5 pages, no figures, contribution to the proceedings of the CRAL-IPNL conference "Dark Energy and Dark Matter", Lyon 2008
发表信息：
Dark Energy and Dark Matter: Observations, Experiments and Theories, E. Pécontal, T. Buchert, Ph. Di Stefano and Y. Copin (eds), EAS Publications Series, 36 (2009) 167-171.
A new approach to extraction of quantum vacuum energy, in the context of the accelerated expansion, is proposed, and it is shown that experimentally realistic orders of values can be derived. The idea has been implemented in the framework of the Friedmann-Lemaitre-Robertson-Walker geometry in the language of the effective action in the relativistic formalism of Schwinger's proper time and Seeley-DeWitt's heat kernel expansion.
It has been shown that an improved estimation of quantum vacuum energy can yield not only acceptable but also experimentally sensible results. The very idea consists in a straightforward extraction of gravitationally interacting part of the full quantum vacuum energy by means of gauge transformations. The implementation of the idea has been performed in the formalism of effective action, in the language of Schwinger's proper time and the Seeley-DeWitt heat kernel expansion, in the background of the Friedmann-Robertson-Walker geometry.
The principles of General Relativity allow for a non-vanishing cosmological constant, which can possibly be interpreted at least partially in terms of quantum-fluctuations of matter fields. Depending on sign and magnitude it can cause accelerated or decelerated expansion at certain stages of cosmic evolution. Recent observations in cosmology seem to indicate that we presently live in an accelerated phase. We recall the history and fundamental issues connected with the cosmological constant and then discuss present evidences for a positive value, which causes the accelerated expansion.
We show that the recently suggested explanations of cosmic acceleration by the modification of gravity at small curvature suffer violent instabilities and strongly disagree with the known properties of gravitational interactions.
Interactions between dark matter and dark energy with a given equation of state are known to modify the cosmic dynamics. On the other hand, the strength of these interactions is subject to strong observational constraints. Here we discuss a model in which the transition from decelerated to accelerated expansion of the Universe arises as a pure interaction phenomenon. Various cosmological scenarios that describe a present stage of accelerated expansion, like the LCDM model or a (generalized) Chaplygin gas, follow as special cases for different interaction rates. This unifying view on the homoge…
In this paper we show that by considering a universe dominated by two interacting components a superaccelerated expansion can be obtained from a decelerated one by applying a dual transformation that leaves the Einstein's field equations invariant.
After a short history of the $Λ$-term it is explained why the (effective) cosmological constant is expected to obtain contributions from short-distance-physics, corresponding to an energy scale of at least 100 GeV. The actual tiny value of the cosmological constant in any natural scale of units represents, therefore, one of the deepest mysteries of present day fundamental physics. We also briefly discuss recent astronomical evidence for a cosmologically significant vacuum energy density causing an accelerating expansion of the universe. This arises mainly from the Hubble diagram of type Ia sup…
A vacuum instability due to a massless ghost in a hidden sector can lead to an effective equation of state for dark energy that changes smoothly from $w=-3/2$ at large redshifts, to $w\approx-1.2$ today, to $w=-1$ in the future. We discuss how this ghost can be the Goldstone boson of Lorentz symmetry breaking, and we find that this breaking in the hidden sector should occur at a scale below $\sim$10 KeV. The normal particles that are produced along with the ghosts are then predominantly neutrinos.
A new exactly solvable model for the evolution of relativistic kinetic system interacting with an internal stochastic reservoir under the influence of a gravitational background expansion is established. This model of self-interaction is based on the relativistic kinetic equation for the distribution function defined in the extended phase space. The supplementary degree of freedom is described by the scalar stochastic variable (Langevin source), which is considered to be the constructive element of the effective one-particle force. The expansion of the Universe is shown to be accelerated for t…
We discuss the physics of backreaction-driven accelerated expansion. Using the exact equations for the behaviour of averages in dust universes, we explain how large-scale smoothness does not imply that the effect of inhomogeneity and anisotropy on the expansion rate is small. We demonstrate with an analytical toy model how gravitational collapse can lead to acceleration. We find that the conjecture of the accelerated expansion being due to structure formation is in agreement with the general observational picture of structures in the universe, and more quantitative work is needed to make a det…
It is an accepted practice in cosmology to invoke a scalar field with potential $V(ϕ)$ when observed evolution of the universe cannot be reconciled with theoretical prejudices. Since one function-degree-of-freedom in the expansion factor $a(t)$ can be traded off for the function $V(ϕ)$, it is {\it always} possible to find a scalar field potential which will reproduce a given evolution. I provide a recipe for determining $V(ϕ)$ from $a(t)$ in two cases:(i) Normal scalar field with Lagrangian ${\cal L} = (1/2)\partial_aϕ\partial^aϕ- V(ϕ)$ used in quintessence/dark energy models. (ii) A tachyonic…
Acceleration of the universe is obtained from a model of non-relativistic particles with a short-range attractive interaction, at low enough temperature to produce a Bose-Einstein condensate. Conditions are derived for negative-pressure behavior. In particular, we show that a phantom-accelerated regime at the beginning of the universe solves the horizon problem, consistently with nucleosynthesis.
We find exact exponentially expanding and contracting de Sitter solutions of the spatially homogeneous TeVeS cosmological equations of motion in the vacuum TeVeS model and a power law accelerated expanding solution in thepresence of an additional ideal fluid with equation of state parameter $-5/3 < ω< -1$. A preliminary stability analysis shows that the expanding vacuum solution is stable, while in the ideal fluid case stability depends on model parameter values. These solutions might provide a basis for incorporating early-time inflation or late-time accelerated expansion in TeVeS cosmology.
We study the quantized Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) model minimally coupled to a free massless scalar field. In a previous paper, \cite{fab2}, solutions of this model were constructed as gaussian superpositions of negative and positive modes solutions of the Wheeler-DeWitt equation, and quantum bohmian trajectories were obtained in the framework of the Bohm-de Broglie (BdB) interpretation of quantum cosmology. In the present work, we analyze the quantum bohmian trajectories of a different class of gaussian packets. We are able to show that this new class generates bohmian traje…
A general approach to viable modified $f(R)$ gravity is developed in both the Jordan and the Einstein frames. A class of exponential, realistic modified gravities is introduced and investigated with care. Special focus is made on step-class models, most promising from the phenomenological viewpoint and which provide a natural way to classify all viable modified gravities. One- and two-steps models are explicitly considered, but the analysis is extensible to $N$-step models. Both inflation in the early universe and the onset of recent accelerated expansion arise in these models in a natural, un…
We show that the accelerated expansion of the Universe can be viewed as a crossover phenomenon where the Newton constant and the Cosmological constant are actually scaling operators, dynamically evolving in the attraction basin of a non-Gaussian infrared fixed point, whose existence has been recently discussed. By linearization of the renormalized flow it is possible to evaluate the critical exponents, and it turns out that the approach to the fixed point is ruled by a marginal and a relevant direction. A smooth transition between the standard Friedmann--Lemaitre--Robertson--Walker (FLRW) cosm…
We calculate the current power spectrum of the gravitational waves created at the big bang (and later amplified by the different transitions during the Universe expansion) taking into account the present stage of accelerated expansion. Likewise, we determine the power spectrum in a hypothetical second dust era that would follow the present one if at some future time the dark energy, that supposedly drives the current accelerated expansion, evolved in such a way that it became dynamically equivalent to cold dark matter. The calculated power spectrum as well as the evolution of the density param…
We study inflationary cosmology and the late-time accelerated expansion of the universe in non-minimal Yang-Mills (YM) theory, in which the YM field couples to a function of the scalar curvature. It is shown that power-law inflation can be realized due to the non-minimal YM field-gravitational coupling which maybe caused by quantum corrections. Moreover, it is demonstrated that both inflation and the late-time accelerated expansion of the universe can be realized in a modified YM-$F(R)$ gravity which is consistent with solar system tests. Furthermore, it is shown that this result can be realiz…
It is shown that observed peculiarities of the Universe expansion are an inevitable consequence of the gravitational force properties following from gauge-invariant gravitation equations considered in detail in an author's paper in Annalen der Physik, v.17, 28 (2008).
Recently, there has been suggestions that the apparent accelerated expansion of the universe is due not to a cosmological constant, but rather to inhomogeneities in the distribution of matter. In this work, we investigate a specific class of inhomogeneous models that can be solved analytically, namely the dust-dominated Lemaitre-Tolman-Bondi universe models. We show that they do not permit accelerated cosmic expansion.
The concept of the scalar graviton as the source of the dark matter and dark energy of the gravitaional origin is applied to study the evolution of the isotropic homogeneous Universe. A realistic self-consistent solution to the modified pure gravity equations, which correctly describes the accelerated expansion of the spatially flat Universe, is found and investigated. It is argued that the scenario with the scalar gravitons filling up the Universe may emulate the LCDM model, reducing thus the true dark matter to an artefact.
The fact that the LambdaCDM model fits the observations does not necessarily imply the physical existence of `dark energy'. Dropping the assumption that cold dark matter (CDM) is a perfect fluid opens the possibility to fit the data without dark energy. For imperfect CDM, negative bulk pressure is favoured by thermodynamical arguments and might drive the cosmic acceleration. The coincidence between the onset of accelerated expansion and the epoch of structure formation at large scales might suggest that the two phenomena are linked. A specific example is considered in which effective (anti-fri…
We consider a single field governed expansion of the universe from a five dimensional (5D) vacuum state. Under an appropiate change of variables the universe can be viewed in a effective manner as expanding in 4D with an effective equation of state which describes different epochs of its evolution. In the example here worked the universe fistly describes an inflationary phase, followed by a decelerated expansion. Thereafter, the universe is accelerated and describes a quintessential expansion to finally, in the future, be vacuum dominated.
A set of cosmological models that takes into account the variation of the particle number is presented. In this context both dark matter and dark energy can be explained using a single component, without assuming any exotic equation of state, solving directly the cosmic coincidence problem.
We study the dynamics of the quantum scalar field responsible for inflation in different epochs of the evolution of the universe by using a recently introduced nonperturbative formalism from a 5D apparent vacuum.
We explain an accelerated expansion of the present universe, suggested from observations of supernovae of type Ia at high redshift, by introducing an anti-frictional force that is self-consistently exerted on the particles of the cosmic substratum. Cosmic anti-friction, which is intimately related to ``particle production'', is shown to give rise to an effective negative pressure of the cosmic medium. While other explanations for an accelerated expansion (cosmological constant, quintessence) introduce a component of dark energy besides ``standard'' cold dark matter (CDM) we resort to a phenome…
The present paradigm in cosmology is the usual Big-Bang Cosmology in which two stages of accelerated expansion are incorporated: the inflationary phase in the very early universe which produces the classical inhomogeneities observed in the universe, and a second stage of acceleration at the present time as the latest Supernovae observations seem to imply. Both stages could be produced by a scalar field and observations will strongly constrain the microscopic lagrangian of any proposed model.
The Universe is filled with relic neutrinos, remnants from the Leptonic Era. Since the formation of galaxies started, gravitation has modified the Fermi-Dirac momentum distribution of these otherwise decoupled particles. Decelerated neutrinos moving toward the field-free regions between galaxies could violate the Pauli principle. The fermion degeneracy pressure resulting from this leads to an accelerated motion of galaxies away from one another. We show that this model not only offers a natural explanation for the accelerated expansion of the Universe, but also allows a straightforward calcula…
A model of an accelerated expansion of the Crab Nebula powered by the spinning-down Crab pulsar is proposed, in which time dependence of the acceleration is connected with evolution of pulsar luminosity. Using recent observational data, we derive estimates of the Crab neutron-star moment of inertia. Correlations between the neutron star moment of inertia and its mass and radius allow for rough estimates of the Crab neutron-star radius and mass. In contrast to the previously used constant-acceleration approximation, even for the expanding nebula mass ~7 M_sun results obtained within our model d…
We test the present expansion of the universe using supernova type Ia data without making any assumptions about the matter and energy content of the universe or about the parameterization of the deceleration parameter. We assume the cosmological principle to apply in a strict sense. The result strongly depends on the data set, the light-curve fitting method and the calibration of the absolute magnitude used for the test, indicating strong systematic errors. Nevertheless, in a spatially flat universe there is at least a 5 sigma evidence for acceleration which drops to 1.8 sigma in an open unive…
Rather uncomplicated calculations by hand display a surprising connection between the energy density of the vacuum and the diameter and age of the universe. Among other things, the result explains the observation of the accelerated expansion of the universe.
In a recent paper, a cosmological model based on El Naschie {\it E} infinity cantorian spacetime was presented [Iovane, G.; Chaos, Solitons and Fractals, 20: 657-667, (2004)]. In that work it was claimed that the present accelerated expansion of the Universe can be obtained as the effect of a scaling law on newtonian cosmology with a certain time dependent gravitational constant ($G$). In the present work we show that it may be problematic to explain the present accelerated expansion of the Universe using the approach presented in \cite{iovane}. As a better alternative we apply the same scalin…
In a class of spatially homogeneous cosmologies including those of Bianchi type I--VIII mathematical results are presented which show that a scalar field non-minimally coupled to the scalar curvature of spacetime can dynamically yield a positive cosmological constant without the potential being required to include one. More precisely, it is shown that in an exponential potential any positive coupling constant leads eventually to late-time de Sitter expansion and isotropization corresponding to a positive cosmological constant and that this behaviour is independent of the steepness of the poten…
