共查询到20条相似文献,搜索用时 78 毫秒
1.
Yousef Bisabr 《General Relativity and Gravitation》2010,42(5):1211-1219
We introduce a dynamical model to reduce a large cosmological constant to a sufficiently small value. The basic ingredient
in this model is a distinction which has been made between the two unit systems used in cosmology and particle physics. We
have used a conformal invariant gravitational model to define a particular conformal frame in terms of large scale properties
of the universe. It is then argued that the contributions of mass scales in particle physics to the vacuum energy density
should be considered in a different conformal frame. In this manner, a decaying mechanism is presented in which the conformal
factor appears as a dynamical field and plays a key role to relax a large effective cosmological constant. Moreover, we argue
that this model also provides a possible explanation for the coincidence problem. 相似文献
2.
Yousef Bisabr 《International Journal of Theoretical Physics》2004,43(10):2137-2148
We investigate a conformal invariant gravitational model which is taken to hold at early universe. The conformal invariance allows us to make a dynamical distinction between the two unit systems (or conformal frames) usually used in cosmology and elementary particle physics. In this model we argue that when the universe suffers phase transition, the resulting mass scale introduced by particle physics should have a variable contribution to vacuum energy density. This variation is controlled by the conformal factor which is taken as a dynamical field. We then deal with the cosmological consequences of this model. In particular, we shall show that there is an inationary phase at early times. At late times, on the other hand, it provides a mechanism which makes a large effective cosmological constant relax to a sufficiently small value. Moreover, we shall show that the conformal factor acts as a quintessence field that leads the universe to accelerate at late times. 相似文献
3.
Wetterich C 《Physical review letters》2003,90(23):231302
We connect a possible solution for the "cosmological constant problem" to the existence of a (postulated) conformal fixed point in a fundamental theory. The resulting cosmology leads to quintessence, where the present acceleration of the expansion of the universe is linked to a crossover in the flow of coupling constants. 相似文献
4.
Philip D. Mannheim 《General Relativity and Gravitation》2011,43(3):703-750
We present a solution to the cosmological constant, the zero-point energy, and the quantum gravity problems within a single
comprehensive framework. We show that in quantum theories of gravity in which the zero-point energy density of the gravitational
field is well-defined, the cosmological constant and zero-point energy problems solve each other by mutual cancellation between
the cosmological constant and the matter and gravitational field zero-point energy densities. Because of this cancellation,
regulation of the matter field zero-point energy density is not needed, and thus does not cause any trace anomaly to arise.
We exhibit our results in two theories of gravity that are well-defined quantum-mechanically. Both of these theories are locally
conformal invariant, quantum Einstein gravity in two dimensions and Weyl-tensor-based quantum conformal gravity in four dimensions
(a fourth-order derivative quantum theory of the type that Bender and Mannheim have recently shown to be ghost-free and unitary).
Central to our approach is the requirement that any and all departures of the geometry from Minkowski are to be brought about
by quantum mechanics alone. Consequently, there have to be no fundamental classical fields, and all mass scales have to be
generated by dynamical condensates. In such a situation the trace of the matter field energy-momentum tensor is zero, a constraint
that obliges its cosmological constant and zero-point contributions to cancel each other identically, no matter how large
they might be. In our approach quantization of the gravitational field is caused by its coupling to quantized matter fields,
with the gravitational field not needing any independent quantization of its own. With there being no a priori classical curvature,
one does not have to make it compatible with quantization. 相似文献
5.
We study the spontaneous symmetry breaking in a conformally invariant gravitational theory. We particularly emphasize on the
nonminimal coupling of matter fields to gravity. By the nonminimal coupling we consider a local distinction between the conformal
frames of metric of matter fieldsand the metric explicitly entering the vacuum sector. We suppose that these two frames are
conformally related by a dilaton field. We show that the imposition of a condition on the variable mass term of a scalar field
may lead to the spontaneous symmetry breaking. In this way the scalar field may imitate the Higgs field behavior. Attributing
a constant configuration to the ground state of the Higgs field, a Higgs conformal frame is specified. We define the Higgs
conformal frame as a cosmological frame which describes the large scale characteristics of the observed universe. In the cosmological
frame the gravitational coupling acquires a correct value and one no longer deals with the vacuum energy problem. We then
study a more general case by considering a variable configuration for the ground state of Higgs field. In this case we introduce
a cosmological solution of themodel. 相似文献
6.
The double constraint equations in the self-dual gravitational theory containing the cosmological term are derived in 3 + 1 gravity. Furthermore, in order to deeply study the Lorentzian and Euclidean reality conditions for this theory, the relations between constraints are discussed by introducing the double constant conformal transformation and the double complex function method. 相似文献
7.
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
8.
ZHA Chao-Zheng 《中国物理C(英文版)》1987,11(5):699-703
The cosmological term is necessaritly vanishing in a conformal gravitational theory, and the sum of the induced cosmological constants of all the brocken gauge symmetries cancels the curvature of the background spacetime. Thus the sum of all the cosmological constants is vanishing when the background spacetime is Minkowskian. Furthermore, the spontaneous breaking of a gauge symmetry through the Higgs mechanism is always accompanied by a phase transition of the background spacetime. 相似文献
9.
An excruciating issue that arises in mathematical, theoretical and astro-physics concerns the possibility of regularizing classical singular black hole solutions of general relativity by means of quantum theory. The problem is posed here in the context of a manifestly covariant approach to quantum gravity. Provided a non-vanishing quantum cosmological constant is present, here it is proved how a regular background space-time metric tensor can be obtained starting from a singular one. This is obtained by constructing suitable scale-transformed and conformal solutions for the metric tensor in which the conformal scale form factor is determined uniquely by the quantum Hamilton equations underlying the quantum gravitational field dynamics. 相似文献
10.
Ilija Lukačević 《General Relativity and Gravitation》1990,22(7):721-734
Space-times conformal to physical space-time are considered, assuming the nongravitational energy is conservative after the conformal transformation. Necessary and sufficient conditions satisfied by the conformal factor axe found for a given type of transformation of the energy tensor. The weak gravitational field is defined and the coordinate conditions for the existence of conformal factors in such a field are obtained. 相似文献
11.
Solar System tests give nowadays constraints on the estimated value of the cosmological constant, which can be accurately
derived from different experiments regarding gravitational redshift, light deflection, gravitational time-delay and geodesic
precession. Assuming that each reasonable theory of gravitation should satisfy Solar System tests, we use these limits on
the estimated value of the cosmological constant to constrain extended theories of Gravity, which are nowadays studied as
possible theories for cosmological models and provide viable solutions to the cosmological constant problem and the explanation
of the present acceleration of the Universe. We obtain that the estimated values, from Solar System tests, for the parameters
appearing in the extended theories of Gravity are orders of magnitude bigger than the values obtained in the framework of
cosmologically relevant theories. 相似文献
12.
Ken-ji Hamada 《Foundations of Physics》2011,41(5):863-882
We study vertex operators in 4D conformal field theory derived from quantized gravity, whose dynamics is governed by the Wess-Zumino
action by Riegert and the Weyl action. Conformal symmetry is equal to diffeomorphism symmetry in the ultraviolet limit, which
mixes positive-metric and negative-metric modes of the gravitational field and thus these modes cannot be treated separately
in physical operators. In this paper, we construct gravitational vertex operators such as the Ricci scalar, defined as space-time
volume integrals of them are invariant under conformal transformations. Short distance singularities of these operator products
are computed and it is shown that their coefficients have physically correct signs. Furthermore, we show that conformal algebra
holds even in the system perturbed by the cosmological constant vertex operator as in the case of the Liouville theory shown
by Curtright and Thorn. 相似文献
13.
Gianluca Allemandi Monica Capone Salvatore Capozziello Mauro Francaviglia 《General Relativity and Gravitation》2006,38(1):33-60
The debate on the physical relevance of conformal transformations can be faced by taking the Palatini approach into account
gravitational theories. We show that conformal transformations are not only a mathematical tool to disentangle gravitational
and matter degrees of freedom (passing from the Jordan frame to the Einstein frame) but they acquire a physical meaning considering
the bi-metric structure of Palatini approach which allows to distinguish between spacetime structure and geodesic structure.
These facts are relevant at least at cosmological scales, while at small scale (i.e. in the spacetime regions relevant for
observations) the conformal factor is slowly varying and its effects are not relevant. Examples of higher-order and non-minimally
coupled theories are worked out and relevant cosmological solutions in Einstein frame and Jordan frame are discussed showing
that also the interpretation of cosmological observations can drastically change depending on the adopted frame. 相似文献
14.
Hadi Salehi P. Moyassari R. Rashidi 《International Journal of Theoretical Physics》2006,45(9):1757-1763
The aim of this paper is to study the triviality of λ ϕ4 theory in a classical gravitational model. Starting from a conformal invariant scalar tensor theory with a self-interaction term λ ϕ4, we investigate the effect of a conformal symmetry breaking emerging from the gravitational coupling of the large-scale distribution of matter in the universe. Taking in this cosmological symmetry breaking phase the infinite limit of the maximal length (the size of the universe) and the zero limit of the minimal length (the Planck length) implies triviality, i.e. a vanishing coupling constant λ. It suggests that the activity of the self-interaction term λ ϕ4 in the cosmological context implies that the universe is finite and a minimal fundamental length exists. 相似文献
15.
H. Fritzsch 《Progress in Particle and Nuclear Physics》2011,66(2):193-196
We discuss the fundamental constants of physics within the Standard Model of particle physics. In this model there are 28 fundamental constants, e.g. the constant of gravity or the fine-structure constant. We consider possible changes of these constants on the cosmological time scale. 相似文献
16.
Kangujam Priyokumar Singh Ningombam Ibotombi Singh 《International Journal of Theoretical Physics》2011,50(8):2486-2492
We have considered some cosmological solutions with variable gravitational and cosmological constants with bulk viscosity.
It is found that the solutions are singularity free and the deceleration parameter is in general not a constant unless we
assume perfect fluid with equation of state in the standard cosmologies. Moreover, the deceleration parameter is a function
of the scale factor and changes sign with evolution, so our solution is a generalization of those obtained by Arbab I. Arbab.
The introduction of viscosity not only free from singularity but also give the deceleration parameter a freedom to vary with
scale factor. Thus, a viscous cosmological fluid gives a more general situation in the early universe. 相似文献
17.
Naresh Dadhich 《General Relativity and Gravitation》2000,32(6):1009-1023
By resolving the Riemann curvature relative to a unit timelike vector into electric and magnetic parts, we consider duality relations analogous to those in electromagnetic theory. It turns out that the duality transformation implies the Einstein vacuum equation without the cosmological term. The vacuum equation is invariant under interchange of active and passive electric parts, giving rise to the same vacuum solutions but with the opposite sign for the gravitational constant. Further, by modifying the equation it is possible to construct interesting dual solutions to vacuum as well as to flat spacetimes. 相似文献
18.
Peter J. Adams 《International Journal of Theoretical Physics》1983,22(5):461-467
In standard physics quantum field theory is based on a flat vacuum space-time. This quantum field theory predicts a nonzero cosmological constant. Hence the gravitational field equations do not admit a flat vacuum space-time. This dilemma is resolved using the units covariant gravitational field equations. This paper shows that the field equations admit a flat vacuum space-time with nonzero cosmological constant if and only if the canonical LNH is valid. This allows an interpretation of the LNH phenomena in terms of a time-dependent vacuum state. If this is correct then the cosmological constant must be positive. 相似文献
19.
Durmu? A. Demir 《Physics letters. [Part B]》2011,701(4):496-502
We study gravitational properties of vacuum energy by erecting a geometry on the stress-energy tensor of vacuum, matter and radiation. Postulating that the gravitational effects of matter and radiation can be formulated by an appropriate modification of the spacetime connection, we obtain varied geometrodynamical equations which properly comprise the usual gravitational field equations with, however, Planck-suppressed, non-local, higher-dimensional additional terms. The prime novelty brought about by the formalism is that, the vacuum energy does act not as the cosmological constant but as the source of the gravitational constant. The formalism thus deafens the cosmological constant problem by channeling vacuum energy to gravitational constant. Nevertheless, quantum gravitational effects, if any, restore the problem via the graviton and graviton-matter loops, and the mechanism proposed here falls short of taming such contributions to cosmological constant. 相似文献
20.
We study models where the gauge coupling constants, masses and the gravitational constant are functions of some conserved charge in the universe, and furthermore a cosmological constant that depends on the total charge of the universe. We first consider the standard Dirac action, but where the mass and the electromagnetic coupling constant are a function of the charge in the universe and afterwards extend this to curved spacetime and consider gauge coupling constants, the gravitational constant and the mass as a function of the charge of the universe, which represent a sort of Mach principle for all the constants of nature. In the flat space formulation, the formalism is not manifestly Lorentz invariant, however Lorentz invariance can be restored by performing a phase transformation of the Dirac field. One interesting model of this type is one where the action is invariant under rescalings of the Dirac wave function. In the curved space time formulation, there is the additional feature that some of the equations of motion break the general coordinate invariance also, but in a way that can be understood as a coordinate choice only, so the equations are still of the General Relativity type, but with a certain natural coordinate choice, where there is no current of the charge. We have generalized what we have done and also constructed a cosmological constant which depends on the total charge of the universe. We discuss how these ideas work when the space where the charges live is finite. If we were to use some only approximately conserved charge for these constructions, like say baryon number (in the context of the standard model), this will lead to corresponding violations of Lorentz symmetry in the early universe for example. We also briefly discuss another non-local formulations where the coupling constants are functions of the Pontryagin index of some non-abelian gauge field configurations. The construction of charge dependent contributions can also be motivated from the structure of the “infra-red counter terms” needed to cancel infra red divergences for example in three dimensions. 相似文献