Hrvoje Stefancic

Wikipedias: collaborative web-based encyclopedias as complex networks

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.

Effective equation of state for dark energy : Mimicking quintessence and phantom energy through a variable Λ

Abstract 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.

Running G and Lambda at low energies from physics at M(X): Possible cosmological and astrophysical implications

The renormalization group (RG) approach to cosmology is an efficient method for studying 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 . 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 . 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 Keplers 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 GeV.

Generalized phantom energy

Abstract We examine cosmological models with generalized phantom energy (GPE). Generalized phantom energy satisfies the supernegative equation of state, but its evolution with the scale factor is generally independent, i.e., not determined by its equation of state. The requirement of general covariance makes the gravitational constant time-dependent. It is found that a large class of distinct GPE models with different evolution of generalized phantom energy density and gravitational constant, but the same equation of state of GPE have the same evolution of the scale factor of the universe in the distant future. The time dependence of the equation of state parameter determines whether the universe will end in a de Sitter-like phase or diverge in finite time with the accompanying “big rip” effect on the bound structures.

Renormalization group running of the cosmological constant and its implication for the Higgs boson mass in the standard model

AbstractThe renormalization-group equation for the zero-point energies associated with vacuum fluctuationsof massive fields from the Standard Model is examined. Our main observation is that at any scale therunning is necessarily dominated by the heaviest degrees of freedom, in clear contradistinction with theAppelquist & Carazzone decoupling theorem. Such an enhanced running would represent a disaster forcosmology, unless a fine-tuned relation among the masses of heavy particles is imposed. In this way, weobtain m H ≃ 550 GeV for the Higgs mass, a value safely within the unitarity bound, but far above themore stringent triviality bound for the case when the validity of the Standard Model is pushed up to thegrand unification (or Planck) scale. PACS: 14.80.Bn; 95.30.Cq; 98.80.CqKeywords: Cosmological constant; Zero-point energy; Renormalization-group equation; Running;Higgs bosonThere 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 backgroundanisotropy [3], that the today’s universe is undergoing a phase of accelerated expansion. Thisis usually attributed to the presence of a cosmological constant. Although the simplest explana-tion is a time-independent (i.e. “true”) cosmological constant Λ, many scenarios have also beendiscussed involving a dynamical cosmological constant Λ(t). There have recently been a numberof suggestions regarding the nature of the latter, the most popular candidate being known under

Transition from a matter-dominated era to a dark energy universe

We develop a general program of the unification of a matter-dominated era with an acceleration epoch for scalar-tensor theory or a dark fluid. The general reconstruction of the scalar-tensor theory is fulfilled. The explicit form of the scalar potential for which the theory admits a matter-dominated era, a transition to an acceleration, and an (asymptotically de Sitter) acceleration epoch consistent with Wilkinson Microwave Anisotropy Probe data is found. The interrelation of the epochs of deceleration-acceleration transition and matter dominance-dark energy transition for dark fluids with a general equation of state (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 a by-product, 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.

Dark energy transition between quintessence and phantom regimes - an equation of state analysis

The dark energy transition between quintessence (

Renormalization group running of the cosmological constant and the fate of the universe

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Renormalization-group running cosmologies. A Scale-setting procedure

) and phantom (

Hint for quintessence-like scalars from holographic dark energy

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