Stability of accelerating cosmology in two scalar-tensor theory: Little Rip versus de Sitter

We develop the general reconstruction scheme in two scalar model. The quintom-like theory which may describe (different) non-singular Little Rip or de Sitter cosmology is reconstructed. (In)stability of such dark energy cosmologies as well as the flow to fixed points is studied. The stability of Little Rip universe which leads to dissolution of bound objects sometime in future indicates that no classical transition to de Sitter space occurs.Comment: LaTeX 27 pages, 12 figures, version appeared in Entrop

Modified gravity as realistic candidate for dark energy, inflation and dark matter

We review the unification of early-time inflation with late-time acceleration in several local modified gravity models which pass Solar System and cosmological tests. It is also demonstrated that account of non-local gravitational corrections to the action does not destroy the possibility of such unification. Dark matter effect is caused by composite graviton degree of freedom in such models.Comment: LaTeX 6 pages, based on the talk given at DSU08 conference, June 200

Modified f(R) gravity unifying R^m inflation with \LambdaCDM epoch

We consider modified $f(R)$ gravity which may unify $R^m$ early-time inflation with late-time $\Lambda$CDM epoch. It is shown that such model passes the local tests (Newton law, stability of Earth-like gravitational solution, very heavy mass for additional scalar degree of freedom) and suggests the realistic alternative for General Relativity. Various scenarios for future evolution of $f(R)$ $\Lambda$CDM era are discussed.Comment: LaTeX 10 pages, version to appear in PR

Properties of singularities in (phantom) dark energy universe

The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. We show the existence of a stable fixed point with an equation of state $w<-1$ and numerically confirm that this is actually a late-time attractor in the phantom-dominated universe. We also construct a phantom dark energy scenario coupled to dark matter that reproduces singular behaviors of the Big Rip type for the energy density and the curvature of the universe. The effect of quantum corrections coming from conformal anomaly can be important when the curvature grows large, which typically moderates the finite-time singularities.Comment: 17 pages, 6 figures, references are added, version to appear in Physical Review

Covariant power-counting renormalizable gravity: Lorentz symmetry breaking and accelerating early-time FRW universe

We continue the study of covariant power-counting renormalizable gravity constrained by scalar Lagrange multiplier. Lorentz symmetry breaking is investigated in such a theory in comparison with the one in ghost condensation model. Covariant power-counting renormalizable vector gravity which is invariant under $U(1)$ gauge symmetry is proposed. Several forms of vector Lagrange multiplier in this theory are discussed. It is shown that covariant scalar/vector gravity under consideration may have power-law or de Sitter accelerating cosmological solution corresponding to inflationary era. Simplest black hole solution is obtained and dispersion relations for graviton are presented.Comment: LaTeX, 13 page

Gauss-Bonnet Chameleon Mechanism of Dark Energy

As a model of the current accelerated expansion of the universe, we consider a model of the scalar-Einstein-Gauss-Bonnet gravity. This model includes the propagating scalar modes, which might give a large correction to the Newton law. In order to avoid this problem, we propose an extension of the Chameleon mechanism where the scalar mode becomes massive due to the coupling with the Gauss-Bonnet term. Since the Gauss-Bonnet invariant does not vanish near the earth or in the Solar System, even in the vacuum, the scalar mode is massive even in the vacuum and the correction to the Newton law could be small. We also discuss about the possibility that the model could describe simultaneously the inflation in the early universe, in addition to the current accelerated expansion.Comment: LaTeX 11 pages, no figur

Critical gravity with a scalar field in four dimensions

We consider the critical gravity theory with a scalar field in four dimensions. We find that this theory has the solution corresponding to the de Sitter (dS), anti-de Sitter (AdS), and Minkowski background depending on whether the action includes the cosmological term or not. The Minkowski background is the solution which cannot be obtained in the model without a scalar field. At the critical point, we show that the Abbott-Deser (AD) mass of the Schwarzschild-de Sitter (SdS) black hole and the energy for the massless graviton vanish, whose situation is not changed from the model without a scalar field.Comment: 6 page

Inhomogeneous Equation of State of the Universe: Phantom Era, Future Singularity and Crossing the Phantom Barrier

The dark energy universe equation of state (EOS) with inhomogeneous,Hubble parameter dependent term is considered. The motivation to introduce such a term comes from time-dependent viscosity considerations and modifications of general relativity. For several explicit examples of such EOS it is demonstrated how the type of future singularity changes, how the phantom epoch emerges and how crossing of phantom barrier occurs. Similar cosmological regimes are considered for the universe with two interacting fluids and for universe with implicit EOS. For instance, the crossing of phantom barrier is realized in easier way, thanks to the presence of inhomogeneous term. The thermodynamical dark energy model is presented where the universe entropy may be positive even at phantom era as a result of crossing of w=-1 barrier.Comment: LaTeX, 13 pages, refs. added, version to appear in PR

Is brane cosmology predictable?

The creation of the inflationary brane universe in 5d bulk Einstein and Einstein-Gauss-Bonnet gravity is considered. We demonstrate that emerging universe is ambigious due to arbitrary function dependence of the junction conditions (or freedom in the choice of boundary terms). We argue that some fundamental physical principle (which may be related with AdS/CFT correspondence) is necessary in order to fix the 4d geometry in unique way.Comment: LaTeX file, 4 page

The future evolution and finite-time singularities in F(R)F(R)-gravity unifying the inflation and cosmic acceleration

We study the future evolution of quintessence/phantom dominated epoch in modified $F(R)$-gravity which unifies the early-time inflation with late-time acceleration and which is consistent with observational tests. Using the reconstruction technique it is demonstrated that there are models where any known (Big Rip, II, III or IV Type) singularity may classically occur. From another side, in Einstein frame (scalar-tensor description) only IV Type singularity occurs. Near the singularity the classical description breaks up, it is demonstrated that quantum effects act against the singularity and may prevent its appearance. The realistic $F(R)$-gravity which is future singularity free is proposed. We point out that additional modification of any $F(R)$-gravity by the terms relevant at the early universe is possible, in such a way that future singularity does not occur even classically.Comment: 16 pages, discussion is enlarged, version to appear in PR