We develop the general program of the unification of matter-dominated era with acceleration epoch for scalar-tensor theory or dark fluid. The general reconstruction of single scalar-tensor theory is fulfilled. The explicit form of scalar potential for which the theory admits matter-dominated era, transition to acceleration and (asymptotically deSitter) acceleration epoch consistent with WMAP data is found. The interrelation of the epochs of deceleration-acceleration transition and matter dominance-dark energy transition for dark fluids with general EOS is investigated. We give several examples of such models with explicit EOS (using redshift parametrization) where matter-dark energy domination transition may precede the deceleration-acceleration transition. As some byproduct, the reconstruction scheme is applied to scalar-tensor theory to define the scalar potentials which may produce the dark matter effect. The obtained modification of Newton potential may explain the rotation curves of galaxies.

The renormalization group (RG) approach to cosmology is an efficient method to study the possible evolution of the cosmological parameters from the point of view of quantum field theory (QFT) in curved space-time. In this work we continue our previous investigations of the RG method based on potential low-energy effects induced from physics at very high energy scales M X M P . In the present instance we assume that both the Newton constant, G, and the cosmological term, Λ, can be functions of a scale parameter µ. It turns out that G(µ) evolves according to a logarithmic law which may lead to asymptotic freedom of gravity, similar to the gauge coupling in QCD. At the same time Λ(µ) evolves quadratically with µ. We study the consistency and cosmological consequences of these laws when µ ≃ H. Furthermore, we propose to extend this method to the astrophysical domain after identifying the local RG scale at the galactic level. It turns out that Kepler's third law of celestial mechanics receives quantum corrections that may help to explain the flat rotation curves of the galaxies without introducing the dark matter hypothesis. The origin of these effects (cosmological and astrophysical) could be linked, in our framework, to physics at M X ∼ 10 16−17 GeV .

For cosmologies including scale dependence of both the cosmological and the gravitational constant, an additional consistency condition dictated by the Bianchi identities emerges, even if the energy-momentum tensor of ordinary matter stays individually conserved. For renormalization-group (RG) approaches it is shown that such a consistency relation ineluctably fixes the RG scale (which may have an explicit as well as an implicit time dependence), provided that the solutions of the RG equation for both quantities are known. Hence, contrary to the procedures employed in the recent literature, we argue that there is no more freedom in identification of the RG scale in terms of the cosmic time in such cosmologies.We carefully set the RG scale for the RG evolution phrased in a quantum gravity framework based on the hypothetical existence of an infrared (IR) fixed point, for the perturbative regime within the same framework, as well as for an evolution within quantum field theory (QFT) in a curved background. In the latter case, the implications of the scale setting for the particle spectrum are also briefly discussed. Recently, indisputable evidence has been mounting to suggest that the expansion of our universe is accelerating owing to the nonvanishing value of unclustered dark energy with negative pressure, see [1]. The crucial evidence for the existence of dark energy [or the cosmological constant (CC)] relies on the CMB observations [2] which strongly support a spatially flat universe as predicted by inflationary models. By combinations of all data a current picture of the universe emerges, in which about 2/3 of the critical energy density of the present universe is made up by a background dark energy with the parameter of the equation of state −1.38 ≤ w ≤ −0.82 at 95% confidence level [3]. Pressed by these data, theorists now need explain not only why the CC is small, but also why dark-energy domination over ordinary matter density has occurred for redshifts z < ∼ 1 (the coincidence problem). Although models with a truly static CC fit these data well, they have two additional drawbacks (besides the coincidence problem): (1) they cannot theoretically explain why the CC is today small but nonvanishing; (2) they have a problem to ensure a phase of inflation, an epoch in the early universe when the CC dominated other forms of energy density. Assessing the possibility to have dynamical dark energy, rolling scalar field models (quintessence fields) [4] with generic attractor properties [5] that make the present dark energy density insensitive to the broad range of unknown initial conditions, have been aimed at dealing with the coincidence problem. Still, a quintessence potential has to be fine-tuned to yield the present ratio of ordinary matter to quintessence energy density, at the same time allowing a phase dominated by matter so that structure can form; therefore these models cannot address the coincidence problem. In addition, such models may have difficulties in achieving the current quintessence equatio...

We consider the possibility that the total dark energy (DE) of the Universe is made out of two dynamical components of different nature: a variable cosmological term, Λ, and a dynamical "cosmon", X, possibly interacting with Λ but not with matter -which remains conserved. We call this scenario the ΛXCDM model. One possibility for X would be a scalar field χ, but it is not the only one. The overall equation of state (EOS) of the ΛXCDM model can effectively appear as quintessence or phantom energy depending on the mixture of the two components. Both the dynamics of Λ and of X could be linked to high energy effects near the Planck scale. In the case of Λ it may be related to the running of this parameter under quantum effects, whereas X might be identified with some fundamental field (say, a dilaton) left over as a low-energy "relic" by e.g. string theory. We find that the dynamics of the ΛXCDM model can trigger a future stopping of the Universe expansion and can keep the ratio ρ D /ρ m (DE density to matter-radiation density) bounded and of order 1. Therefore, the model could explain the so-called "cosmological coincidence problem". This is in part related to the possibility that the present value of the cosmological term can be Λ 0 < 0 in this framework (the current total DE density nevertheless being positive). However, a cosmic halt could occur even if Λ 0 > 0 because of the peculiar behavior of X as "Phantom Matter". We describe various cosmological scenarios made possible by the composite and dynamical nature of ΛXCDM, and discuss in detail their impact on the cosmological coincidence problem.For instance, consider the so-called "cosmological coincidence problem" [40], to wit: why do we find ourselves in an epoch t = t 0 where the DE density is similar to the matter density (ρ D (t 0 ) ≃ ρ M (t 0 ))? In the ΛCDM model this is an especially troublesome problem because ρ Λ remains constant throughout the entire history of the Universe. In the ordinary dynamical DE models the problem has also its own difficulties. Thus, in a typical quintessence model the matter-radiation energy density ρ m = ρ M + ρ R decreases (with the expansion) faster than the DE density and we expect that in the early epochs ρ m ≫ ρ D , whereas at present and in the future ρ m ≪ ρ D . Therefore, why do we just happen to live in an epoch t 0 where the two functions ρ D (t 0 ) ≃ ρ m (t 0 )? Is this a mere coincidence or there is some other, more convincing, reason? The next question of course is: can we devise a model where the ratio ρ m /ρ D stays bounded (perhaps even not too far from 1) in essentially the entire span of the Universe lifetime? This is certainly not possible neither in the standard ΛCDM model, nor in standard quintessence models 2 . And it is also impossible for a model whose DE consists only of a running Λ [23]. However, in this paper we will show that we can produce a dynamical DE model with such a property. Specifically, we investigate a minimal realization of a composite DE model made out of just two components: a runnin...

One of the main aims in the next generation of precision cosmology experiments will be an accurate determination of the equation of state (EOS) for the dark energy (DE). If the latter is dynamical, the resulting barotropic index ω should exhibit a non-trivial evolution with the redshift. Usually this is interpreted as a sign that the mechanism responsible for the DE is related to some dynamical scalar field, and in some cases this field may behave non-canonically (phantom field). Present observations seem to favor an evolving DE with a potential phantom phase near our time. In the literature there is a plethora of dynamical models trying to describe this behavior. Here we show that the simplest option, namely a model with a variable cosmological term, Λ = Λ(t), leads in general to a non-trivial effective EOS, with index ω e , which may naturally account for these data features. We prove that in this case there is always a "crossing" of the ω e = −1 barrier near our time. We also show how this effect is modulated (or even completely controlled) by a variable Newton's gravitational coupling G = G(t).

The renormalization-group equation for the zero-point energies associated with vacuum fluctuations of massive fields from the Standard Model is examined. Our main observation is that at any scale the running is necessarily dominated by the heaviest degrees of freedom, in clear contradistinction with the Appelquist & Carazzone decoupling theorem. Such an enhanced running would represent a disaster for cosmology, unless a fine-tuned relation among the masses of heavy particles is imposed. In this way, we obtain m H ≃ 550 GeV for the Higgs mass, a value safely within the unitarity bound, but far above the more stringent triviality bound for the case when the validity of the Standard Model is pushed up to the grand unification (or Planck) scale.There are now increasing indications, based on observations on rich clusters of galaxies [1], searches for Type Ia Supernovae [2] and measurements of the cosmic microwave background anisotropy [3], that the today's universe is undergoing a phase of accelerated expansion. This is usually attributed to the presence of a cosmological constant. Although the simplest explanation is a time-independent (i.e. "true") cosmological constant Λ, many scenarios have also been discussed involving a dynamical cosmological constant Λ(t). There have recently been a number of suggestions regarding the nature of the latter, the most popular candidate being known under *

Wikipedia is a popular web-based encyclopedia edited freely and collaboratively by its users. In this paper we present an analysis of Wikipedias in several languages as complex networks. The hyperlinks pointing from one Wikipedia article to another are treated as directed links while the articles represent the nodes of the network. We show that many network characteristics are common to different language versions of Wikipedia, such as their degree distributions, growth, topology, reciprocity, clustering, assortativity, path lengths and triad significance profiles. These regularities, found in the ensemble of Wikipedias in different languages and of different sizes, point to the existence of a unique growth process. We also compare Wikipedias to other previously studied networks.

While there is mounting evidence in all fronts of experimental cosmology for a non-vanishing dark energy component in the Universe, we are still far away from understanding its ultimate nature. A fundamental cosmological constant, Λ, is the most natural candidate, but many dynamical mechanisms to generate an effective Λ have been devised which postulate the existence of a peculiar scalar field (so-called quintessence, and generalizations thereof). These models are essentially ad hoc, but they lead to the attractive possibility of a time-evolving dark energy with a non-trivial equation of state (EOS). Most, if not all, future experimental studies on precision cosmology (e.g. the SNAP and PLANCK projects) address very carefully the determination of an EOS parametrized a la quintessence. Here we show that by fitting cosmological data to an EOS of that kind can also be interpreted as a hint of a fundamental, but time-evolving, cosmological term: Λ = Λ(t). We exemplify this possibility by studying the effective EOS associated to a renormalization group (RG) model for Λ. We find that the effective EOS can correspond to both normal quintessence and phantom dark energy, depending on the value of a single parameter of the RG model. We conclude that behind a non-trivial EOS of a purported quintessence or phantom scalar field there can actually be a running cosmological term Λ of a fundamental quantum field theory.