共查询到20条相似文献,搜索用时 15 毫秒
1.
The scalar–tensor theory of gravity with the Higgs field as scalar field is presented. For central symmetry it reproduces
the empirically measured flat rotation curves of galaxies. We approximate the galaxy by a polytropic gas sphere with the polytropic
index γ = 2 and a massive core. 相似文献
2.
Naresh Dadhich 《Pramana》2010,74(6):875-882
We prove the theorem: The second-order quasilinear differential operator as a second-rank divergence-free tensor in the equation
of motion for gravitation could always be derived from the trace of the Bianchi derivative of the fourth-rank tensor, which
is a homogeneous polynomial in curvatures. The existence of such a tensor for each term in the polynomial Lagrangian is a
new characterization of the Lovelock gravity. 相似文献
3.
In this paper, we reconstruct cosmological models in the framework of f(R,T) gravity, where R is the Ricci scalar and T is the trace of the stress-energy tensor. We show that the dust fluid reproduces ΛCDM, phantom–non-phantom era and phantom cosmology. Further, we reconstruct different cosmological models, including the Chaplygin
gas, and scalar field with some specific forms of f(R,T). Our numerical simulation for the Hubble parameter shows good agreement with the BAO observational data for low redshifts,
z<2. 相似文献
4.
In this paper, we study the perturbation property of a scalar field coupling to Einstein’s tensor in the background of the regular black hole spacetimes. Our calculations show that the the coupling constant \(\eta \) imprints in the wave equation of a scalar perturbation. We calculated the quasinormal modes of scalar field coupling to Einstein’s tensor in the regular black hole spacetimes by the 3rd order WKB method. 相似文献
5.
Alejandro?Guarnizo Leonardo?Casta?eda Juan?M.?Tejeiro 《General Relativity and Gravitation》2010,42(11):2713-2728
The main goal of this paper is to get in a straightforward form the field equations in metric f (R) gravity, using elementary variational principles and adding a boundary term in the action, instead of the usual treatment
in an equivalent scalar–tensor approach. We start with a brief review of the Einstein–Hilbert action, together with the Gibbons–York–Hawking
boundary term, which is mentioned in some literature, but is generally missing. Next we present in detail the field equations
in metric f (R) gravity, including the discussion about boundaries, and we compare with the Gibbons–York–Hawking term in General Relativity.
We notice that this boundary term is necessary in order to have a well defined extremal action principle under metric variation. 相似文献
6.
N Banerjee 《Pramana》1985,24(5):701-706
An isotropic homogeneous cosmological model with Robertson-Walker line element is studied in general scalar tensor theory
where the parameterω is a function of the scalar field. The model consists of perfect fluid with the equation of statep=ερ. Exact solutions are obtained in Dicke’s conformally transformed units forε=1 andε=1/3 assuming a functional relationship betweenω and the scalar fieldφ. The properties are compared with vacuum models in this theory. 相似文献
7.
It is well known that the vacuum in the Einstein gravity, which is linear in the Riemann curvature, is trivial in the critical (2+1=3) dimension because vacuum solution is flat. It turns out that this is true in general for any odd critical d=2n+1 dimension where n is the degree of homogeneous polynomial in Riemann defining its higher order analogue whose trace is the nth order Lovelock polynomial. This is the “curvature” for nth order pure Lovelock gravity as the trace of its Bianchi derivative gives the corresponding analogue of the Einstein tensor as defined by Dadhich (2010) [1]. Thus the vacuum in the pure Lovelock gravity is always trivial in the odd critical (2n+1) dimension which means it is pure Lovelock flat but it is not Riemann flat unless n=1 and then it describes a field of a global monopole. Further by adding Λ we obtain the Lovelock analogue of the BTZ black hole. 相似文献
8.
Han Dong Ying-bin Wang Xin-he Meng 《The European Physical Journal C - Particles and Fields》2012,72(5):2002
f(T) theory, a generally modified teleparallel gravity, has been proposed as an alternative gravity model to account for the
dark energy phenomena. Following our previous work [Xin-he Meng and Ying-bin Wang, Eur. Phys. J. (2011), ], we prove that Birkhoff’s theorem holds in a more general context, specifically with the off diagonal tetrad case, in this
communication letter. Then, we discuss, respectively, the results of the external vacuum and internal gravitational field
in the f(T) gravity framework, as well as the extended meaning of this theorem. We also investigate the validity of Birkhoff’s theorem
in the frame of f(T) gravity via a conformal transformation by regarding the Brans–Dicke-like scalar as effective matter, and study the equivalence
between both Einstein frame and Jordan frame. 相似文献
9.
We investigate Einstein theories of gravity, coupled to a scalar field j{\varphi} and point-like matter, which are characterized by a scalar field-dependent matter coupling function eH(j){e^{H(\varphi)}} . We show that under mild constraints on the form of the potential for the scalar field, there are a broad class of Einstein-like
gravity models—characterized by the asymptotic behavior of H—which allow for a non-Newtonian weak-field limit with the gravitational potential behaving for large distances as ln r. The Newtonian term GM/r appears only as sub-leading. We point out that this behavior is also shared by gravity models described by f (R) Lagrangians. The relevance of our results for the building of infrared modified theories of gravity and for modified Newtonian
dynamics is also discussed. 相似文献
10.
Nabamita Banerjee Rajeev Kumar Jain Dileep P. Jatkar 《General Relativity and Gravitation》2008,40(1):93-105
We study interacting scalar field theory non-minimally coupled to gravity in the FRW background. We show that for a specific
choice of interaction terms, the energy–momentum tensor of the scalar field ϕ vanishes, and as a result the scalar field does
not gravitate. The naive space dependent solution to equations of motion gives rise to singular field profile. We carefully
analyze the energy–momentum tensor for such a solution and show that the singularity of the solution gives a subtle contribution
to the energy–momentum tensor. The space dependent solution therefore is not non-gravitating. Our conclusion is applicable
to other space–time dependent non-gravitating solutions as well. We study hybrid inflation scenario in this model when purely
time dependent non-gravitating field is coupled to another scalar field χ. 相似文献
11.
I. Kulnazarov K. Yerzhanov O. Razina Sh. Myrzakul P. Tsyba R. Myrzakulov 《The European Physical Journal C - Particles and Fields》2011,71(7):1698
We study the g-essence model with Yukawa interactions between a scalar field φ and a Dirac field ψ. For the homogeneous, isotropic and flat Friedmann–Robertson–Walker universe filled with the such g-essence, the exact solution
of the model is found. Moreover, we reconstruct the corresponding scalar and fermionic potentials which describe the coupled
dynamics of the scalar and fermionic fields. It is shown that some particular g-essence models with Yukawa interactions correspond
to the usual and generalized Chaplygin gas unified models of dark energy and dark matter. Also we present some scalar–fermionic
Dirac–Born–Infeld models corresponding g-essence models with Yukawa interactions which again describe the unified dark energy–dark
matter system. 相似文献
12.
Sayan K. Chakrabarti 《General Relativity and Gravitation》2007,39(5):567-582
We obtain the quasinormal modes for tensor perturbations of Gauss–Bonnet (GB) black holes in d = 5, 7, 8 dimensions and vector perturbations in d = 5, 6, 7 and 8 dimensions using third order WKB formalism. The tensor perturbation for black holes in d = 6 is not considered because of the fact that the black hole is unstable to tensor mode perturbations. In the case of uncharged
GB black hole, for both tensor and vector perturbations, the real part of the QN frequency increases as the Gauss–Bonnet coupling
(α′) increases. The imaginary part first decreases upto a certain value of α′ and then increases with α′ for both tensor and vector perturbations. For larger values of α′, the QN frequencies for vector perturbation differs slightly from the QN frequencies for tensorial one. It has also been
shown that as α′ → 0, the quasinormal frequencies for tensor and vector perturbations of the Schwarzschild black hole can be obtained. We
have also calculated the quasinormal spectrum of the charged GB black hole for tensor perturbations. Here we have found that
the real oscillation frequency increases, while the imaginary part of the frequency falls with the increase of the charge.
We also show that the quasinormal frequencies for scalar field perturbations and the tensor gravitational perturbations do
not match as was claimed in the literature. The difference in the result increases if we increase the GB coupling. 相似文献
13.
The static spherically symmetric solution for R ± μ
4/R model of f(R) gravity is investigated. We obtain the metric for space-time in the solar system that reduces to the Schwarzschild metric,
when μ tends to zero. For the obtained metric, the deviation from Einstein gravity is very small. This result is different from
the other results have been obtained by equivalence between f(R) gravity and scalar tensor theory. Also it is shown that the vacuum solution in the solar system depends on the shape of
matter distribution which differ from the Einstein’s gravity. 相似文献
14.
It has been shown [Chin. Phys. Lett.25 (2008) 4199] that the generalized second law of thermodynamics holds in Einstein gravity. Here we extend this procedure for Gauss-Bonnet and Lovelock gravities. It is shown that by employing the general expression for temperature Th =|κ|/2π= 1/2πτA (1-τA/2HτA) associated with the apparent horizon of a Friedman Robertson-Walker (FRW) universe and assuming Tm = bTh, we are able to construct conditions for which the generalized second law holds in Gauss Bonnet and Lovelock gravities, where Tm and Th are the temperatures of the source and the horizon respectively. 相似文献
15.
We obtain exact cosmological solutions of a higher derivative theory described by the Lagrangian L=R+2αR
2 in the presence of interacting scalar field. The interacting scalar field potential required for a known evolution of the
FRW universe in the framework of the theory is obtained using a technique different from the usual approach to solve the Einstein
field equations. We follow here a technique to determine potential similar to that used by Ellis and Madsen in Einstein gravity.
Some new and interesting potentials are noted in the presence of R
2 term in the Einstein action for the known behaviours of the universe. These potentials in general do not obey the slow rollover
approximation. 相似文献
16.
Taeyoon Moon Yun Soo Myung Edwin J. Son 《The European Physical Journal C - Particles and Fields》2011,71(10):1777
We study the stability of the f(R)-AdS (Schwarzschild–AdS) black hole obtained from f(R) gravity. In order to resolve the difficulty of solving fourth-order linearized equations, we transform f(R) gravity into scalar–tensor theory by introducing two auxiliary scalars. In this case, the linearized curvature scalar becomes
a dynamical scalaron, showing that all linearized equations are second order. Using the positivity of gravitational potentials
and S-deformed technique allows us to guarantee the stability of f(R)-AdS black hole if the scalaron mass squared satisfies the Breitenlohner–Freedman bound. This is confirmed by computing quasinormal
frequencies of the scalaron for the f(R)-AdS black hole. 相似文献
17.
A generalized geometric Reissner–Nordstrom problem taking into account a geometric scalar field G
44 (x) is treated in the context of the five-dimensional geometric theory of gravitation and electromagnetism. A general solution
is obtained for the corresponding five-dimensional Einstein vacuum equations. The essential contribution of the geometric
scalar field, which can give rise to wormholes, is shown. 相似文献
18.
Christian?G.?B?hmer Francisco?S.?N.?Lobo 《The European Physical Journal C - Particles and Fields》2010,70(4):1111-1118
Recently, Hořava proposed a power counting renormalizable theory for (3+1)-dimensional quantum gravity, which reduces to Einstein
gravity with a non-vanishing cosmological constant in IR, but possesses improved UV behaviors. In this work, we analyze the
stability of the Einstein static universe by considering linear homogeneous perturbations in the context of an IR modification
of Hořava gravity, which implies a ‘soft’ breaking of the ‘detailed balance’ condition. The stability regions of the Einstein
static universe is parameterized by the linear equation of state parameter w=p/ρ and the parameters appearing in the Hořava theory, and it is shown that a large class of stable solutions exists in the respective
parameter space. 相似文献
19.
Sudeshna Mukerji Subenoy Chakraborty 《International Journal of Theoretical Physics》2010,49(10):2446-2455
In this work, Emergent Universe scenario has been developed in Einstein-Gauss-Bonnet (EGB) theory. The universe is chosen
as homogeneous and isotropic FRW model and the matter in the universe has two components—the first one is a perfect fluid
with barotropic equation of state p=ω
ρ (ω, a constant) and the other component is a real or phantom (or tachyonic) scalar field. Various possibilities for the existence
of emergent scenario has been discussed and the results are compared with those in Einstein gravity. 相似文献
20.
The well-known energy problem is discussed in f (R) theory of gravity. We use the generalized Landau–Lifshitz energy–momentum complex in the framework of metric f (R) gravity to evaluate the energy density of plane symmetric solutions for some general f (R) models. In particular, this quantity is found for some popular choices of f (R) models. The constant scalar curvature condition and the stability condition for these models are also discussed. Further,
we investigate the energy distribution of cosmic string spacetime. 相似文献