approaches a small positive
value at late time which is supported by recent Supernovae Ia (SN Ia) observations. Also it is evident that the distance modulus
curve of derived model matches with observations perfectly. 相似文献
6.
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
7.
The present study deals with spatially homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi type I cosmological
model with dominance of dark energy. To get the deterministic model of Universe, we assume that the shear scalar (σ) in the model is proportional to expansion scalar (θ). This condition leads to A=B
n
, where A, B are metric potential and n is positive constant. It has been found that the anisotropic distribution of dark energy leads to the present accelerated
expansion of Universe. The physical behavior of the Universe has been discussed in detail. 相似文献
8.
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
9.
Zi Zhu 《Astrophysics and Space Science》2000,271(4):353-363
Combining Hipparcos proper motions and the radial velocity data, we have studied the Cepheid kinematics on the basis of the
three-dimensional Ogorodnikov-Milne model. The results seem to show a slight contracting motion of the Galaxy in the solar
neighbourhood, ∂ V
θ / ∂θ / R = −2.60 ± 1.07 km s-1 kpc-1, which is along the solar circle. Under the hypothesis of a circular stream model, we have determined the galactic rotation
V
θ = −240.5 ± 10.2 km s-1 for the classical Cepheids.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
The present study deals with locally rotationally symmetric (LRS) Bianchi type II cosmological model representing massive
string. The energy-momentum tensor for such string as formulated by Letelier (Phys. Rev. D 28:2414, 1983) is used to construct massive string cosmological model for which we assume that the expansion (θ) in the model is proportional to the shear (σ). This condition leads to A=B
m
, where A and B are the metric coefficients and m is proportionality constant. For suitable choice of constant m, it is observed
that in early stage of the evolution of the universe string dominates over the particle whereas the universe is dominated
by massive string at the late time. Our model is in accelerating phase which is consistent to the recent observations of type
Is supernovae. Some physical and geometric behavior of the model is also discussed. 相似文献
11.
Anirudh Pradhan Vineet K. Yadav Lallan Yadav A. K. Yadav 《Astrophysics and Space Science》2007,312(3-4):267-273
Cylindrically symmetric inhomogeneous cosmological model for perfect fluid distribution with electromagnetic field is obtained.
The source of the magnetic field is due to an electric current produced along the z-axis. F
12 is the non-vanishing component of electromagnetic field tensor. To get the deterministic solution, it has been assumed that
the expansion θ in the model is proportional to the shear σ. Physical and geometric aspects of the models are also discussed in presence and absence of magnetic field.
相似文献
12.
In the present investigation we measure the differential rotation of strong magnetic flux during solar cycles 21 – 23 with
the method of wavelet transforms. We find that the cycle-averaged synodic rotation rate of strong magnetic flux can be written
as ω=13.47−2.58sin 2
θ or ω=13.45−2.06sin 2
θ−1.37sin 4
θ, where θ is the latitude. They agree well with the results derived from sunspots. A north–south asymmetry of the rotation rate is
found at high latitudes (28°<θ<40°). The strong flux in the southern hemisphere rotates faster than that in the northern hemisphere by 0.2 deg day−1. The asymmetry continued for cycles 21 – 23 and may be a secular property. 相似文献
13.
We have investigated Bianchi type III non-static magnetized cosmological model for perfect fluid distribution in general relativity.
We assume that F
12 is the only non-vanishing component of F
ij
. Maxwell’s equation
leads to
where K and α are constants.
To get a deterministic model, we assume that σ
11
∝
θ which leads to A=C
n
where n is a constant, σ
11 the x-component of shear tensor σ
ij
and theta is the expansion in the model. The behaviour of the model in absence of magnetic field is discussed. The other
physical and geometrical aspects of the model are also discussed. 相似文献
14.
We estimate the values of the cosmological parameters using the data about peculiar velocities of 1493 flat edge-on spirals
from the RFGC catalogue. The obtained values Ω
m
= 0.21− 0.09
+ 0.22, σ8 = 1.07− 0.24
+ 0.28 differ from the WMAP values by approximately 2–3σ, but well agree with modern constraints on these parameters. Due to a strong
correlation between these quantities the shape of the 1σ, 2σ and 3σ-boundaries are rather narrow. This gives us the opportunity
to use this estimation to verify the corresponding values, obtained by different methods. 相似文献
15.
Based on a new geometric diagnostic method-Om, we consider a new independent-model parametrization . When we work in potential W σ [1+(σ−A)2]e (−Bσ), we investigate the evolutional behavior of Om with respect to red-shift z and the influence of coupling parameter α on the trajectory of Om with respect to z. We get that phantom model of Dilaton dark energy can avoid the future singularity “Big Rip”. The numerical results give current value of EOS which fits the latest observational data WMAP5+BAO+SNe very well. 相似文献
16.
Xingfen Zhu 《Astrophysics and Space Science》1999,268(4):483-495
Using the Hewitt-Burbidge QSO Catalogue (1993) and all-sky catalogue of Abell clusters (ACO, 1989) at the region |b| > 40° we analyze the cross correlation function and find anti-correlation between them at angular separations 3° < θ < 10° , which is mainly caused by optical-selected QSOs, rather than radio-selected QSOs. There is no such anti-correlation between
QSOs and Abell clusters at smaller separations θ < 3°. Considering that this phenomenon may be caused by different characters of the objects, we further estimate the correlation
function with various subsamples. We find that the correlation is independent of the redshift of QSOs, but depends upon the
type of Abell clusters: for the D ≤ 4 clusters there is an obvious tendency of overdensity of quasars at 0° < θ < 5°; around the R ≥ 2 Abell clusters there is about an 18.7% deficit of quasars in the region 3° < θ < 7°. K-S Test shows the overdensity or deficit of quasars around different types of clusters cannot be explained by the projection
effect of background quasars. We get the enhancement factor of quasar overdensity (for D ≤ 4 clusters) q =1.13, and the extinction magnitude factor of QSO deficiency (for R ≥ 2 clusters) Av= 0.14.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
John E. Norris 《Astrophysics and Space Science》1999,265(1-4):213-220
The status of the Galactic thick disk is reviewed. Consideration of the recent literature suggests that its vertical scale
height and normalisation with respect to the thin disk remain uncertain to within a factor two, with values reported in the
ranges 750–1500 pc, and 0.02–0.13, respectively. The bulk of the thick disk has kinematics (σU, σV, σW) = (65, 54, 38 km s-1), and lags the thin disk by some 40 km s-1; differences of opinion exists as to whether kinematics change with distance from the Galactic plane. The bulk of the thick
disk has [Fe/H] ∼ −0.6, with little or no evidence for a vertical gradient. The question of gradients is critical for an understanding
of thick disk cosmogony and needs closer attention. The reality of the so-called metal-weak thick disk (material having disklike
kinematics and [Fe/H] ≤ −1.0) is also considered. The case for such material seems to be steadily growing: in the range −1.6
≤ [Fe/H] ≤ −1.0, recent estimates suggest ρMWTD/ρHalo ∼ 0.1-0.3. While many workers regard the thick disk as a discrete entity, the caveat is made that this is a sufficient condition,
but not one necessarily required by the observations. Best practice requires that both the discrete model and the alternative
extended configuration be compared with observational data to examine the relative likelihood of their relevance. Recent theoretical
advances are also discussed, together with the need for in situ measurements of the thick disk away from the Galactic plane.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
Empirical evidence for both stellar mass black holes (M
•<102
M
⊙) and supermassive black holes (SMBHs, M
•>105
M
⊙) is well established. Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is correlated with the bulge
mass, and even more strongly with the central stellar velocity dispersion σ
c
, the M
•–σ relation. On the other hand, evidence for “intermediate-mass” black holes (IMBHs, with masses in the range 100–105 M
⊙) is relatively sparse, with only a few mass measurements reported in globular clusters (GCs), dwarf galaxies and low-mass
AGNs. We explore the question of whether globular clusters extend the M
•–σ relationship for galaxies to lower black hole masses and find that available data for globular clusters are consistent with
the extrapolation of this relationship. We use this extrapolated M
•–σ relationship to predict the putative black hole masses of those globular clusters where existence of central IMBH was proposed.
We discuss how globular clusters can be used as a constraint on theories making specific predictions for the low-mass end
of the M
•–σ relation. 相似文献
19.
V. I. Burnashev B. A. Burnasheva 《Bulletin of the Crimean Astrophysical Observatory》2010,106(1):63-67
From the results of long-term observations, data on the energy distribution in the spectra of 116 variable stars of different
types have been obtained. From the published response curves of the UBV system, the magnitudes and color indexes were calculated. The mean accuracy of the calculated values is σ
v
≃ 0.
m
04, σ
bv
≃ 0.
m
03, and σ
ub
≃ 0.
m
06. 相似文献
20.
In this paper we examine the recently introduced Dvali-Gabadadze-Porrati (DGP) gravity model. We use a space-time metric in which the local gravitation source dominates the metric over the contributions from the cosmological flow. Anticipating ideal possible solar system effects, we derive expressions for the signal time delays in the vicinity of the Sun. and for various ranges of the angle θ of the signal approach, The time contribution due to DGP correction to the metric is found to be proportional to b 3/2/c 2 r 0. For r 0 equal to 5 Mpc and θ in the range [−π/3,π/3], Δt is equal to 0.0001233 ps. This delay is extremely small to be measured by today’s technology but it could be probably measurable by future experiments. 相似文献