共查询到20条相似文献,搜索用时 31 毫秒
1.
The European Physical Journal C - Oscillations of the F(R) dark energy around the phantom divide line, ω DE=−1, both during the matter era and also in the de Sitter epoch are... 相似文献
2.
Guido Cognola Monica Gastaldi Sergio Zerbini 《International Journal of Theoretical Physics》2008,47(4):898-910
Motivated by the dark energy issue, a minisuperspace approach to the stability for modified gravitational models in a four
dimensional cosmological setting is investigated. Specifically, after revisiting the f(R) case, R being the Ricci curvature, we present a stability condition around a de Sitter solution valid for modified gravitational
models of generalized type F(R,G,Q), G and Q being the Gauss-Bonnet and quadratic Riemann invariants respectively. A generalization to higher order invariants is presented. 相似文献
3.
4.
J. Klusoň S. Nojiri S. D. Odintsov D. Sáez-Gómez 《The European Physical Journal C - Particles and Fields》2011,71(7):1690
This paper is devoted to the study of various aspects of projectable F(R) Hořava–Lifshitz (HL) gravity. We show that some versions of F(R) HL gravity may have stable de Sitter solution and unstable flat-space solution. In this case, the problem of scalar graviton
does not appear because flat space is not vacuum state. Generalizing the U(1) HL theory proposed in , we formulate U(1) extension of scalar theory and of F(R) Hořava–Lifshitz gravity. The Hamiltonian approach for such the theory is developed in full detail. It is demonstrated that
its Hamiltonian structure is the same as for the non-relativistic covariant HL gravity. The spectrum analysis performed around
the flat background indicates the consistency of the theory because it contains a graviton with only transverse polarization.
Finally, we analyze the spatially flat FRW equations for U(1) invariant F(R) Hořava–Lifshitz gravity. 相似文献
5.
Generalized from the so-called teleparallel gravity, which is exactly equivalent to general relativity, f(T) gravity has been proposed as an alternative gravity model to account for the dark energy phenomena. In this letter we prove
that the external vacuum gravitational field for a spherically symmetric distribution of source matter in the f(T) gravity framework must be static. The conclusion is independent of the radial distribution and spherically symmetric motion
of the source matter, that is, whether it is in motion or static. As a consequence, the Birkhoff’s theorem is valid in the
general nonsingular f(T) theory at the un-perturbative level. We also discuss its application in the de Sitter spacetime evolution phase as preferred
by present dark energy observations. 相似文献
6.
A. A. Starobinsky 《JETP Letters》2007,86(3):157-163
For higher-derivative f(R) gravity, where R is the Ricci scalar, a class of models is proposed, which produce viable cosmology different from the ACDM at recent times
and satisfy cosmological, Solar System, and laboratory tests. These models have both flat and de Sitter spacetimes as particular
solutions in the absence of matter. Thus, a cosmological constant is zero in a flat spacetime, but appears effectively in
a curved one for sufficiently large R. A “smoking gun” for these models would be a small discrepancy in the values of the slope of the primordial perturbation
power spectrum determined from galaxy surveys and CMB fluctuations. On the other hand, a new problem for dark energy models
based on f(R) gravity is pointed out, which is connected with the possible overproduction of new massive scalar particles (scalarons)
arising in this theory in the very early Universe.
The text was submitted by the author in English. 相似文献
7.
8.
Salvatore Capozziello 《Journal of Russian Laser Research》2010,31(2):129-138
The so-called f(R)-gravity could, in principle, explain the accelerated expansion of the Universe without adding unknown forms of dark energy/dark
matter, but more simply extending the general relativity by generic functions of the Ricci scalar. However, as a part of several
phenomenological models, there is no final f(R)-theory capable of fitting all the observations and addressing all the issues related to the presence of dark energy and
dark matter. Astrophysical observations are pointing out huge amounts of “dark matter” and “dark energy” needed to explain
the observed large-scale structures and cosmic accelerating expansion. Up to now, no experimental evidence has been found,
at a fundamental level, to explain such mysterious components. The problem could be completely reversed considering dark matter
and dark energy as “shortcomings” of general relativity. 相似文献
9.
We investigate propagations of graviton and additional scalar on four-dimensional anti-de Sitter (AdS4) space using f(R) gravity models with external sources. It is shown that there is the van Dam–Veltman–Zakharov (vDVZ) discontinuity in f(R) gravity models because f(R) gravity implies GR with additional scalar. This clearly indicates a difference between general relativity and f(R) gravity. 相似文献
10.
Zhe Chang Ming-Hua Li Xin Li Sai Wang 《The European Physical Journal C - Particles and Fields》2012,72(3):1915
We investigate the possible effect of cosmological-constant type dark energy during the inflation period of the early universe.
This is accommodated by a new dispersion relation in de Sitter space. The modified inflation model of a minimally coupled
scalar field is still able to yield an observation-compatible scale-invariant primordial spectrum, simultaneously having the
potential to generate a spectrum with lower power at large scales. A qualitative match to the WMAP 7-year data is presented.
We obtain an Ω
Λ
of the same order of that in the Λ-CDM model. Possible relations between the de Sitter scenario and Doubly Special Relativity (DSR) are also discussed. 相似文献
11.
Yue-Yue Zhao Ya-Bo Wu Jian-Bo Lu Zhuo Zhang Wei-Li Han Liang-Liang Lin 《The European Physical Journal C - Particles and Fields》2012,72(3):1924
A modified f(G) gravity model with coupling between matter and geometry is proposed, which is described by the product of the Lagrange density
of the matter and an arbitrary function of the Gauss–Bonnet term. The field equations and the equations of motion corresponding
to this model show the non-conservation of the energy-momentum tensor, the presence of an extra force acting on test particles
and non-geodesic motion. Moreover, the energy conditions and the stability criterion at the de Sitter point in modified f(G) gravity models with curvature–matter coupling are derived, which can degenerate to the well-known energy conditions in general
relativity. Furthermore, in order to get some insight in the meaning of these energy conditions, we apply them to the specific
models of f(G) gravity and the corresponding constraints on the models are given. In addition, the conditions and the candidate for late-time
cosmic accelerated expansion in modified f(G) gravity are studied by means of conditions of power-law expansion and the equation of state of matter ω smaller than
-\frac13-\frac{1}{3}. 相似文献
12.
Lixin Xu Jianbo Lu Wenbo Li 《The European Physical Journal C - Particles and Fields》2009,64(1):89-95
In this paper, we consider generalized holographic and Ricci dark energy models where the energy densities are given as ρ
R
=3c
2
M
pl2
Rf(H
2/R) and ρ
h
=3c
2
M
pl2
H
2
g(R/H
2), respectively; here f(x), g(y) are positive defined functions of the dimensionless variables H
2/R or R/H
2. It is interesting that holographic and Ricci dark energy densities are recovered or recovered interchangeably when the function
f(x)=g(y)≡1 or f(x)=Id and g(y)=Id are taken, respectively (for example f(x),g(x)=1−ε(1−x), ε=0or1, respectively). Also, when f(x)≡xg(1/x) is taken, the Ricci and holographic dark energy models are equivalent to a generalized one. When the simple forms f(x)=1−ε(1−x) and g(y)=1−η(1−y) are taken as examples, by using current cosmic observational data, generalized dark energy models are considered. As expected,
in these cases, the results show that they are equivalent (ε=1−η=1.312), and Ricci-like dark energy is more favored relative to the holographic one where the Hubble horizon was taken as
an IR cut-off. And the suggested combination of holographic and Ricci dark energy components would be 1.312R−0.312H
2, which is
2.312H2+1.312[(H)\dot]2.312H^{2}+1.312\dot{H}
in terms of H
2 and
[(H)\dot]\dot{H}
. 相似文献
13.
S. K. Srivastava 《International Journal of Theoretical Physics》2008,47(7):1966-1978
Using modified gravity with non-linear terms of curvature, R
2 and R
(2+r) (with r being a positive real number and R being the scalar curvature), cosmological scenario, beginning at the Planck scale, is obtained. Here a unified picture of
cosmology is obtained from f(R)-gravity. In this scenario, universe begins with power-law inflation followed by deceleration and acceleration in the late universe
as well as possible collapse of the universe in future. It is different from f(R)-dark energy models with non-linear curvature terms assumed as dark energy. Here, dark energy terms are induced by linear as well as non-linear
terms of curvature in Friedmann equation being derived from modified gravity. It is also interesting to see that, in this
model, dark radiation and dark matter terms emerge spontaneously from the gravitational sector. It is found that dark energy,
obtained here, behaves as quintessence in the early universe and phantom in the late universe. Moreover, analogous to brane-tension
in brane-gravity inspired Friedmann equation, a tension term λ arises here being called as cosmic tension, It is found that, in the late universe, Friedmann equation (obtained here) contains
a term −ρ
2/2λ (ρ being the phantom energy density) analogous to a similar term in Friedmann equation with loop quantum effects, if λ>0 and brane-gravity correction when λ<0. 相似文献
14.
Renée Hlozek Marina Cortês Chris Clarkson Bruce Bassett 《General Relativity and Gravitation》2008,40(2-3):285-300
We discuss degeneracies between dark energy and cosmic parameters using a fully non-perturbative and non-parametric approach.
This allows us to examine the knock-on bias induced in the reconstructed dark energy equation of state, w(z), if there is a bias in the cosmic curvature or dark matter content. Assuming perfect Hubble, distance and volume measurements,
we show that for z > 1, the bias in w(z) is up to two orders of magnitude larger than the corresponding errors in Ω
k
or Ω
m
. 相似文献
15.
In this paper we put forward a running coupling scenario for describing the interaction between dark energy and dark matter.
The dark sector interaction in our scenario is free of the assumption that the interaction term Q is proportional to the Hubble expansion rate and the energy densities of dark sectors. We only use a time-variable coupling
b(a) (with a the scale factor of the universe) to characterize the interaction Q. We propose a parametrization form for the running coupling b(a)=b
0
a+b
e
(1−a) in which the early-time coupling is given by a constant b
e
, while today the coupling is given by another constant, b
0. For investigating the feature of the running coupling, we employ three dark energy models, namely, the cosmological constant
model (w=−1), the constant w model (w=w
0), and the time-dependent w model (w(a)=w
0+w
1(1−a)). We constrain the models with the current observational data, including the type Ia supernova, the baryon acoustic oscillation,
the cosmic microwave background, the Hubble expansion rate, and the X-ray gas mass fraction data. The fitting results indicate
that a time-varying vacuum scenario is favored, in which the coupling b(z) crosses the noninteracting line (b=0) during the cosmological evolution and the sign changes from negative to positive. The crossing of the noninteracting line
happens at around z=0.2–0.3, and the crossing behavior is favored at about 1σ confidence level. Our work implies that we should pay more attention to the time-varying vacuum model and seriously consider
the phenomenological construction of a sign-changeable or oscillatory interaction between dark sectors. 相似文献
16.
Astrophysical observations are pointing out huge amounts of “dark matter” and “dark energy” needed to explain the observed
large scale structure and cosmic dynamics. The emerging picture is a spatially flat, homogeneous Universe undergoing the today
observed accelerated phase. Despite of the good quality of astrophysical surveys, commonly addressed as Precision Cosmology, the nature and the nurture of dark energy and dark matter, which should constitute the bulk of cosmological matter-energy,
are still unknown. Furthermore, up to now, no experimental evidence has been found, at fundamental level, to explain such
mysterious components. The problem could be completely reversed considering dark matter and dark energy as “shortcomings”
of General Relativity in its simplest formulation (a linear theory in the Ricci scalar R, minimally coupled to the standard perfect fluid matter) and claiming for the “correct” theory of gravity as that derived
by matching the largest number of observational data, without imposing any theory a priori. As a working hypothesis, accelerating
behavior of cosmic fluid, large scale structure, potential of galaxy clusters, rotation curves of spiral galaxies could be
reproduced by means of extending the standard theory of General Relativity. In other words, gravity could acts in different ways at different scales and the
above “shortcomings” could be due to incorrect extrapolations of the Einstein gravity, actually tested at short scales and
low energy regimes. After a survey of what is intended for Extended Theories of Gravity in the so called “metric” and “Palatini” approaches, we discuss some cosmological and astrophysical applications where the
issues related to the dark components are addressed by enlarging the Einstein theory to more general f (R) Lagrangians, where f (R) is a generic function of Ricci scalar R, not assumed simply linear. Obviously, this is not the final answer to the problem of “dark-components” but it can be considered
as an operative scheme whose aim is to avoid the addition of unknown exotic ingredients to the cosmic pie. 相似文献
17.
M. Hamani Daouda Manuel E. Rodrigues M. J. S. Houndjo 《The European Physical Journal C - Particles and Fields》2012,72(2):1893
We develop the reconstruction of the f(T) gravity model according to the holographic dark energy. T is the torsion scalar and its initial value from the teleparallel gravity is imposed for fitting the initial value of the
function f(T). The evolutionary nature of the holographic dark energy is essentially based on two important parameters, Ω
V
and ω
V
, respectively, the dimensionless dark energy and the parameter of the equation of state, related to the holographic dark
energy. The result shows a polynomial function for f(T), and we also observe that, when Ω
V
→1 at the future time, ω
V
may cross −1 for some values of the input parameter b. Another interesting aspect of the obtained model is that it provides a unification scenario of dark matter with dark energy. 相似文献
18.
A plane-symmetric magnetized inhomogeneous cosmological model of the universe with time dependent gauge function β for perfect fluid distribution with variable magnetic permeability within the framework of Lyra geometry is investigated.
The source of the magnetic field is due to an electric current produced along the z-axis. Thus F
12 is the only non-vanishing component of electromagnetic field tensor F
ij
. To get a deterministic solution of Einstein’s modified field equations, the free gravitational field is assumed to be Petrov
type-II non-degenerate. For our derived model we obtain the deceleration parameter q=−1 as in the case of de Sitter universe. It has been found that the displacement vector β(t) behaves like cosmological term Λ in the normal gauge treatment and the solution is consistent with the observations. The
displacement vector β(t) affects entropy. Some physical and geometric properties of the model are also discussed. 相似文献
19.
Lixin Xu Wenbo Li Jianbo Lu 《The European Physical Journal C - Particles and Fields》2009,60(1):135-140
In this paper, the holographic dark-energy model is considered in Brans–Dicke theory, where the holographic dark-energy density
ρ
Λ
=3c
2
M
pl2
L
−2 is replaced by ρ
h=3c
2
Φ(t)L
−2. Here
is the time-variable Newton constant. With this replacement, it is found that no accelerated expansion for the universe will
be achieved when the Hubble horizon is taken to play the role of an IR cut-off. When the event horizon is adopted as the IR
cut-off, accelerated expansion for the universe is obtained. In this case, the equation of state of holographic dark energy,
w
h, takes the modified form
. In the limit α→0, the ‘standard’ holographic dark energy is recovered. In the holographic dark-energy dominated epoch, power-law and de
Sitter time-space solutions are obtained. 相似文献
20.
Piyali Bagchi Khatua Shuvendu Chakraborty Ujjal Debnath 《International Journal of Theoretical Physics》2012,51(2):405-417
In this work, we have considered dilaton dark energy model in Weyl-scaled induced gravitational theory in presence of barotropic
fluid. It is to be noted that the dilaton field behaves as a quintessence. Here we have discussed the role of dilaton dark
energy in modified gravity theories, namely f(R),f(T) and Hořava-Lifshitz gravities and analyzed the behavior of the dilaton field and the corresponding potential in respect
to these modified gravity theories instead of Einstein’s gravity. In f(R) and f(T) gravities, we have considered some particular forms of f(R) and f(T) and we have shown that the potentials always increase with the dilaton fields. But in Hořava-Lifshitz gravity, it has been
seen that the potential always decreases as dilation field increases. 相似文献