共查询到20条相似文献,搜索用时 0 毫秒
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D. R. K. Reddy CH. C. S. V. V. R. Murthy R. Venkateswarlu 《Astrophysics and Space Science》2006,301(1-4):79-82
Field equations in the presence of a perfect fluid distribution are obtained in a scalar-tensor theory of gravitation proposed
by Saez and Ballester (Phys. Lett. 113, 1985, 467) with the aid of Einstein–Rosen cylindrically symmetric metric. A static vacuum model and a non-static stiff fluid
model are presented. The physical and geometrical properties of the stiff fluid model are studied. 相似文献
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Morteza Yavari 《Astrophysics and Space Science》2013,348(1):293-302
The f(R) theories of gravity have been interested in recent years. A considerable amount of work has been devoted to the study of modified field equations with the assumption of constant Ricci scalar which may be zero or nonzero. In this paper, the exact vacuum solutions of plane symmetric spacetime are analyzed in f(R) theory of gravity. The modified field equations are studied not only for R=constant but also for general case R≠constant. In particular, we show that the Novotný-Horský and anti-de Sitter spacetimes are the exact solutions of the field equations with the non-zero constant Ricci scalar. Finally, the family of solutions with R≠constant is obtained explicitly which includes the Novotný-Horský, Kottler-Whittaker, Taub and conformally flat spacetimes. 相似文献
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Spatially-homogeneous and anisotropic Bianchi type-III, V, VI0 cosmological models in Rosen's (1973) bimetric theory of gravitation are considered. It is shown that, in each case, when
the soure of the gravitation field is a perfect fluid distribution Bianchi type cosmological models do not exist. Hence vacuum
models are presented and studied.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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D. R. K. Reddy 《Astrophysics and Space Science》2006,305(2):139-141
Explicit field equations of a scalar tensor theory of gravitation proposed by Saez and Ballester are obtained with the aid of Einstein–Rosen cylindrically symmetric metric in the presence of cosmic string source. The field equations being highly non–linear static and non–static cases have been considered separately. It is observed that in the static case the geometric strings do not exist while in the non–static case cosmological model does not exist in this theory. 相似文献
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We compare the moment of inertia (MOI) of a simple hydrostatic, two layer body as determined by the Radau–Darwin Approximation (RDA) to its exact hydrostatic MOI calculated to first order in the parameter q = Ω2R3/GM, where Ω, R, and M are the spin angular velocity, radius, and mass of the body, and G is the gravitational constant. We show that the RDA is in error by less than 1% for many configurations of core sizes and layer densities congruent with those of solid bodies in the Solar System. We then determine the error in the MOI of icy satellites calculated with the RDA due to nonhydrostatic effects by using a simple model in which the core and outer shell have slight degree 2 distortions away from their expected hydrostatic shapes. Since the hydrostatic shape has an associated stress of order ρΩ2R2 (where ρ is density) it follows that the importance of nonhydrostatic effects scales with the dimensionless number σ/ρΩ2R2, where σ is the nonhydrostatic stress. This highlights the likely importance of this error for slowly rotating bodies (e.g., Titan and Callisto) and small bodies (e.g., Saturn moons other than Titan). We apply this model to Titan, Callisto, and Enceladus and find that the RDA-derived MOI can be 10% greater than the actual MOI for nonhydrostatic stresses as small as ∼0.1 bars at the surface or ∼1 bar at the core–mantle boundary, but the actual nonhydrostatic stresses for a given shape change depends on the specifics of the interior model. When we apply this model to Ganymede we find that the stresses necessary to produce the same MOI errors as on Titan, Callisto, and Enceladus are an order of magnitude greater due to its faster rotation, so Ganymede may be the only instance where RDA is reliable. We argue that if satellites can reorient to the lowest energy state then RDA will always give an overestimate of the true MOI. Observations have shown that small nonhydrostatic gravity anomalies exist on Ganymede and Titan, at least at degree 3 and presumably higher. If these anomalies are indicative of the nonhydrostatic anomalies at degree 2 then these imply only a small correction to the MOI, even for Titan, but it is possible that the physical origin of nonhydrostatic degree 2 effects is different from the higher order terms. We conclude that nonhydrostatic effects could be present to an extent that allows Callisto and Titan to be fully differentiated. 相似文献
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We study the dynamical evolution of an f(R) model of gravity in a viscous and anisotropic background which is given by a Bianchi type-I model of the Universe. We find viable forms of f(R) gravity in which one is exactly the Einsteinian model of gravity with a cosmological constant and other two are power law f(R) models. We show that these two power law models are stable with a suitable choice of parameters. We also examine three potentials which exhibit the potential effect of f(R) models in the context of scalar tensor theory. By solving different aspects of the model and finding the physical quantities in the Jordan frame, we show that the equation of state parameter satisfy the dominant energy condition. At last we show that the two power law f(R) models behave like quintessence model at late times and also the shear coefficient viscosity tends to zero at late times. 相似文献
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Field equations in a scalar-tensor theory of gravitation proposed by Saezand Ballester (1985) are obtained with the aid of
(i) Friedmann-type metric (ii) a non static plane symmetric metric and (iii) spatially homogeneous Bianchi type – III metric.
Some cosmological models corresponding to perfect fluid and bulk viscous fluid are presented. Physical and kinematical properties
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
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Field equations in the presence of a perfect fluid distributionc for Einstein-Rosen cylindrically symmetric metric are obtained in the scale-covariant theory of gravitation proposed by Canuto et al. (1977a). A static vacuum model and a non-static cosmological model corresponding to perfect fluid are presented. Physical and Kinematical properties of the models are also discussed. 相似文献
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Spherically symmetric kink space-time is considered in the framework of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) in the presence of a cloud of massive strings with perfect fluid. Solving the field equations of this modified theory of gravity, we observe that cosmic strings and perfect fluid do not survive in this theory of gravitation and in this particular space-time. Hence a vacuum kink model, which is asymptotically flat, is presented. 相似文献
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V. Papoyan 《Astrophysics and Space Science》1986,124(2):335-344
The problem of stationary axisymmetric gravitational fields is formulated within the framework of Generalized Theory of Gravitation. It is shown that solutions of the problem mentioned above may be found, if analogous solutions in General Relativity are obtained. As an illustration a Kerr-like solution is offered. A generation theorem for finding magnetostatic solution from stationary vacuum solutions is proposed. 相似文献
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A relativistic, first-order differential equation is derived for the accumulated moment of inertia of a spherically symmetric
celestial body. An approximate equation is proposed to describe the contribution of relativistic effects to the moment of
inertia of a superdense star. For configurations of an incompressible fluid, this approximation describes the results of the
numerical calculations of Chandrasekhar and Miller to within 5% in the entire range of central pressures from 0 to ∞.
Translated from Astrofizika, Vol. 40, No. 1, pp. 87–96, January–March, 1997. 相似文献
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An axially symmetric Bianchi type-I space-time is considered in the presence of perfect fluid source in the scale-covariant
theory of gravitation formulated by Canuto et al. [1977a, Phys. Rev. Lett. 39, 429]. With the help of special law of variation for Hubble’s parameter proposed by Bermann [1983, Nuovo Cimento 74B, 182] a cosmological model with a negative constant declaration parameter is obtained in this theory. Some physical properties
of the model are also discussed. 相似文献
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W. Petry 《Astrophysics and Space Science》2000,272(4):353-368
The 1-post-Newtonian approximation of perfect fluid in cosmological models of the theory of gravitation in flat space-time
is studied. The equations of motion are given in evolution form. At high redshifts the terms of post-Newtonian approximation
are important in studying the development of inhomogeneities on scales smaller than galaxies but at present time these terms
only take effect on very large scales.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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Field equations are obtained with the aid of higher dimensional Bianchi type-I cosmological model in scale covariant theory
of gravitation in the context of cosmic strings. We present here isotropic and anisotropic solutions of the field equations
and some physical implications of these solutions are briefly discussed. 相似文献