arXiv subjects

Akika Nakamichi

Publications and source records attributed to Akika Nakamichi.

23 records · Page 2Linked to original sources

arXiv1996

Late-Time Mild Inflation --- a possible solution of dilemma: cosmic age and the Hubble parameter ---

We explore the cosmological model in which a late-time mild inflation is realized after the star formation epoch. Non-vanishing curvature coupling of a classical boson field yields this mild inflation without a cosmological constant. Accordingly the lifetime of the present Universe is remarkably increased in our model. Thus we show that the present observed high value of the Hubble parameter $H_0 \approx 70-80{\rm km/sec/Mpc}$ is compatible with the age of the oldest stars $14{\rm Gyr}$ without introducing the cosmological constant or the open Universe model. Moreover in our model, the local Hubble parameter becomes larger than the global one. Thus we show that the present observed local Hubble parameter measured by using the Cepheid variables is compatible with the global Hubble parameter measured by using the Sunyaev-Zeldovich effect. We also examine the energy conditions, evolution of the effective gravitational constant and the nucleosynthesis process.

astro-ph↗

arXiv1994

Quantized Topological 2-form Gravity

We develop a perturbation theory of four-dimensional topological 2-form gravity without cosmological constant. A 2-form and an $SU(2)$ connection 1-form are used as fundamental variables instead of metric. There is no quantum correction from two-loop and higher orders in covariant gauge, in Landau gauge and in background covariant gauge. We improve naive dimensional regularization and calculate an exact quantum correction at one-loop order. We observe that the renormalizability depends on gauge choice in topological 2-form gravity. It is unrenormalizable in the covariant gauge and in the background covariant gauge. On the other hand in the Landau gauge, we obtain finite but $SU(2)$ non-covariant quantum corrections. There may exist anomaly for the $SU(2)$ gauge symmetry. (Talk given by A. Nakamichi at the Workshop on General Relativity and Gravitation held at Tokyo University, January 17-20,1994.)

hep-th↗

arXiv1993

Cosmological Model in 2d Dilaton Gravity

We apply CGHS-type dilaton gravity model to (1+1)-dimensional cosmological situations. First the behavior of a compact 1-dimensional universe (i.e. like a closed string) is classified on the assumption of homogeneity of universe. Several interesting solutions are found, which include a Misner-type universe having closed time-like curves, and an asymptotically de Sitter universe first pointed out by Yoshimura. In the second half of this talk, we discuss the modification of the classical homogeneous solutions, considering inhomogeneity of classical conformal matters and also quantum back-reaction respectively. (An expanded version of the talk presented by T. Mishima at Yukawa Institute of Theoretical Physics workshop `Quantum Gravity' 24-27, November 1992.)

hep-th↗

arXiv1993

Chronology Protection in Two-Dimensional Dilaton Gravity

The global structure of 1 + 1 dimensional compact Universe is studied in two-dimensional model of dilaton gravity. First we give a classical solution corresponding to the spacetime in which a closed time-like curve appears, and show the instability of this spacetime due to the existence of matters. We also observe quantum version of such a spacetime having closed timelike curves never reappear unless the parameters are fine-tuned.

hep-th↗

arXiv1993

Wave Function of the Universe in Topological and in Einstein 2-form Gravity

We clarify the relation between 2-form Einstein gravity and its topological version. The physical space of the topological version is contained in that of the Einstein gravity. Moreover a new vector field is introduced into 2-form Einstein gravity to restore the large symmetry of its topological version. The wave function of the universe is obtained for each model. (Talk given at the Workshop on General Relativity and Gravitation held at Waseda University, January 18-20, 1993.)

hep-th↗