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Subjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We study asymptotically AdS topological black hole solutions with k = 0 (plane symmetric) in the Einstein gravity with Gauss-Bonnet term, the dilaton and a “cosmo-logical constant” in various dimensions. We derive the field equations for suitable ansatz for general D dimensions. We determine the parameter regions including dilaton cou-plings where such solutions exist and construct black hole solutions of various masses numerically in D = 4, 5, 6 and 10 dimensional spacetime with (D ? 2)-dimensional hypersurface of zero curvature. |

Accelerating Cosmologies in theEinstein-Gauss-Bonnet Theory with Dilaton

Kazuharu BAMBA; Zong-Kuan Guo; Nobuyoshi OhtaSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We study cosmological solutions in the low-energy e?ective heterotic string theory, which is the Einstein gravity with Gauss-Bonnet term and the dilaton. We show that the field equations are cast into an autonomous system for flat internal and external spaces, and derive all the fixed points in the system. We also examine the time evolution of the solutions and whether the solutions can give (transient) accelerated expansion of our four-dimensional space in the Einstein frame. |

Probing the Coupling between Dark Components of the Universe

Zong-Kuan Guo; Nobuyoshi Ohta; Shinji TsujikawaSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We place observational constraints on a coupling between dark energy and dark matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the five-year Supernova Legacy Survey (SNLS), the cosmic microwave background (CMB) shift parameter from the three-year Wilkinson Microwave Anisotropy Probe (WMAP), and the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS). The interactions we study are (i) constant coupling #14; and (ii) varying coupling #14;(z) that depends on a redshift z, both of which have simple parametrizations of the Hubble parameter to confront with observational data. We find that the combination of the three databases marginalized over a present dark energy density gives stringent constraints on the coupling,?0.08 < #14; < 0.03 (95% CL) in the constant coupling model and ?0.4 < #14;0 < 0.1 (95% CL) in the varying coupling model, where #14;0 is a present value. The uncoupled #3;CDM model (wX = ?1 and #14; = 0) still remains a good fit to the data, but the negative coupling (#14; < 0) with the equation of state of dark energy wX < ?1 is slightly favoured over the #3;CDM model. |

Realizing Scale-invariant Density Perturbations in Low-energy Effective String Theory

Zong-Kuan Guo; Nobuyoshi Ohta; Shinji TsujikawaSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We discuss the realization of inflation and resulting cosmological perturbations in the low-energy effective string theory. In order to obtain nearly scale-invariant spectra of density perturbations and a suppressed tensor-to-scalar ratio, it is generally necessary that the dilaton field #30; is effectively decoupled from gravity together with the existence of a slowly varying dilaton potential. We also study the effect of second-order corrections to the tree-level action which are the sum of a Gauss-Bonnet term coupled to #30; and a kinetic term (?#30;)4. We find that it is possible to realize observationally supported spectra of scalar and tensor perturbations provided that the correction is dominated by the (?#30;)4 term even in the absence of the dilaton potential. When the Gauss-Bonnet term is dominant, tensor perturbations exhibit violent negative instabilities on small-scales about a de Sitter background in spite of the fact that scale-invariant scalar perturbations can be achieved. |

Parametrization of Quintessence and Its Potential

Zong-Kuan Guo; Nobuyoshi Ohta; Yuan-Zhong ZhangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We develop a theoretical method of constructing the quintessence potential directly from the effective equation of state function w(z), which describes the properties of the dark energy. We apply our method to four parametrizations of equation of state parameter and discuss the general features of the resulting potentials. In particular, it is shown that the constructed quintessence potentials are all in the form of a runaway type. |

Subjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We study spherically symmetric, asymptotically flat black hole solutions in the low-energy e ective heterotic string theory, which is the Einstein gravity with Gauss-Bonnet term and the dilaton, in various dimensions. We derive the field equations for suitable ansatz for general D dimensions and construct black hole solutions of various masses numerically in D = 4, 5, 6 and 10 dimensional spacetime with (D ? 2)-dimensional hypersurface with positive constant curvature. A detailed comparison with the non-dilatonic solutions is made. We also examine the thermodynamic properties of the solutions. It is found that the dilaton has significant e ects on the black hole solutions, and we discuss physical consequences. |

Cosmological Evolution of Dirac-Born-Infeld Field

Zong-Kuan Guo; Nobuyoshi OhtaSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We investigate the cosmological evolution of the system of a Dirac-Born-Infeld field plus a perfect fluid. We analyze the existence and stability of scaling solutions for the AdS throat and the quadratic potential. We find that the scaling solutions exist when the equation of state of the perfect fluid is negative and in the ultra-relativistic limit. |

[1 Pages/ 7 Totals]