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

The Abnormally Weighting Energy Hypothesis: The origin of the cosmic acceleration

We generalize tensor-scalar theories of gravitation by the introduction of an abnormally weighting type of energy. This theory of tensor-scalar anomalous gravity is based on a relaxation of the weak equivalence principle that is now restricted to ordinary visible matter only. As a consequence, the convergence mechanism toward general relativity is modified and produces naturally cosmic acceleration as an inescapable gravitational feedback induced by the mass-variation of some invisible sector. The cosmological implications of this new theoretical framework are studied. This glimpses at an enticing new symmetry between the visible and invisible sectors, namely that the scalar charges of visible and invisible matter are exactly opposite.Comment: 10 pages, 4 figures, to appear in the AIP proceedings of the 'Invisible Universe International Conference', UNESCO-Paris, June 29-July 3, 200

Non-Abelian Einstein-Born-Infeld-Dilaton Cosmology

The non-abelian Einstein-Born-Infeld-Dilaton theory, which rules the dynamics of tensor-scalar gravitation coupled to a $su(2)$-valued gauge field ruled by Born-Infeld lagrangian, is studied in a cosmological framework. The microscopic energy exchange between the gauge field and the dilaton which results from a non-universality of the coupling to gravity modifies the usual behaviour of tensor-scalar theories coupled to matter fluids. General cosmological evolutions are derived for different couplings to gravitation and a comparison to universal coupling is highlighted. Evidences of cosmic acceleration are presented when the evolution is interpreted in the Jordan physical frame of a matter respecting the weak equivalence principle. The importance for the mechanism of cosmic acceleration of the dynamics of the Born-Infeld gauge field, the attraction role of the matter fluid and the non-universality of the gravitational couplings is briefly outlined.Comment: 31 pages, 9 figures, minor changes, accepted for publication in Phys. Rev. D1

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