共查询到20条相似文献，搜索用时 15 毫秒
1.
M. Farasat Shamir 《Astrophysics and Space Science》2010,330(1):183-189
The modified theories of gravity, especially the f(R) gravity, have attracted much attention in the last decade. In this context, we study the exact vacuum solutions of Bianchi
type I, III and Kantowski-Sachs spacetimes in the metric version of f(R) gravity. The field equations are solved by taking expansion scalar θ proportional to shear scalar σ which gives A=B
n
, where A and B are the metric coefficients. The physical behavior of the solutions has been discussed using some physical quantities. Also,
the function of the Ricci scalar is evaluated in each case. 相似文献
2.
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
3.
Self-consistent system of spinor, scalar and BI gravitational fields in presence of magneto-fluid and Λ term is considered.
Assuming that the expansion of the BI universe is proportional to the σ1 component of the shear tensor, exact solutions for the metric functions, as well as for scalar and spinor fields are obtained.
For a non-positive Λ, the initially anisotropic space–time becomes isotropic one in the process of expansion; whereas for
Λ > 0, an oscillatory mode of expansion of the BI model occurs.
PACS numbers: 03.65.Pm and 04.20.Ha 相似文献
4.
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
5.
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
6.
Periodic orbits in the photogravitational restricted problem with the smaller primary an oblate body
In this paper, we have studied periodic orbits generated by Lagrangian solutions of the restricted three body problem when
more massive body is a source of radiation and the smaller primary is an oblate body. We have determined periodic orbits for
fixed values of μ, σ and different values of p and h (μ mass ratio of the two primaries, σ oblate parameter, p radiation parameter and h energy constant). These orbits have been determined by giving displacements along the tangent and normal to the mobile co-ordinates
as defined by Karimov and Sokolsky (in Celest. Mech. 46:335, 1989). These orbits have been drawn by using the predictor-corrector method. We have also studied the effect of radiation pressure
on the periodic orbits by taking some fixed values of μ and σ. 相似文献
7.
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
8.
A perturbation in the ratio of the matter temperature to the radiation temperature in the form of a Gaussian with amplitude
A and width σ (in units of the redshift z) centered at some redshift z
c is considered, with some “standard” temperature ratio obtained from a simultaneous solution of the cosmological recombination
kinetics and energy equations being taken as the initial (unperturbed) one. Comparatively small (A = ± 0.01), fast (σ = 17) perturbations are shown to give rise to distinct narrow absorption (for A > 0) or emission (for A < 0) quasi-lines in each of the subordinate continua. The positions of these quasi-lines correlate with the position of the
perturbation center, while their intensities are very sensitive to the perturbation amplitude. At the same time, the manifestation
of the perturbation is much less clear in hydrogen lines (subordinate ones and the Ly-α line) and two-photon emission. As a result, the full perturbed spectrum is characterized by the presence of the narrow quasi-lines
mentioned above and by a general decrease (for A > 0) or increase (for A < 0) in intensity with increasing wavelength. 相似文献
9.
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
10.
We obtain a new parametric class of exact solutions of Einstein–Maxwell field equations which are well behaved. We present a charged super-dense star model after prescribing particular forms of the metric potential and electric intensity. The metric describing the super dense stars joins smoothly with the Reissner–Nordstrom metric at the pressure free boundary. The electric density assumed is where n may take the values 0,1,2,3,4 and so on and K is a positive constant. For n=0,1 we rediscover the solutions by Gupta and Maurya (Astrophys. Space Sci. 334(1):155, 2011) and Fuloria et al. (J. Math. 2:1156, 2011) respectively. The solution for n=2 have been discussed extensively keeping in view of well behaved nature of the charged solution of Einstein–Maxwell field equations. The solution for n=3 and n=4 can be also studied likewise. In absence of the charge we are left behind with the regular and well behaved fifth model of Durgapal (J. Phys. A 15:2637, 1982). The outmarch of pressure, density, pressure-density ratio and the velocity of sound is monotonically decreasing, however, the electric intensity is monotonically increasing in nature. For this class of solutions the mass of a star is maximized with all degree of suitability, compatible with Neutron stars and Pulsars. 相似文献
11.
M. Mehdipoor 《Astrophysics and Space Science》2012,338(1):73-79
Korteweg-de-Vries-Burger (K-dVB) equation is derived for ion acoustic shock waves in electron-positron-ion plasmas. Electrons
and positrons are considered superthermal and are effectively modeled by a kappa distribution in which ions are as cold fluid.
The analytical traveling wave solutions of the K-dVB equation investigated, through the (G′/G)-expansion method. These traveling wave solutions are expressed by hyperbolic function, trigonometric functions are rational
functions. When the parameters are taken special values, the shock waves are derived from the traveling waves. It is observed
that the amplitude ion acoustic shock waves increase as spectral index κ and kinematic viscosity η
i,0 increases in which with increasing positron density β and electron temperature σ the shock amplitude decreases. Also, numerically the effect different parameters on the nonlinearity A and dispersive B terms and wave velocity V investigated. 相似文献
12.
In the present article, we have obtained a class of charged superdense star models, starting with a static spherically symmetric
metric in curvature coordinates by considering Durgapal (J. Phys. A 15:2637, 1982) type metric i.e. g
44=B(1+Cr
2)
n
, where n being any positive integer. It is observed that the maximum mass of the charged fluid models is monotonically increasing
with the increasing values of n≤4. For n≥4, the maximum mass of the charged fluid models is throughout monotonically decreasing and over all maximum mass is attained
at n=4. The present metric tends to another metric which describes the charged analogue of Kuchowicz neutral solution as n→∞. Consequently the lower limit of maximum mass of the charged fluid models could be determined and found to be 5.1165 solar
mass with corresponding radius 18.0743 Km. While the upper limit of maximum mass of the model of this category is already
known to be 5.7001 solar mass with corresponding radius 17.1003 Km for n=4. The solutions so obtained are well behaved. 相似文献
13.
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
14.
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
15.
The vertical stability character of the families of short and long period solutions around the triangular equilibrium points
of the restricted three-body problem is examined. For three values of the mass parameter less than equal to the critical value
of Routh (μ
R
) i.e. for μ = 0.000953875 (Sun-Jupiter), μ = 0.01215 (Earth-Moon) and μ = μ
R
= 0.038521, it is found that all such solutions are vertically stable. For μ > (μ
R
) vertical stability is studied for a number of ‘limiting’ orbits extended to μ = 0.45. The last limiting orbit computed by
Deprit for μ = 0.044 is continued to a family of periodic orbits into which the well known families of long and short period
solutions merge. The stability characteristics of this family are also studied. 相似文献
16.
In this paper we present a new class of nonsingular solutions representing time dependent balls of perfect fluid with matter-radiation
in general relativity. The solution of the class is suitable for interior modeling of a quasar i.e. a massive radiating star.
The interior solution is matched with a zero pressure Vaidya metric. From this solution we constructed a quasar model by assuming
the life time of the quasar of ≈107 year. We obtained a mass of the quasar of ≈109 M
θ
, linear dimension ≈1017 km and a rate of emission L
∞≈1047 erg/s. 相似文献
17.
Koijam Manihar Singh 《Astrophysics and Space Science》2010,325(1):127-135
Some new interesting solutions, the dynamics, behaviour and phenomena of rotating charged perfect fluid models are investigated,
and their physical and geometrical properties are studied in order to substantiate the possibility of the existence of such
astrophysical bodies in this Universe. The nature and role of the metric rotation Ω(r,t) as well as that of the matter rotation ω(r,t) are studied for uniform and non-uniform motions. The reactions of the gravitational and charged fields with respect to the
rotational motion are studied and possible results are explored for real astrophysical situations, and in some solutions we
find the spatial restrictions on the models for realistic conditions. Rotating models which are expanding are obtained in
which the rotational motions are decaying with time. 相似文献
18.
The energy density of Vaidya-Tikekar isentropic superdense star is found to be decreasing away from the center, only if the
parameter K is negative. The most general exact solution for the star is derived for all negative values of K in terms of circular and inverse circular functions. Which can further be expressed in terms of algebraic functions for K = 2-(n/δ)2 < 0 (n being integer andδ = 1,2,3 4). The energy conditions 0 ≤ p ≤ αρc
2, (α = 1 or 1/3) and adiabatic sound speed conditiondp dρ ≤ c
2, when applied at the center and at the boundary, restricted the parameters K and α such that .18 < −K −2287 and.004 ≤ α ≤ .86. The maximum mass of the star satisfying the strong energy condition (SEC),
(α = 1/3) is found to be3.82 Mq· at K=−2/3, while the same for the weak energy condition (WEC), (α =1) is 4.57 M_
⊙ atK=−>5/2. In each case the surface density is assumed to be 2 × 1014 gm cm-3. The solutions corresponding to K>0 (in fact K>1) are also made meaningful by considering the hypersurfaces t= constant as 3-hyperboloid by replacing the parameter R
2 by −R2 in Vaidya-Tikekar formalism. The solutions for the later case are also expressible in terms of algebraic functions for K=2-(n/δ2 > 1 (n being integer or zero and δ =1,2,3 4). The cases for which 0 < K < 1 do not possess negative energy density gradient and therefore are incapable of representing any physically plausible
star model. In totality the article provides all the physically plausible exact solutions for the Buchdahl static perfect
fluid spheres.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
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
20.
Properties of three-dimensional ion-acoustic solitary and shock waves accompining electron-positron-ion magnetoplasma with
high-energy (superthermal) electrons and positrons are investigated. For this purpose, a Zakharov-Kuznetsov-Burgers (ZKB)
equation is derived from the ion continuity equation, ion momentum equation with kinematic viscosity among ions fluid, electrons,
and positrons having kappa distribution together with the Poisson equation. The dependence of the solitary and shock excitations
characteristics on the parameter measuring the superthermality κ, the ion gyrofrequency Ω, the unperturbed positrons-to-ions density ratio ν, the viscosity parameter η, the direction cosine ℓ, the ion-to-electron temperature ratio σ
i
, and the electron-to-positron temperature ratio σ
p
have been investigated. Moreover, it is found that the parameters κ, Ω, ν, η, and ℓ lead to accelerate the particles, whereas the parameters σ
i
and σ
p
would lead to decelerate them. Numerical calculations reveal that the nonlinear pulses polarity are always positive. This
study could be useful to understand the nonlinear electrostatic excitations in interstellar medium. 相似文献