arXiv subjects

Hrvoje Stefancic

Publications and source records attributed to Hrvoje Stefancic.

46 records · Page 3Linked to original sources

arXiv2005

Effective equation of state for dark energy: mimicking quintessence and phantom energy through a variable Lambda

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, Lambda, is the most natural candidate, but many dynamical mechanisms to generate an effective Lambda 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: Lambda=Lambda(t). We exemplify this possibility by studying the effective EOS associated to a renormalization group (RG) model for Lambda. 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 of a fundamental quantum field theory.

astro-ph↗

arXiv2005

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

The dark energy transition between quintessence ($w>-1$) and phantom ($w<-1$) regimes (the crossing of the cosmological constant boundary) is studied using the dark energy equation of state. Models characterized by this type of transition are explicitly constructed and their equation of state is found to be {\em implicitly} defined. The behavior of the more general models with the implicitly defined equation of state, obtained by the generalization of the explicitly constructed models, is studied to gain insight into the necessary conditions for the occurrence of the transition, as well as to investigate the mechanism behind the transition. It is found that the parameters of the generalized models need to satisfy special conditions for the transition to happen and that the mechanism behind the transition is the cancellation of the contribution of the cosmological constant boundary. The aspects of the behavior of the generalized models which are not related to the transition are briefly discussed and the role of the implicitly defined dark energy equation of state in the description of the dark energy evolution is emphasized.

astro-ph↗

arXiv2005

"Expansion" around the vacuum equation of state - sudden future singularities and asymptotic behavior

The dark energy model with the equation of state p_d=-rho_d - A rho_d^alpha is studied. The model comprises and provides realization of several types of singularities in different parameter regimes: the divergence of the dark energy density and pressure at finite time and finite value of the scale factor, the singularity of the "big rip" type and the sudden future singularity recently introduced by Barrow. For parameter choices which lead to a nonsingular expansion of the universe, various types of the asymptotic evolution are found. The entire time evolution of the universe is described both analytically and numerically. The advantages of this dark energy EOS as a parametrization of dark energy are discussed.

astro-ph↗

arXiv2005

Running G and Λat low energies from physics at M_X: possible cosmological and astrophysical implications

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 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 near 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.

hep-ph↗

arXiv2004

Preferential attachment with information filtering - node degree probability distribution properties

A network growth mechanism based on a two-step preferential rule is investigated as a model of network growth in which no global knowledge of the network is required. In the first filtering step a subset of fixed size $m$ of existing nodes is randomly chosen. In the second step the preferential rule of attachment is applied to the chosen subset. The characteristics of thus formed networks are explored using two approaches: computer simulations of network growth and a theoretical description based on a master equation. The results of the two approaches are in excellent agreement. Special emphasis is put on the investigation of the node degree probability distribution. It is found that the tail of the distribution has the exponential form given by $exp(-k/m)$. Implications of the node degree distribution with such tail characteristics are briefly discussed.

cond-mat.dis-nn↗

arXiv2004

The Robin Hood method - a novel numerical method for electrostatic problems based on a non-local charge transfer

We introduce a novel numerical method, named the Robin Hood method, of solving electrostatic problems. The approach of the method is closest to the boundary element methods, although significant conceptual differences exist with respect to this class of methods. The method achieves equipotentiality of conducting surfaces by iterative non-local charge transfer. For each of the conducting surfaces non-local charge transfers are performed between surface elements which differ the most from the targeted equipotentiality of the surface. The method is tested against analytical solutions and its wide range of application is demonstrated. The method has appealing technical characteristics. For the problem with N surface elements, the computational complexity of the method essentially scales with N^alpha, where alpha < 2, the required computer memory scales with N, while the error of the potential decreases exponentially with the number of iterations for many orders of magnitude of the error, without the presence of the Critical Slowing Down. The Robin Hood method has a large potential of application in other classical as well as quantum problems. Some possible applications outside electrostatics are outlined.

physics.comp-ph↗

arXiv2004

Phantom appearance of non-phantom matter

Two cosmological models with non-phantom matter having the same expansion of the universe as phantom cosmologies are constructed. The first model is characterized by the evolving gravitational "constant" $G$ and a dark energy component with a non-conserved energy-momentum tensor. The second model includes two interacting components, the dark energy component and the matter component. Closed form solutions are obtained for the constant values of model parameters and constraints on the parameters of each model from cosmological observations are outlined. For both models it is explicitly shown how the components of each model produce the expansion of the universe characteristic of phantom cosmologies, despite the absence of phantom energy. These findings stress the interpretation of phantom energy as an effective description of the more complex dynamics of non-phantom matter.

astro-ph↗

arXiv2004

Partial rip scenario - a cosmology with a growing cosmological term

A cosmology with the growing cosmological term is considered. If there is no exchange of energy between vacuum and matter components, the requirement of general covariance implies the time dependence of the gravitational constant $G$. Irrespectively of the exact functional form of the cosmological term growth, the universe ends in a de Sitter regime with a constant asymptotic $Λ$, but vanishing $G$. Although there is no divergence of the scale factor in finite time, such as in the "Big Rip" scenario, gravitationally bound systems eventually become unbound. In the case of systems bound by non-gravitational forces, there is no unbounding effect, as the asymptotic $Λ$ is insufficiently large to disturb these systems.

astro-ph↗

arXiv2004

Comparison of social and physical free energies on a toy model

Social free energy has been recently introduced as a measure of social action obtainable in a given social system, without changes in its structure. The authors of this paper argue that social free energy surpasses the gap between the verbally formulated value sets of social systems and the quantitatively based predictions. This point is further developed by analyzing the relation between the social and the physical free energy. Generically, this is done for a particular type of social dynamics. The extracted type of social dynamics is one of many realistic types of the differing proportion of social and economic elements. Numerically, this has been done for a toy model of interacting agents. The values of the social and physical free energies are, within the numerical accuracy, equivalent in the class of non-trivial, quasi-stationary model states.

cond-mat.stat-mech↗

arXiv2004

Generalized phantom energy

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.

astro-ph↗