共查询到20条相似文献，搜索用时 15 毫秒
1.
Luis O. Pimentel 《Astrophysics and Space Science》1985,112(1):175-183
Under the assumption of a power-law between the expansion factor of the Universe, and the scalar field (a
n=c=const.) tensor theory with cosmological constant are reduced to quadrature. Several exact solutions are obtained, among them inflationary universes that have barotropic equation of state. 相似文献
2.
D. Lorenz-Petzold 《Astrophysics and Space Science》1984,106(1):211-212
We consider the non-flatk
2=1 Friedmann-Robertson-Walker models with a non-vanishing cosmological constant in the Brans-Dicke theory of gravitation. Some special solutions are given in case of =0, which may be considered as exact solutions of the projective theory of Jordan and Thiry. 相似文献
3.
A. A. Saaryan 《Astrophysics》1995,38(2):164-175
We consider multi-dimensional cosmological models in the low-energy field theory of strings with a boson gravitational sector containing a metric, dilaton field, and antisymmetric Kalb-Ramon field. We study the conformal properties of the action and show that in the general conformal representation the theory is equivalent to a generalized scalar-tensor theory with a Lagrangian of nongravitating matter dependent on the dilaton. We find exact solutions of the flat homogeneous anisotropic model with structure R×M1×...×Mn and with equation of state pi=ai in the space Mi. We discuss the picture of cosmological evolution in different conformal representations.Translated fromAstrofizika, Vol. 38, No. 2, 1995. 相似文献
4.
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
5.
The perturbation by a spherical rotating shell is investigated in a homogeneous and isotropic cosmological model of viscous fluid distribution to first order in angular velocity (r, t) of matter and the metric rotation function (r, t) which is uniform and non-uniform the exact solutions for (r, t) are obtained for all cosmological models. The physical properties of these solutions are discussed. 相似文献
6.
In this paper basic cosmological consequences of zatrikean pregeometry are studied. It is shown that many universal quantities can be calculated with simple applications of this theory. An equation directly linking the angular velocity of the universe, the Hubble constantH and the mean density of the universe is derived. The relation between the massM and the radiusR of the universe is examined. This relation leads to various cosmological scenarios, including variations in physical constants and/or violation of the mass conservation and/or variable geometrical properties. 相似文献
7.
The angular size-redshift test for quasars was compared with various cosmological models including non standard models. The possible effects of radio source orientation and relativistic beaming were taken into account in the analysis.It was found that orientation effects alone were not sufficient to explain the observed-z relation in terms of Friedmann models. In addition, linear size evolution of the formD ~ (1 +z)–n
, with 0.75 n 1.2 would be required for 0 1.0, or possibly an inverse correlation between luminosity and linear size. The non-standard cosmological models all gave better fits to the deprojected data than the Friedmann models in the absence of evolutionary effects, with the tired light effect providing the best fit. 相似文献
8.
In addition to our previous paper (Dehnen and Obregón, 1971) the exact cosmological solutions of Brans and Dicke's scalar-tensor theory allowing a power law between the gravitational constant and the radius of curvatureR of the universe are sought in case that — in contrast to our previous paper — the initial conditionR(t=0)=0 is avoided. There exist two different types of solutions especially for the closed space of positive curvature and for positive values of the freely available parameter of the scalar-tensor theory. The radius of curvature and also the gravitational constant increase at first with respect to time and decrease after reaching a maximum value, in contradiction to Dirac's hypothesis whereafter the gravitational constant should decrease with time in an expanding universe. The age of the universe following from these solutions is in accordance with the observations. 相似文献
9.
Subenoy Chakraborty 《Astrophysics and Space Science》1991,180(2):293-303
In this paper the inflationary solutions are studied for the Bianchi-IX space-time in presence of a massless scalar field with a flat potential. Also a class of cosmological solutions of massive strings are obtained following the techniques used by Letelier and Stachel. Some solutions are calculated for pure massive strings following the Takabayashi equation of state =(1+w). 相似文献
10.
Exact solutions of the semi-classical Einstein equations with cosmological constant for conformally invariant free quantum fields in a spatially flat Robertson-Walker metric are found when a classical perfect fluid is present. The stability of the asymptotically Friedmann-de Sitter solutions is studied. The formers are found to be stable if < 0, while the stability of the latter depends of the signs of , and A.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas.Supported by scholarship of the Universidad de Buenos Aires. 相似文献
11.
Corrado Massa 《Astrophysics and Space Science》1993,209(2):309-312
The Weinberg relation (which connects the Hubble constantH to the mass of a typical elementary particle) is an empirical relation hitherto unexplained. I suggest an explanation based on the Zel'dovich energy tensor of vacuum in a Robertson-Walker universe with constant deceleration parameter,q = const. This model leads to
- the Weinberg relation,
- a varying cosmological term Λ scaling asH 2,
- a varying gravitational constantG scaling asH,
- a matter creation process throughout the universe at the rate 10?47 g s?1 cm3,
- a deceleration parameter in the range -1 to 1/2, which allows a horizon-free universe and makes the lawG/H = constant, consistent with the Viking lander data on the orbit of planet Mars.
12.
The Einstein field equations for a perfect fluid distribution representing slowly-rotating fluid spheres are investigated. By imposing restrictions on the matter rotation (r, t) which is related to the dragging of inertial frames, and a uniform rotation which is a function of time, the general solutions for (r, t) are obtained for all cosmological models. In the case of closed models the solutions for (r, t) may represent realistic astrophysical situations only when the radial distance is greater than –1 and less than +1. 相似文献
13.
Differential equations are derived, following the methods ofLifshitz (1946) andLifshitz andKhalatnikov (1963), for density perturbations in isotropic, spatially homogeneous cosmological models of arbitrary space curvature. The unperturbed models contain both matter and radiation. An explicit third-order equation is obtained for the time development of the perturbations, and it is shown that one of the solutions is not covariantly defined. The two remaining solutions are compared with existing solutions for the limiting cases of radiation-filled and dust-filled models. The results ofBonnor's (1957) Newtonian analysis are shown to be a valid limiting case of our equation when the pressurep is finite, but small compared with the densityp timesc
2.A detailed analysis is given of a model containing coupled radiation (p=pc
2/3) and dust (p=0). It is shown that density perturbations with long wavelengths are unstable (with slow growth rate) for all time. The instability exists because for a long-wavelength disturbance, the time scale governing the propagation of pressure effects (which stabilize perturbations) is longer than the time scale for which pressure falls to the point of ineffectiveness. The present value of the critical wavelength is 60 Mpc in models based on flat space sections in which the present background radiation temperature is 3 °K.The research reported herein was supported in part by the Atomic Energy Commission under contract number AT(11-1)-34, Project Agreement No. 125, and by the National Science Foundation, under Grant GP-4975. 相似文献
14.
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
15.
U. S. Pandey G. S. Dubey S. N. Singh D. R. Singh 《Astrophysics and Space Science》1988,150(2):205-212
The solutions of the Einstein and Einstein-Maxwell equations for magnetofluid with cosmological constant are obtained for plane-symmetric space-time. We recover the results of Hojman and Santamarina in a special case when the magnetic field is made to vanish. It has also been shown that under a suitable transformation the cases =0 and 0 are mathematically equivalent. 相似文献
16.
In this paper we discuss Bianchi type-VI0 cosmological models with perfect fluid distribution and magnetic field directed along the axial direction. Einstein's equations have been solved for the condition that the expansion scalar bears a constant ratio to the anisotropy in the direction of a space-like unit vector i. 相似文献
17.
We use the generalized Brans-Dicke theory, in which the Pauli metric is identified to be the physical space-time metric, to study the Universe in different epochs. Exact analytical expressions for dilaton field , cosmological radiusR and density parameter are obtained fork=+1,0,–1 Universe in the radiation-dominated epoch. For matter dominated Epoch, exact analytical expressions for Hubble parameterH, cosmological radius, dilaton field, deceleration factorq, density parameter and the gravitational coupling of the ordinary matter are obtained for the flat Universe. Other important results are: (1) the density parameter is always less than unity for the flat Universe because the dilaton field plays a role as an effective dark matter, and (2) the new Brans-Dicke parameter must be larger than 31.75 in order to consistent with the observed data. 相似文献
18.
C. Sivaram 《Astrophysics and Space Science》1985,116(1):39-42
Recently it was pointed out that a non-zero cosmological constant can play a role in the formation of neutrino halos only in the case of neutrinos of very low rest mass (m
v
<-0.1eV). However, phase-space considerations would requirem
v
>50 eV if neutrinos dominate the missing mass in halos of large spiral galaxies and moreoverm
v
>200 eV is implied in the case of dwarf spheroidals. These larger neutrino masses would be in conflict with observed constraints on the age of the Universe unless a cosmological constant is invoked. 相似文献
19.
A. Ivanchik P. Petitjean E. Rodriguez D. Varshalovich 《Astrophysics and Space Science》2003,283(4):583-588
The possible cosmological variation of the proton-to-electron mass ratio μ = m
p
/m
e was estimated by measuring the H2 wavelengths in the high-resolution spectrum of the quasar Q 0347-382. Our analysis yielded an estimate for the possible deviation
ofμ value in the past, 10 Gyr ago: for the unweighted valueΔ μ / μ = (3.0±2.4)×10-5; for the weightedvalueΔ μ / μ = (5.02±1.82)×10-5.Since the significance of the both results does not exceed3σ, further observations are needed to increase the statistical
significance. In any case, this result may be considered as the most stringent estimate on an upper limit of a possible variation
of μ (95% C.L.):|Δ μ / μ| < 8× 10-5 .This value serves as an effective tool for selection of models determining a relation between possible cosmological deviations
of the fine-structure constant α and the elementary particle masses (mp, me, etc.).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
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