Some impacts of quintessence models on cosmic structure formation

Some physical imprints of quintessence scalar fields on dark matter (DM) clustering are illustrated, and a comparison with the concordance model $\Lambda CDM$ is highlighted. First, we estimate the cosmological parameters for two quintessence models, based on scalar fields rolling down the Ratra-Peebles or Sugra potential, by a statistical analysis of the Hubble diagram of type Ia supernovae. Then, the effect of these realistic dark energy models on large-scale DM clustering is established through N-body simulations. Various effects like large-scale distribution of DM, cluster mass function and halos internal velocities are illustrated. It is found that realistic dark energy models lead to quite different DM clustering, due to a combination of the variation of the equation of state and differences in the cosmological parameters, even at $z=0$. This conclusion contradicts other works in the recent litterature and the importance of considering more realistic models in studying the impact of quintessence on structure formation is highlighted.Comment: 9 pages, 5 figures, in "Albert Einstein Century International Conference", Paris, 18-22 July 2005, AIP Proceedings Conference 861, Pages 858-866, Edited by Jean-Michel ALIMI & Andre FUZF

An Awesome Hypothesis for Dark Energy : The Abnormally Weighting Energy

We introduce the Abnormally Weighting Energy (AWE) hypothesis in which dark energy (DE) is presented as a consequence of the violation of the weak equivalence principle (WEP) at cosmological scales by some dark sector. Indeed, this implies a violation of the strong equivalence principle (SEP) for ordinary matter and consequent cosmic acceleration in the observable frame as well as variation of the gravitational constant. The consequent DE mechanism build upon the AWE hypothesis (i) does not require a violation of the strong energy condition $p<-\rho c^2/3$, (ii) assumes rather non-negligible direct couplings to the gravitational scalar field (iii) offers a natural convergence mechanism toward general relativity (iv) accounts fairly for supernovae data from various couplings and equations of state of the dark sector as well as density parameters very close to the ones of the concordance model $\Lambda CDM$. Finally (v), this AWE mechanism typically ends up with an Einstein-de Sitter expansion regime once the attractor is reached.Comment: 3 pages, 1 figure, prepared for the Proceedings of the 11th Marcel Grossmann Conference, held in Berlin, Germany, July 200

University Optical Fibre Network Access Optimisation: A Case Study

For network access, it is always desired that a fibre optic network connectivity be the major area of cost minimization, while keeping quality of service as high as possible. This is because overall cost of setting up a fibre optic cable route is usually very high in relation to other components of the network. In this study, three buildings clusters were mapped to the University Network Operating Center, and the network was developed as a fail-safe network, so that in the case of damage to a cable or connecting component, users are not cut out. The problem was formulated as a Mathematical Programming problem and solved the resulting transportation problem using MATLAB linear programming solution. Results from the mathematical model shows an optimized cost for which a fibre Optic network connectivity can be further develope

1/R multidimensional gravity with form-fields: stabilization of extra dimensions, cosmic acceleration and domain walls

We study multidimensional gravitational models with scalar curvature nonlinearity of the type 1/R and with form-fields (fluxes) as a matter source. It is assumed that the higher dimensional space-time undergoes Freund-Rubin-like spontaneous compactification to a warped product manifold. It is shown that for certain parameter regions the model allows for a freezing stabilization of the internal space near the positive minimum of the effective potential which plays the role of the positive cosmological constant. This cosmological constant provides the observable late-time accelerating expansion of the Universe if parameters of the model is fine tuned. Additionally, the effective potential has the saddle point. It results in domain walls in the Universe. We show that these domain walls do not undergo inflation.Comment: 10 pages, revtex, 5 eps figures, footnotes and references adde

Convergence of Scalar-Tensor theories toward General Relativity and Primordial Nucleosynthesis

In this paper, we analyze the conditions for convergence toward General Relativity of scalar-tensor gravity theories defined by an arbitrary coupling function $\alpha$ (in the Einstein frame). We show that, in general, the evolution of the scalar field $(\phi)$ is governed by two opposite mechanisms: an attraction mechanism which tends to drive scalar-tensor models toward Einstein's theory, and a repulsion mechanism which has the contrary effect. The attraction mechanism dominates the recent epochs of the universe evolution if, and only if, the scalar field and its derivative satisfy certain boundary conditions. Since these conditions for convergence toward general relativity depend on the particular scalar-tensor theory used to describe the universe evolution, the nucleosynthesis bounds on the present value of the coupling function, $\alpha_0$, strongly differ from some theories to others. For example, in theories defined by $\alpha \propto \mid\phi\mid$ analytical estimates lead to very stringent nucleosynthesis bounds on $\alpha_0$ ($\lesssim 10^{-19}$). By contrast, in scalar-tensor theories defined by $\alpha \propto \phi$ much larger limits on $\alpha_0$ ($\lesssim 10^{-7}$) are found.Comment: 20 Pages, 3 Figures, accepted for publication in Class. and Quantum Gravit