Solar System experiments do not yet veto modified gravity models

The dynamical equivalence between modified and scalar-tensor gravity theories is revisited and it is concluded that it breaks down in the limit to general relativity. A gauge-independent analysis of cosmological perturbations in both classes of theories lends independent support to this conclusion. As a consequence, the PPN formalism of scalar-tensor gravity and Solar System experiments do not veto modified gravity, as previously thought.Comment: 7 pages, latex, submitted to Phys. Rev.

Generalization of Einstein-Lovelock theory to higher order dilaton gravity

A higher order theory of dilaton gravity is constructed as a generalization of the Einstein-Lovelock theory of pure gravity. Its Lagrangian contains terms with higher powers of the Riemann tensor and of the first two derivatives of the dilaton. Nevertheless, the resulting equations of motion are quasi-linear in the second derivatives of the metric and of the dilaton. This property is crucial for the existence of brane solutions in the thin wall limit. At each order in derivatives the contribution to the Lagrangian is unique up to an overall normalization. Relations between symmetries of this theory and the O(d,d) symmetry of the string-inspired models are discussed.Comment: 18 pages, references added, version to be publishe

On the way from matter-dominated era to dark energy universe

We develop the general program of the unification of matter-dominated era with acceleration epoch for scalar-tensor theory or dark fluid. The general reconstruction of single scalar-tensor theory is fulfilled. The explicit form of scalar potential for which the theory admits matter-dominated era, transition to acceleration and (asymptotically deSitter) acceleration epoch consistent with WMAP data is found. The interrelation of the epochs of deceleration-acceleration transition and matter dominance-dark energy transition for dark fluids with general EOS is investigated. We give several examples of such models with explicit EOS (using redshift parametrization) where matter-dark energy domination transition may precede the deceleration-acceleration transition. As some by-product, the reconstruction scheme is applied to scalar-tensor theory to define the scalar potentials which may produce the dark matter effect. The obtained modification of Newton potential may explain the rotation curves of galaxies.Comment: LaTeX 12 pages, 1 figure, extended version to appear in PR

Effects of modified gravity on the turnaround radius in cosmology

We revisit the concept of turnaround radius in cosmology, in the context of modified gravity. While preliminary analyses were limited to scalar-tensor/$F(R)$ gravity, we extend the definition and the study of this quantity to a much broader class of theories including also quantum $R^2$ gravity. The turnaround radius is computed in terms of the parameters of the theory and it is shown that a deviation not larger than 10\% of this quantity from its value in Einstein's theory could constrain the model parameters and even rule out some current theories.Comment: LaTeX, version to appear in PR

Generalized Slow Roll Conditions and the Possibility of Intermediate Scale Inflation in Scalar-Tensor Theory

Generalized slow roll conditions and parameters are obtained for a general form of scalar-tensor theory (with no external sources), having arbitrary functions describing a nonminimal gravitational coupling F(\phi), a Kahler-like kinetic function k(\phi), and a scalar potential V(\phi). These results are then used to analyze a simple toy model example of chaotic inflation with a single scalar field \phi and a standard Higgs potential and a simple gravitational coupling function. In this type of model inflation can occur with inflaton field values at an intermediate scale of roughly 10^{11} GeV when the particle physics symmetry breaking scale is approximately 1 TeV, provided that the theory is realized within the Jordan frame. If the theory is realized in the Einstein frame, however, the intermediate scale inflation does not occur.Comment: 14 pages, no figs. Accepted to Classical and Quantum Gravit

Brans-Dicke theory: Jordan vs Einstein Frame

It is well known that, in contrast to general relativity, there are two conformally related frames, the Jordan frame and the Einstein frame, in which the Brans-Dicke theory, a prototype of generic scalar-tensor theory, can be formulated. There is a long standing debate on the physical equivalence of the formulations in these two different frames. It is shown here that gravitational deflection of light to second order accuracy may observationally distinguish the two versions of the Brans-Dicke theory.Comment: 10 pages, Accepted by Mod. Phys. Letts.

The final state and thermodynamics of dark energy universe

As it follows from the classical analysis, the typical final state of the dark energy universe where dominant energy condition is violated is finite time, sudden future singularity (Big Rip). For a number of dark energy universes (including scalar phantom and effective phantom theories as well as specific quintessence model) we demonstrate that quantum effects play the dominant role near Big Rip, driving the universe out of future singularity (or, at least, making it milder). As a consequence, the entropy bounds with quantum corrections become well-defined near Big Rip. Similarly, black holes mass loss due to phantom accretion is not so dramatic as it was expected: masses do not vanish to zero due to transient character of phantom evolution stage. Some examples of cosmological evolution for negative, time-dependent equation of state are also considered with the same conclusions. The application of negative entropy (or negative temparature) occurence in the phantom thermodynamics is briefly discussed.Comment: LaTeX file 36 pages, version to appear in PR