Generalized Brans-Dicke cosmology in the presence of matter and dark energy

We study the Generalized Brans-Dicke cosmology in the presence of matter and dark energy. Of particular interest for a constant Brans-Dicke parameter, the de Sitter space has also been investigated.Comment: 9 page

Cosmic Evolution in Brans-Dicke Chameleon Cosmology

We have investigated the Brans-Dicke Chameleon theory of gravity and obtained exact solutions of the scale factor $a(t)$, scalar field $\phi(t)$, an arbitrary function $f(\phi)$ which interact with the matter Lagrangian in the action of the Brans-Dicke Chameleon theory and potential $V(\phi)$ for different epochs of the cosmic evolution. We plot the functions $a(t)$, $\phi(t)$, $f(t)$ and $V(\phi)$ for different values of the Brans-Dicke parameter. In our models, there is no accelerating solution, only decelerating one with $q>0$. The physical cosmological distances have been investigated carefully. Further the statefinder parameters pair and deceleration parameter are discussed.Comment: To be appear in "The European Physical Journal - Plus (EPJ Plus)",Extended version,15 pages, 17eps figure

Reconciling MOND and dark matter?

Observations of galaxies suggest a one-to-one analytic relation between the inferred gravity of dark matter at any radius and the enclosed baryonic mass, a relation summarized by Milgrom's law of modified Newtonian dynamics (MOND). However, present-day covariant versions of MOND usually require some additional fields contributing to the geometry, as well as an additional hot dark matter component to explain cluster dynamics and cosmology. Here, we envisage a slightly more mundane explanation, suggesting that dark matter does exist but is the source of MOND-like phenomenology in galaxies. We assume a canonical action for dark matter, but also add an interaction term between baryonic matter, gravity, and dark matter, such that standard matter effectively obeys the MOND field equation in galaxies. We show that even the simplest realization of the framework leads to a model which reproduces some phenomenological predictions of cold dark matter (CDM) and MOND at those scales where these are most successful. We also devise a more general form of the interaction term, introducing the medium density as a new order parameter. This allows for new physical effects which should be amenable to observational tests in the near future. Hence, this very general framework, which can be furthermore related to a generalized scalar-tensor theory, opens the way to a possible unification of the successes of CDM and MOND at different scales.Comment: 9 page

Statistics of Cosmological Inhomogeneities

This contribution to the Proceedings is based on the talk given at the Conference on Birth of the Universe and Fundamental Physics, Rome, May 18-21, 1994. Some selected topics of the subject are reviewed: Models of Primordial Fluctuations; Reconstruction of the Cosmological Density Probability Distribution Function (PDF) from Cumulants; PDFs from the Zel'dovich Approximation and from Summarizing Perturbation Series; Fitting by the Log-normal Distribution.Comment: 11 pages, 3 figures (available from the author), LaTe

Large scale density profiles surrounding halos as cosmological probe

International audienceThe understanding of the cosmic acceleration led to the introduction of the cosmological constant behind the standard ΛCDM model. However, there are many hints of the possibility of more complex Dark Energy which could be understand, at first order, as a Dark Fluid with constant equation of state pDE = w × ρDE. It could also results from a modification of the essence of Gravity such as described in modified gravity models

Ponderomotive effects in a magnetized plasma

Results concerning the ponderomotive effects in a collisionless magnetized plasma are presented. The main result concerns the fundamental difference between a three-dimensional plasma, which follows a Boltzmann equilibrium, and a two-dimensional plasma, where the high frequency field and the plasma inhomogeneities are perpendicular to the background magnetic field, and where the nonlinear response of the plasma depends on the value of the parameter ω p L/c, where L is the scale length of the envelope of the high frequency field. Previous results of Porkolab and Goldman are corrected and generalized. Finite Larmor radius effects are also studied, and a connexion with the work of Cary and Kaufman on ponderomotive Hamiltonian is made.Des résultats concernant les effets pondéromoteurs pour un plasma magnétisé sans collision sont présentés. Le principal résultat concerne la différence fondamentale entre un plasma à trois dimensions, qui suit l'équilibre de Boltzmann, et un plasma à deux dimensions, c'est-à-dire un plasma où le champ haute fréquence et l'inhomogénéité du plasma sont perpendiculaires au champ magnétique extérieur. Pour ce dernier la réponse non linéaire dépend de la valeur du paramètre ωp L/c, où L est l' échelle de longueur de l'enveloppe du champ haute fréquence. Les précédents résultats de Porkolab et Goldman sont corrigés et généralisés. Les effets de rayon de Larmor finis sont également étudiés, et un lien avec les travaux de Cary et Kaufman sur l'hamiltonien pondéromoteur est fait

Elliptic solutions of generalized Brans–Dicke gravity with a non-universal coupling

We study a model of the generalized Brans–Dicke gravity presented in both the Jordan and in the Einstein frames, which are conformally related. We show that the scalar field equations in the Einstein frame are reduced to the geodesics equations on the target space of the nonlinear sigma model. The analytical solutions in elliptical functions are obtained when the conformal couplings are given by reciprocal exponential functions. The behavior of the scale factor in the Jordan frame is studied using numerical computations. For certain parameters the solutions can describe an accelerated expansion. We also derive an analytical approximation in exponential functions. © 2014, The Author(s)

Elliptic solutions of generalized Brans–Dicke gravity with a non-universal coupling

We study a model of the generalized Brans–Dicke gravity presented in both the Jordan and in the Einstein frames, which are conformally related. We show that the scalar field equations in the Einstein frame are reduced to the geodesics equations on the target space of the nonlinear sigma model. The analytical solutions in elliptical functions are obtained when the conformal couplings are given by reciprocal exponential functions. The behavior of the scale factor in the Jordan frame is studied using numerical computations. For certain parameters the solutions can describe an accelerated expansion. We also derive an analytical approximation in exponential functions. © 2014, The Author(s)