We review the underpinnings of the standard Newton-Einstein theory of gravity, and identifywhere it could possibly go wrong. In particular, we discuss the logical independence from eachother of the general covariance principle, the equivalence principle and the Einstein equations, anddiscuss how to constrain the matter energy-momentum tensor which serves as the source of gravity.<lb...

High-precision observational data have confirmed with startling evidence that the Universe is currently undergoing a phase of accelerated expansion. This phase, one of the most important and challenging current problems in cosmology, represents a new imbalance in the governing gravitational equations. Historically, physics has addressed such imbalances by either identifying sources that were pr...

Recent observation of supernovae type Ia show clearly that there is a large scale repulsive force in the Universe. Neither of the four known fundamental interactions can account for this repulsive force. Gravity is known to be the interaction responsible for the large scale structure and evolution of the Universe. The problem with gravity is that it gives rise to a force which is attractive onl...

We show that growing neutrino models, in which the growing neutrino mass stops the dynamical evolution of a dark energy scalar field, lead to a substantial neutrino clustering on the scales of superclusters. Nonlinear neutrino lumps form at redshift z ≈ 1 and could partially drag the clustering of dark matter. If observed, large scale non-linear structures could be an indication for a new attra...

Within the context of modified gravity and dark energy scenarios of the accelerated universe, we study the stability of de Sitter space with respect to inhomogeneous perturbations using a gauge– independent formalism. In modified gravity the stability condition is exactly the same that one obtains from a homogeneous perturbation analysis, while the stability condition in scalar–tensor gravity i...

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...

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...