共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
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
3.
Gabriele Umberto Varieschi 《General Relativity and Gravitation》2010,42(4):929-974
We present an alternative cosmology based on conformal gravity, as originally introduced by H. Weyl and recently revisited
by P. Mannheim and D. Kazanas. Unlike past similar attempts our approach is a purely kinematical application of the conformal
symmetry to the Universe, through a critical reanalysis of fundamental astrophysical observations, such as the cosmological
redshift and others. As a result of this novel approach we obtain a closed-form expression for the cosmic scale factor R(t) and a revised interpretation of the space–time coordinates usually employed in cosmology. New fundamental cosmological parameters
are introduced and evaluated. This emerging new cosmology does not seem to possess any of the controversial features of the
current standard model, such as the presence of dark matter, dark energy or of a cosmological constant, the existence of the
horizon problem or of an inflationary phase. Comparing our results with current conformal cosmologies in the literature, we
note that our kinematic cosmology is equivalent to conformal gravity with a cosmological constant at late (or early) cosmological
times. The cosmic scale factor and the evolution of the Universe are described in terms of several dimensionless quantitites,
among which a new cosmological variable δ emerges as a natural cosmic time. The mathematical connections between all these quantities are described in details and
a relationship is established with the original kinematic cosmology by L. Infeld and A. Schild. The mathematical foundations
of our kinematical conformal cosmology will need to be checked against current astrophysical experimental data, before this
new model can become a viable alternative to the standard theory. 相似文献
4.
C. P. Singh 《International Journal of Theoretical Physics》2006,45(3):519-528
Einstein field equations are considered in zero-curvature Robertson–Walker (R–W) cosmology with perfect fluid source and time-dependent gravitational and cosmological “constants.” Exact solutions of the field equations are obtained by using the ’gamma-law' equation of state p = (γ − 1)ρ in which γ varies continuously with cosmological time. The functional form of γ (R) is used to analyze a wide range of cosmological solutions at early universe for two phases in cosmic history: inflationary phase and Radiation-dominated phase. The corresponding physical interpretations of the cosmological solutions are also discussed. 相似文献
6.
Patricio S. Letelier 《International Journal of Theoretical Physics》2008,47(5):1312-1315
A geometric flow based in the Riemann-Christoffel curvature tensor that in two dimensions has some common features with the
usual Ricci flow is presented. For n dimensional spaces this new flow takes into account all the components of the intrinsic curvature. For four dimensional Lorentzian
manifolds it is found that the solutions of the Einstein equations associated to a “detonant” sphere of matter, as well, as
a Friedman-Roberson-Walker cosmological model are examples of Riemann-Christoffel flows. Possible generalizations are mentioned. 相似文献
7.
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
8.
T. Padmanabhan 《General Relativity and Gravitation》2008,40(2-3):529-564
I review the problem of dark energy focussing on cosmological constant as the candidate and discuss what it tells us regarding
the nature of gravity. Section 1 briefly overviews the currently popular “concordance cosmology” and summarizes the evidence
for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as a
candidate and emphasizes why no other approach really solves the conceptual problems usually attributed to cosmological constant.
Section 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract certain
key ingredients which must be present in any viable solution. In the conventional approach, the equations of motion for matter
fields are invariant under the shift of the matter Lagrangian by a constant while gravity breaks this symmetry. I argue that
until the gravity is made to respect this symmetry, one cannot obtain a satisfactory solution to the cosmological constant
problem. Hence cosmological constant problem essentially has to do with our understanding of the nature of gravity. Section 3 discusses such an alternative perspective on gravity in which the gravitational interaction—described in terms
of a metric on a smooth spacetime—is an emergent, long wavelength phenomenon, and can be described in terms of an effective
theory using an action associated with normalized vectors in the spacetime. This action is explicitly invariant under the
shift of the matter energy momentum tensor T
ab
→ T
ab
+ Λ
gab
and any bulk cosmological constant can be gauged away. Extremizing this action leads to an equation determining the background geometry which gives Einstein’s theory at the lowest order with Lanczos–Lovelock type corrections. In this approach, the observed
value of the cosmological constant has to arise from the energy fluctuations of degrees of freedom located in the boundary
of a spacetime region. 相似文献
9.
G. Börner 《The European physical journal. Special topics》2007,152(1):139-181
Precise astronomical observations of the cosmic expansion
and the anisotropies of the cosmic microwave background have
confirmed the simple cosmological “big bang” models. They have
also produced evidence for a strange composition of the cosmic
matter and energy density. The known baryons contribute only
5 percent to the cosmic substrate, while 25 percent are due to unknown dark matter particles, and 70 percent seem to come
from
a mysterious dark energy component which presently acts like a
cosmological constant accelerating the cosmic expansion. 相似文献
10.
We consider the astrophysical and cosmological implications of the existence of a minimum density and mass due to the presence
of the cosmological constant. If there is a minimum length in nature, then there is an absolute minimum mass corresponding
to a hypothetical particle with radius of the order of the Planck length. On the other hand, quantum mechanical considerations
suggest a different minimum mass. These particles associated with the dark energy can be interpreted as the “quanta” of the
cosmological constant. We study the possibility that these particles can form stable stellar-type configurations through gravitational
condensation, and their Jeans and Chandrasekhar masses are estimated. From the requirement of the energetic stability of the
minimum density configuration on a macroscopic scale one obtains a mass of the order of 1055 g, of the same order of magnitude as the mass of the universe. This mass can also be interpreted as the Jeans mass of the
dark energy fluid. Furthermore we present a representation of the cosmological constant and of the total mass of the universe
in terms of ‘classical’ fundamental constants. 相似文献
11.
Recently, a renormalizable model of gravity has been proposed by Hoř ava, which might be an ultraviolet completion of general
relativity and it reduces to Einstein gravity with a non-vanishing cosmological constant in infrared approximation. Kehagias
and Sfetsos have added a relevant operator proportional to the 3D geometry Ricci scalar to the original Hoř ava-Lifshitz theory
action, which “softly” deviated from detailed-balance. This does not modify the ultraviolet properties of the theory. However,
it modifies the infrared approximation and the Minkowski vacuum can be allowed in the infrared Hořava-Lifshitz gravity theory.
The static spherical symmetric black hole solutions have been obtained in the Hořava-Lifshitz and infrared Hořava-Lifshitz
gravity theory. Based on the metric of the black holes, Hawking radiation of massless scalar particles is investigated using
Damour-Ruffini method. Then the black hole thermodynamics property will also be discussed. 相似文献
12.
In this work, we examine in depth the physical aspects of the archaic universe described by Euclidean 5-sphere geometry, by
using Projective Relativity techniques. We hypothesize that the expansion of the Universe was “ignited” by primordial R processes,
and that the big bang consisted of a spatially extended nucleation process which took place at the end of a pre-cosmic phase,
characterized by the evolution parameter x0\underline{x}_{0}. This parameter, which can be considered a quantum precursor of ordinary physical time, is a coordinate of Euclidean 5-sphere
metrics. It is so possible to avoid many of the difficulties with standard model and to get rid of ad hoc assumptions. A complete solution to Projective General Relativity (PGR) equations is provided, so as to establish univocal
relations between the scale factor R(τ) and cosmic time τ. In this way, the physics and geometry of the cosmological model are specified completely. 相似文献
13.
14.
We consider a D-dimensional model of gravity with non-linear “scalar fields” as a matter source. The model is defined on the product manifold M, which contains n Einstein factor spaces. General cosmological type solutions to the field equations are obtained when n − 1 factor spaces are Ricci-flat, e.g. when one space M 1 of dimension d 1 > 1 has nonzero scalar curvature. The solutions are defined up to solutions to geodesic equations corresponding to a sigma model target space. Several examples of sigma models are presented. A subclass of spherically symmetric solutions is studied and a restricted version of “no-hair theorem” for black holes is proved. For the case d 1 = 2 a subclass of latent soliton solutions is singled out. 相似文献
15.
16.
P. A. Nakaznoy 《Journal of Experimental and Theoretical Physics》2008,107(3):405-412
Basic properties of the Einstein equations modified by a cosmological Λ-term dependent on the Ricci scalar R are considered. We show that in addition to a nonzero divergence of the energy-momentum tensor of the matter and the consequent cold matter mass nonconservation as the Universe expands, this model suggests a significant modification of the equations for the gravitational potential and particle acceleration in the Newtonian approximation. These circumstances allow the necessary criteria for possible functional dependences Λ(R) to be formulated. Nevertheless, by introducing a variable Λ-term, we can look at the problems of dark matter and dark energy anew. In particular, we show that the model in which the cosmological term depends linearly on the Ricci scalar (this corresponds to the approximation of a more complex dependence in the case of low matter densities) makes it possible to satisfactorily describe the rotation curves of galaxies without invoking the dark matter hypothesis and to construct a cosmological model with a variable vacuum energy density, in qualitative agreement with the present views of the early Universe. 相似文献
17.
We study the cosmological evolution of an induced gravity model with a self-interacting scalar field σ and in the presence of matter and radiation. Such model leads to Einstein gravity plus a cosmological constant as a stable attractor among homogeneous cosmologies and is therefore a viable dark-energy (DE) model for a wide range of scalar field initial conditions and values for its positive γ coupling to the Ricci curvature γσ2R. 相似文献
18.
Recent observations on Type-Ia supernovae and low density (Ω
m
=0.3) measurement of matter including dark matter suggest that the present-day universe consists mainly of repulsive-gravity
type ‘exotic matter’ with negative-pressure often said ‘dark energy’ (Ω
x
=0.7). But the nature of dark energy is mysterious and its puzzling questions, such as why, how, where and when about the
dark energy, are intriguing. In the present paper the authors attempt to answer these questions while making an effort to
reveal the genesis of dark energy and suggest that ‘the cosmological nuclear binding energy liberated during primordial nucleo-synthesis
remains trapped for a long time and then is released free which manifests itself as dark energy in the universe’. It is also
explained why for dark energy the parameter
w=-\frac23w=-\frac{2}{3}
. Noting that w=1 for stiff matter and
w=\frac13w=\frac{1}{3}
for radiation;
w=-\frac23w=-\frac{2}{3}
is for dark energy because “−1” is due to ‘deficiency of stiff-nuclear-matter’ and that this binding energy is ultimately
released as ‘radiation’ contributing “
+\frac13+\frac{1}{3}
”, making
w=-1+\frac13=-\frac23w=-1+\frac{1}{3}=-\frac{2}{3}
. When dark energy is released free at Z=80,
w=-\frac23w=-\frac{2}{3}
. But as on present day at Z=0 when the radiation-strength-fraction (δ), has diminished to δ→0, the
w=-1+d\frac13=-1w=-1+\delta\frac{1}{3}=-1
. This, almost solves the dark-energy mystery of negative pressure and repulsive-gravity. The proposed theory makes several
estimates/predictions which agree reasonably well with the astrophysical constraints and observations. Though there are many
candidate-theories, the proposed model of this paper presents an entirely new approach (cosmological nuclear energy) as a
possible candidate for dark energy. 相似文献
19.
Mark Trodden 《General Relativity and Gravitation》2011,43(12):3367-3379
The discovery of cosmic acceleration has raised the intriguing possibility that we are witnessing the first breakdown of General
Relativity on cosmological scales. In this article I will briefly review current attempts to construct theoretically consistent
and observationally viable modifications of gravity that are capable of describing the accelerating universe. I will discuss
f(R) models, and their obvious extensions, and the DGP model as an example of extra-dimensional implementations. I will then
briefly describe the Galileon models and their very recent multifield and curved space extensions—a class of four-dimensional
effective field theories encoding extra dimensional modifications to gravity. 相似文献
20.
F. Darabi 《International Journal of Theoretical Physics》2010,49(9):2113-2120
A model of Einstein-Hilbert action subject to the scale transformation is studied. By introducing a dilaton field as a means
of scale transformation a new action is obtained whose Einstein field equations are consistent with traceless matter with
non-vanishing modified terms together with dynamical cosmological and gravitational coupling terms. The obtained modified
Einstein equations are neither those in f(R) metric formalism nor the ones in f(ℛ) Palatini formalism, whereas the modified source terms are formally equivalent to those of
f(R)=\frac12R2f({\mathcal{R}})=\frac{1}{2}{\mathcal{R}}^{2} gravity in Palatini formalism. The correspondence between the present model, the modified gravity theory, and Brans-Dicke
theory with
w = -\frac32\omega=-\frac{3}{2} is explicitly shown, provided the dilaton field is condensated to its vacuum state. 相似文献