The modified gravity, which eliminates the need for dark energy and which seems to be stable, is considered. The terms with positive powers of the curvature support the inflationary epoch while the terms with negative powers of the curvature serve as effective dark energy, supporting current cosmic acceleration. The equivalent scalar-tensor gravity may be compatible with the simplest solar syst...

Recently, some efforts focus on differentiating dark energy and modified gravity with the growth function δ(z). In the literature, it is useful to parameterize the growth rate f ≡ d ln δ/d ln a = Ωm with the growth index γ. In this note, we consider the general DGP model with any Ωk. We confront the growth index of DGP model with currently available growth rate data and find that the DGP model ...

The possibility of a connection between dark energy and gravity through a direct coupling in the Lagrangian of the underlying theory has acquired an increasing interest due to the recently discovered capability of the extended quintessence model to encompass the fine-tuning problem of the cosmological constant. The gravity induced “R-boost” mechanism is indeed responsible for an early, enhanced...

In gravitostatics, the minimally generalized Newtonian theory, the simplest cosmological model implies a cosmological redshift with acceleration parameter −1. If the gravitational potential satisfies a wave equation , the total mass density (visible plus dark) is one third of the critical density, in good agreement with observation. There is no horizon problem and no need to invoke dark energy....

The strongest evidence for dark energy comes presently from geometric techniques such as the supernova distance-redshift relation. By combining the measured expansion history with the Friedmann equation one determines the energy density and its time evolution, hence the equation of state of dark energy. Because these methods rely on the Friedmann equation which has not been independently tested...

We compare the WMAP temperature power spectrum and SNIa data to models with a generalized Chaplygin gas as dark energy. The generalized Chaplygin gas is a component with an exotic equation of state, pX = −A/ρ α X (a polytropic gas with negative constant and exponent). Our main result is that, restricting to a flat universe and to adiabatic pressure perturbations for the generalized Chaplygin ga...

We present a very brief and incomplete overview of the on-going theoretical activity aiming at providing an explanation for the (at least apparent) acceleration of the universe. We discuss the issue of the cosmological constant amplitude, describe some inhomogeneous cosmology models, present some quintessence and other dark energy models, and say a few words on non-minimal gravity. We mention t...

It is generally assumed that the two dark components of the energy density of the universe, a smooth component called dark energy and a fluid of nonrelativistic weakly interacting particles called dark matter, are independent of each other and interact only through gravity. In this paper, we consider a class of models in which the dark matter and dark energy interact directly. The dark matter p...

The acceleration of the cosmic expansion is a fundamental challenge to standard models of particle physics and cosmology. The new physics of dark energy may lie in the nature of gravity, the quantum vacuum, or extra dimensions. I give a brief overview of the puzzles and possibilities of dark energy, and discuss the confrontation of a wide variety of “beyond Einstein” models with the latest data...