共查询到20条相似文献，搜索用时 31 毫秒
1.
Recent cosmological observations of large-scale structures (red shift of type Ia supernovae) confirm that the universe is
currently expanding at an accelerating rate and its dominant component is dark energy. This has stimulated the development
of the theory of gravity and led to many alternative variants, including tensor-scalar ones. This paper deals with the role
of conformal transformations in the Jordan-Brans-Dicke theory. Variants of intrinsic, conformally coupled, and Einstein representations
are examined. In the Einstein representation an exact analytic solution for the standard cosmological model is obtained. It
is expressed in terms of the relative energy contributions of ordinary matter Ω
m
, the scalar field Ω
CK
, and a term ΩΛ related to the cosmological constant Λ . Information on the evolution of the universe for the case with a minimally coupled
scalar field is given in the form of graphs. 相似文献
2.
We consider Brans-Dicke theory with a self-interacting potential in Einstein conformal frame. We introduce a class of solutions
in which an accelerating expansion is possible in a spatially flat universe for positive and large values of the Brans-Dicke
parameter consistent with local gravity experiments. In this Einstein frame formulation, the theory appears as an interacting
quintessence model in which the interaction term is given by the conformal transformation. In such an interacting model, we
shall show that the solutions lead simultaneously to a constant ratio of energy densities of matter and the scalar field. 相似文献
3.
This paper discusses the effect of neutrinos with non-zero rest-mass on the clustering process in the early universe. The early universe is regarded as a two-component fluid, one component being the de-coupled neutrinos, and the other being matter and radiation, between the two there is only the gravitational coupling. The main conclusions are: (1) such neutrinos will cause clustering of matter before the epoch of re-combination; (2) the mass so clustered will be in the range of the mass of clusters of galaxies; (3) there exists a preferential clustering scale, corresponding to the earliest onset of Jeans instability; (4) if the rest-mass is below a certain value, then there will be no effect. 相似文献
4.
Evolution of the universe is discussed in the framework of f(R) theory of gravity. The deceleration parameter is used to interpret various phases of the universe. We investigate the future evolution of the flat FRW universe by using observationally viable f(R) models. A numerical technique is applied to solve the evolution equation in terms of Hubble parameter which is used to explore late time acceleration of the universe. Some novel and interesting results based on the choice of coupling parameters in gravitational action are obtained. We can conclude that the considered f(R) models imply unification of matter dominated epoch with present accelerating phase of the universe. 相似文献
5.
V. K. Oikonomou 《Astrophysics and Space Science》2014,352(2):925-935
In this paper we consider the implications that the effect gravitational memory would have on primordial black holes, within the theoretical context of F(R) related scalar-tensor theories. As we will demonstrate, under the assumption that the initial mass of the primordial black hole is such so that it evaporates today, this can potentially constrain the F(R) related theories of gravity. We study two scalar-tensor models and discuss the evolution of primordial black holes created at some initial time t f in the early universe. The results between the two models vary significantly which shows us that, if the effect of gravitational memory is considered valid, some of the scalar-tensor models and their corresponding F(R) theories must be further constrained. 相似文献
6.
The nature of scalar field potentials plays a dominant role in the cosmological dynamics of scalar-tensor gravity. The superpotential reconstruction technique is an interesting way to determine the nature of scalar field potentials by taking the Hubble parameter as a function of scalar field. The present study is an application of this technique in the gravitational framework of scalar-tensor gravity using LRS Bianchi type I universe. We explore the nature of scalar field potentials for some particular cases. It is found that the potentials in all cases turn out to be of polynomial nature and the anisotropy parameter m classifies its degree. The graphical behavior of the directional Hubble parameter shows monotonic behavior which is in contrast to the FRW case. 相似文献
7.
Antonaldo Diaferio Luisa Ostorero 《Monthly notices of the Royal Astronomical Society》2009,393(1):215-223
We run adiabatic N -body/hydrodynamical simulations of isolated self-gravitating gas clouds to test whether conformal gravity, an alternative theory to general relativity, is able to explain the properties of X-ray galaxy clusters without resorting to dark matter. We show that the gas clouds rapidly reach equilibrium with a density profile which is well fitted by a β-model whose normalization and slope are in approximate agreement with observations. However, conformal gravity fails to yield the observed thermal properties of the gas cloud: (i) the mean temperature is at least an order of magnitude larger than the observed and (ii) the temperature profiles increase with the square of the distance from the cluster centre, in clear disagreement with real X-ray clusters. These results depend on a gravitational potential whose parameters reproduce the velocity rotation curves of spiral galaxies. However, this parametrization stands on an arbitrarily chosen conformal factor. It remains to be seen whether a different conformal factor, specified by a spontaneous breaking of the conformal symmetry, can reconcile this theory with observations. 相似文献
8.
We study the dynamical interactions of mass systems in equilibrium under their own gravity that mutually exert and ex‐perience gravitational forces. The method we employ is to model the dynamical evolution of two isolated bars, hosted within the same galactic system, under their mutual gravitational interaction. In this study, we present an analytical treatment of the secular evolution of two bars that oscillate with respect to one another. Two cases of interaction, with and without geometrical deformation, are discussed. In the latter case, the bars are described as modified Jacobi ellipsoids. These triaxial systems are formed by a rotating fluid mass in gravitational equilibrium with its own rotational velocity and the gravitational field of the other bar. The governing equation for the variation of their relative angular separation is then numerically integrated, which also provides the time evolution of the geometrical parameters of the bodies. The case of rigid, non‐deformable, bars produces in some cases an oscillatory motion in the bodies similar to that of a harmonic oscillator. For the other case, a deformable rotating body that can be represented by a modified Jacobi ellipsoid under the influence of an exterior massive body will change its rotational velocity to escape from the attracting body, just as if the gravitational torque exerted by the exterior body were of opposite sign. Instead, the exchange of angular momentum will cause the Jacobian body to modify its geometry by enlarging its long axis, located in the plane of rotation, thus decreasing its axial ratios. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
In this paper, using the energy definition in MØller’s tetrad theory of gravity we calculate the total energy of the universe in Bianchi-type I cosmological models which includes both the matter and gravitational fields. The total energy is found to be zero and this result agrees with a previous works of Banerjee and Sen who investigated this problem using the general relativity version of the Einstein energy-momentum complex and Xulu who investigated same problem using the general relativity versions of the Landau and lifshitz, Papapetrou and Weinberg’s energy-momentum complexes. The result that total energy of the universe in Bianchi-type I universes is zero supports the viewpoint of Tryon. 相似文献
10.
本文讨论了无碰撞粒子系统中简并度对引力不稳定性的影响,计算了塌缩时间的Ｌａｎｄａｕ阻尼时间．在研究费密型暗物质占主导地位的宇宙中星系形成的理论时,系统分布函数的简并度对引力不稳定性的影响有重要意义． 相似文献
11.
On the influence of degeneracy on gravitational instability 总被引：1，自引：0，他引：1
The influence of degeneracy on gravitational instability in a collisionless particle system with a semi-degenerate distribution function is discussed. The collapse time and the Landau damping time are calculated. The influence of degeneracy on Jeans wave number and on stability have important implications in the theory of galaxy formation in a universe dominated by fermionic dark matter. 相似文献
12.
In this paper it is shown that in non-minimally coupled Brans-Dicke theory containing a self-interacting potential, a suitable conformal transformation can automatically give rise to an interaction between the normal matter and the Brans-Dicke scalar field. Considering the scalar field in the Einstein frame as the quintessence matter, it has been shown that such a non-minimal coupling between the matter and the scalar field can give rise to a late time accelerated expansion for the universe preceded by a decelerated expansion for very high values of the Brans-Dicke parameter ω. We have also studied the observational constraints on the model parameters considering the Hubble and Supernova data. 相似文献
13.
In this work the collapsing process of a spherically symmetric star, made of dust cloud, in the background of dark energy is studied for two different gravity theories separately, i.e., DGP Brane gravity and Loop Quantum gravity. Two types of dark energy fluids, namely, Modified Chaplygin gas and Generalised Cosmic Chaplygin gas are considered for each model. Graphs are drawn to characterize the nature and the probable outcome of gravitational collapse. A comparative study is done between the collapsing process in the two different gravity theories. It is found that in case of dark matter, there is a great possibility of collapse and consequent formation of Black hole. In case of dark energy possibility of collapse is far lesser compared to the other cases, due to the large negative pressure of dark energy component. There is an increase in mass of the cloud in case of dark matter collapse due to matter accumulation. The mass decreases considerably in case of dark energy due to dark energy accretion on the cloud. In case of collapse with a combination of dark energy and dark matter, it is found that in the absence of interaction there is a far better possibility of formation of black hole in DGP brane model compared to Loop quantum cosmology model. 相似文献
14.
Farkhat Zaripov 《Astrophysics and Space Science》2014,352(1):289-305
This research is an extension of the author’s works, in which conformally invariant generalization of string theory was suggested to higher-dimensional objects. Special cases of the proposed theory are Einstein’s theory of gravity and string theory. This work is devoted to the formation of self-consistent equations of the theory of induced gravity in the presence of matter in the form of a perfect fluid that interacts with scalar fields. The study is done to solve these equations for the case of the cosmological model. In this model time-evolving gravitational and cosmological “constants” take place which are determined by the square of scalar fields. The values of which can be matched with the observational data. The equations that describe the theory have solutions that can both match with the solutions of the standard theory of gravity as well as it can differ from it. This is due to the fact that the fundamental “constants” of the theory, such as gravitational and cosmological, can evolve over time and also depend of the coordinates. Thus, in a rather general case the theory describes the two systems (stages): Einstein and “evolving”. This process is similar to the phenomenon of phase transition, where the different phases (Einstein gravity system, but with different constants) transit into each other. 相似文献
15.
In this paper, we search the existence of Bianchi type I cosmological model in f(R,T) gravity, where the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar R and of the trace of the stress-energy tensor T. We obtain the gravitational field equations in the metric formalism, and reconstruct the corresponding f(R,T) functions. Attention is attached to the special case, f(R,T)=f 1(R)+f 2(T) and two examples are assumed for this model. In the first example, we consider the unification of matter dominated and accelerated phases with f(R) gravity in anisotropic universe, and in the second instance, model of f(R,T) gravity with transition of matter dominated phase to the acceleration phase is obtained. In both cases, f(R,T) is proportional to a power of R with exponents depending on the input parameters. 相似文献
16.
In this work, we have assumed the generalized Vaidya solution in Lovelock theory of gravity in (n+2)-dimensions. It has been shown that Gauss-Bonnet gravity, dimensionally continued Lovelock gravity and pure Lovelock gravity
can be constructed by suitable choice of parameters. We have investigated the occurrence of singularities formed by the gravitational
collapse in above three particular forms of Lovelock theory of gravity. The dependence of the nature of singularity on the
existence of radial null geodesic for Vaidya space-time has been specially considered. In all the three models, we have shown
that the nature of singularities (naked singularity or black hole) completely depend on the parameters. Choices of various
parameters are shown in tabular form. In Gauss-Bonnet gravity theory, it can be concluded that the possibility of naked singularity
increases with increase in dimensions. In dimensionally continued Lovelock gravity, the naked singularity is possible for
odd dimensions for several values of parameters. In pure Lovelock gravity, only black hole forms due to the gravitational
collapse for any values of parameters. It has been shown that when accretion is taking place on a collapsing object, it is
highly unlikely to get a black hole. Finally on considering the phantom era in the expanding universe it is observed that
there is no possibility of formation of a black hole if we are in the Gauss-Bonnet gravity considering the accreting procedure
upon a collapsing object. 相似文献
17.
Garth A. Barber 《Astrophysics and Space Science》2002,282(4):683-730
A theory is described which produces continuous creation by adapting that of Brans-Dicke. The universe is seen to be created
out of the zero point energy field by self-contained gravitational, scalar, and matter fields. The theory is conformally equivalent
to General Relativity in vacuo.Both the Jordan and the Einstein frames are physical and they conserve energy and four-momentum respectively. The conformal
equivalence has the consequence that predictions of the theory in solar system experiments are identical with General Relativity,
but definitive experiments exist which distinguish between the two theories. The cosmological solution yields a linear expansion
with a dynamical density parameter Omega of
anda cold matter density parameter of
, but the universe is closed. The theory is free of the horizon, smoothness and density problems of GR and therefore does
not need Inflation. It does however require an exotic equation of state with negative pressure and it is suggested that this
is provided by a false vacuum or zero point energy determined, and there forelimited by, its field equations thereby overcoming
the ‘lambda problem’.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
本文利用Hartle和Hawking的方法,讨论了具有旋量场的量子宇宙学,得到了相应的Wheeler-De Witt方程。求出了具有旋量场的宇宙波函数。从波函数可以看出,当标度因子α很小时,旋量场的影响很强,具体的形式与初始条件有关,而当标度因子α很大时,旋量场的行为和标量场一样。 相似文献
19.
G. F. R. Ellis W. R. Stoeger 《Monthly notices of the Royal Astronomical Society》2009,398(3):1527-1536
The causal limit usually considered in cosmology is the particle horizon, delimiting the possibilities of causal connection in the expanding Universe. However, it is not a realistic indicator of the effective local limits of important interactions in space–time. We consider here the matter horizon for the Solar system, i.e. the comoving region which has significantly contributed matter to our local physical environment. This lies inside the effective domain of dependence , which (assuming the universe is dominated by dark matter along with baryonic matter and vacuum-energy-like dark energy) consists of those regions that have had a significant active physical influence on this environment through effects such as matter accretion and acoustic waves. It is not determined by the velocity of light c , but by the flow of matter perturbations along their world lines and associated gravitational effects. We emphasize how small a region the perturbations which became our Galaxy occupied, relative to the observable universe – even relative to the smallest scale perturbations detectable in the cosmic microwave background radiation. Finally, looking to the future of our local cosmic domain, we suggest simple dynamical criteria for determining the present domain of influence and the future matter horizon . The former is the radial distance at which our local region is just now separating from the cosmic expansion. The latter represents the limits of growth of the matter horizon in the far future. 相似文献
20.
V. Fayaz H. Hossienkhani A. Farmany M. Amirabadi N. Azimi 《Astrophysics and Space Science》2014,351(1):299-306
It is shown that the acceleration of the universe can be understood by considering a f(T) gravity models. Modified teleparallel gravity theory with the torsion scalar has recently gained a lot of attention as a possible explanation of dark energy. For these f(T) gravity models, a variant of the accelerating cosmology reconstruction program is developed. We consider spatially homogenous and anisotropic Bianchi type I universe in the context of f(T) gravity. The de Sitter, power-law and general exponential solutions are assumed for the scale factor in each spatial direction and the corresponding cosmological models are reconstructed. We reconstruct f(T) theories from two different holographic dark energy models in different time durations. For the holographic dark energy model, the dark energy dominated era with new setting up is chosen for reconstruction, and the Ricci dark energy model, radiation, matter and dark energy dominated time durations are all investigated. Finally we have obtained a modified gravity action consistent with the holographic dark energy scenario. 相似文献