Stationary vs. singular points in an accelerating FRW cosmology derived from six-dimensional Einstein-Gauss-Bonnet gravity

Six-dimensional Einstein-Gauss-Bonnet gravity (with a linear Gauss-Bonnet term) is investigated. This theory is inspired by basic features of results coming from string and M-theory. Dynamical compactification is carried out and it is seen that a four-dimensional accelerating FRW universe is recovered, when the two-dimensional internal space radius shrinks. A non-perturbative structure of the corresponding theory is identified which has either three or one stable fixed points, depending on the Gauss-Bonnet coupling being positive or negative. A much richer structure than in the case of the perturbative regime of the dynamical compactification recently studied by Andrew, Bolen, and Middleton is exhibited.Comment: 9 pages, 20 figures, accepted in PL

Superstring in Two Dimensional Black Hole

We construct superstring theory in two dimensional black hole background based on supersymmetric $SU(1,1)/U(1)$ gauged Wess-Zumino-Witten model.Comment: 12p

Inverse problem - reconstruction of dark energy models

We review how we can construct the gravity models which reproduces the arbitrary development of the universe. We consider the reconstruction in the Einstein gravity coupled with generalized perfect fluid, scalar-Einstein gravity, scalar-Einstein-Gauss-Bonnet gravity, Einstein-$F(G)$-gravity, and $F(R)$-gravity. Very explicit formulas are given to reconstruct the models, which could be used when we find the detailed data of the development of the universe by future observations. Especially we find the formulas using e-foldings, which has a direct relation with observed redshift. As long as we observe the time development of the Hubble rate $H$, there exists a variety of models describing the arbitrary development of universe.Comment: LaTeX 11 pages, references are added, to appear in Proceedings of International Workshop on Dark Matter, Dark Energy and Matter-Antimatter Asymmetry, National Tsing Hua University, Hsinchu, Taiwan, November 20-21, 200

Cosmological reconstruction of realistic modified F(R) gravities

The cosmological reconstruction scheme for modified $F(R)$ gravity is developed in terms of e-folding (or, redshift). It is demonstrated how any FRW cosmology may emerge from specific $F(R)$ theory. The specific examples of well-known cosmological evolution are reconstructed, including $\Lambda$CDM cosmology, deceleration with transition to phantom superacceleration era which may develop singularity or be transient. The application of this scheme to viable $F(R)$ gravities unifying inflation with dark energy era is proposed. The additional reconstruction of such models leads to non-leading gravitational correction mainly relevant at the early/late universe and helping to pass the cosmological bounds (if necessary). It is also shown how cosmological reconstruction scheme may be generalized in the presence of scalar field.Comment: LaTex 11 page

Multiple Lambda cosmology: dark fluid with time-dependent equation of state as classical analog of cosmological landscape

We discuss FRW universe with time-dependent EoS dark fluid which leads to multiple de Sitter space. This model (as well as its scalar-tensor version) may be considered as some classical analog of cosmological landscape. The universe expansion history may look as transitions between different deSitter eras what suggests the interesting solution for cosmological constant problem. For specific time-dependent EoS dark fluid the possibility of transitions between universe regions with positive and negative cosmological constant is also established.Comment: LaTeX file, 8 pages, no figure, version to appear in PL

Running gauge coupling and quark-antiquark potential from dilatonic gravity

The running gauge coupling and quark-antiquark potential in d-dimensions are calculated from the explicit solution of d+1-dimensional dilatonic gravity. This background interpolates between usual AdS in UV and flat space with singular dilaton in IR and it realizes two-boundaries AdS/CFT correspondence. The behaviour of running coupling and potential is consistent with results following from IIB supergravity.Comment: LaTeX file, 8 pages, more references added. To appear in PL

De Sitter space versus Nariai Black Hole: stability in d5 higher derivative gravity

d5 higher derivative gravity on the Schwarzschild-de Sitter (SdS) black hole background is considered. Two horizons SdS BHs are not in thermal equilibrium and Hawking-Page phase transitions are not expected there, unlike to the case of AdS BHs. It is demonstrated that there exists the regime of d5 theory where Nariai BH which is extremal limit of SdS BH is stable. It is in the contrast with Einstein gravity on such backgroundwhere only pure de Sitter space is always stable. Speculating on the applications in proposed dS/CFT correspondence, these two (de Sitter and Nariai) stable spaces may correspond to confining-deconfining phases in dual CFT.Comment: LaTeX file, 11 page

Covariant Lagrange multiplier constrained higher derivative gravity with scalar projectors

We formulate higher derivative gravity with Lagrange multiplier constraint and scalar projectors. Its gauge-fixed formulation as well as vector fields formulation is developed and corresponding spontaneous Lorentz symmetry breaking is investigated. We show that the only propagating mode is higher derivative graviton while scalar and vector modes do not propagate. Despite to higher derivatives structure of the action, its first FRW equation is the first order differential equation which admits the inflationary universe solution.Comment: Physics Letters B published version. LaTeX 12 page

(Super-) String in Two Dimensional Black Hole and Target Space Dualities

We review the recently proposed string theory in two dimensional black hole background. Especially, the structure of the duality in the target space is discussed. The duality is analogous to \lq\lq $R \rightarrow 1/R$" symmetry of a compactified boson. We consider the duality in more general target space manifolds which have Killing symmetries and we give an explicit formula which connects two different manifolds which are dual to each other. Superstring theory in two dimensional black hole background is also discussed based on supersymmetric $SU(1,1)/U(1)$ gauged Wess-Zumino-Witten model.Comment: 19pp (Talk at YITP workshop, ``Developments in Strings and Field Theories'', Kyoto, JAPAN