Entanglement of Coarse Grained Quantum Field in the Expanding Universe (2009)
We investigate the entanglement of a quantum field in the expanding universe. By introducing a bipartite system using a coarse grained scalar field, we apply the separability criterion based on the...
The Role of Anisotropy in the Void Models without Dark Energy (2009)
Tanimoto, Masayuki, Nambu, Yasusada, Iwata, Kazuhiro
Void models provide a possible explanation of the "accelerated expansion" of the Universe without dark energy. To make the conventional void models more realistic, we allow the void, an underdense...
Kawakami, Hayato, Mitsuda, Eiji, Nambu, Yasusada, Tomimatsu, Akira
Considering gravitational collapse of matter, it is important problem to clarify what kind of conditions leads to the formation of naked singularity. For this purpose, we apply the 1+3 orthonormal...
Entanglement of Quantum Fluctuations in the Inflationary Universe (2008)
We investigate quantum entanglement of a scalar field in the inflationary universe. By introducing a bipartite system using a lattice model of scalar field, we apply the separability criterion based...
Yoshiyuki Y. Yamaguchi, Yasusada Nambu
We investigate perturbed Hamiltonian systems with two degrees of freedom by renormalization group method, which derives a reduced equation called renormalization group equation (RGE) by handling...
Luminosity distance-redshift relation for the LTB solution near the center (2007)
Tanimoto, Masayuki, Nambu, Yasusada
Motivated by the inverse problem for the Lemaitre-Tolman-Bondi dust solution, in which problem the luminosity distance function $D_L(z)$ is taken as an input to select a specific model, we compute...
Can inhomogeneties accelerate the cosmic volume expansion? (2006)
Kai, Tomohiro, Kozaki, Hiroshi, Nakao, Ken-ichi, Nambu, Yasusada, Yoo, Chul-Moon
If expanding and contracting regions coexist in the universe, the speed of the cosmic volume expansion can be accelerated. We construct simple inhomogeneous dust-filled universe models in which the...
Evolution of Non-linear Fluctuations in Preheating after Inflation (2005)
We investigate the evolution of the non-linear long wavelength fluctuations during preheating after inflation. By using the separate universe approach, the temporal evolution of the power spectrum of...
Accelerating Universe via Spatial Averaging (2005)
Nambu, Yasusada, Tanimoto, Masayuki
We present a model of an inhomogeneous universe that leads to accelerated expansion after taking spatial averaging. The model universe is the Tolman-Bondi solution of the Einstein equation and...
The separate universe and the back reaction of long wavelength fluctuations (2005)
We investigate the backreaction of cosmological long wavelength perturbations on the evolution of the Universe. By applying the renormalization group method to a Friedmann-Robertson-Walker universe...
Black ring formation in particle systems (2004)
Yoshino, Hirotaka, Nambu, Yasusada
It is known that the formation of apparent horizons with non-spherical topology is possible in higher-dimensional spacetimes. One of these is the black ring horizon with $S^1\times S^{D-3}$ topology...
Instability of Massive Scalar Fields in Kerr-Newman Spacetime (2004)
Furuhashi, Hironobu, Nambu, Yasusada
We investigate the instability of charged massive scalar fields in Kerr-Newman spacetime. Due to the super-radiant effect of the background geometry, the bound state of the scalar field is unstable,...
Black hole formation in the grazing collision of high-energy particles (2002)
Yoshino, Hirotaka, Nambu, Yasusada
We numerically investigate the formation of D-dimensional black holes in high-energy particle collision with the impact parameter and evaluate the total cross section of the black hole production. We...
Long-wavelength Cosmological Perturbation in the Universe with Multiple Perfect Fluids (2002)
Nambu, Yasusada, Ohokata, Shin-ichi
We investigate the large scale cosmological perturbation in the Universe with multiple perfect fluids. Using the long-wavelength approximation with Hamilton-Jacobi method, we derive the formula for...
High-energy head-on collisions of particles and hoop conjecture (2002)
Yoshino, Hirotaka, Nambu, Yasusada
We investigate the apparent horizon formation for high-energy head-on collisions of particles in multi-dimensional spacetime. The apparent horizons formed before the instance of particle collision...
We investigate the back reaction of cosmological perturbations on the evolution of the Universe using the renormalization group method. Starting from the second order perturbed Einstein's equation,...
Hoop conjecture for colliding black holes : non-time-symmetric initial data (2001)
Yoshino, Hirotaka, Nambu, Yasusada, Tomimatsu, Akira
The hoop conjecture is well confirmed in momentarily static spaces, but it has not been investigated systematically for the system with relativistic motion. To confirm the hoop conjecture for...
Back Reaction Problem in the Inflationary Universe (2000)
We investigate the back reaction of cosmological perturbations on an inflationary universe using the renormalization-group method. The second-order zero mode solution which appears by the...
Renormalization Group Approach to Cosmological Back Reaction Problems (2000)
We investigated the back reaction of cosmological perturbations on the evolution of the universe using the second order perturbation of the Einstein's equation. To incorporate the back reaction...
Vacuum Polarization of Massive Scalar Fields on the Black Hole Horizon (2000)
Koyama, Hiroko, Nambu, Yasusada, Tomimatsu, Akira
Vacuum polarization of massive scalar fields in a thermal state at arbitrary temperature is studied near the horizon of a Reissner-Nordstr\"{o}m black hole. We derived an analytic form of $$...
Renormalization of Long-wavelength Solution of Einstein Equation (1999)
Nambu, Yasusada, Yamaguchi, Yoshiyuki Y.
Using the renormalization group method, we improved the first order solution of the long-wavelength expansion of the Einstein equation. By assuming that the renormalization group transformation has...
Renormalization group method and canonical perturbation theory (1999)
Yamaguchi, Yoshiyuki Y., Nambu, Yasusada
Renormalization group method is one of the most powerful tool to obtain approximate solutions to differential equations. We apply the renormalization group method to Hamiltonian systems whose...
Yamaguchi, Yoshiyuki Y., Nambu, Yasusada
We investigate perturbed Hamiltonian systems with two degrees of freedom by renormalization group method, which derives a reduced equation called renormalization group equation (RGE) by handling...
Renormalization group equations and integrability in Hamiltonian systems (1998)
Yamaguchi, Yoshiyuki Y., Nambu, Yasusada
We investigate Hamiltonian systems with two degrees of freedom by using renormalization group method. We show that the original Hamiltonian systems and the renormalization group equations are...
Solutions of gauge invariant cosmological perturbations in long-wavelength limit (1998)
Nambu, Yasusada, Taruya, Atsushi
We investigate gauge invariant cosmological perturbations in a spatially flat Friedman-Robertson-Walker universe with scalar fields. It is well known that the evolution equation for the gauge...
Moderately Chaotic Hamiltonian Systems: Structure and Motion in High-dimensional Phase Spaces (1997)
Yamaguchi Y. Yoshiyuki, Yasusada Nambu, Atsushi Taruya, Kenji Imai, Masaharu Ishii
tent discussions always gave me useful comments. It is great happiness for me that he was one of my roommates. I thank the Computer Center of the Institute for Molecular Science, for the use of the...
Evolution of Cosmological Perturbation in Reheating Phase of the Universe (1996)
Nambu, Yasusada, Taruya, Atsushi
The evolution of the cosmological perturbation during the oscillatory stage of the scalar field is investigated. For the power law potential of the inflaton field, the evolution equation of the...
Taruya, Atsushi, Nambu, Yasusada
As an application of Gradient Expansion (long-wavelength) approximation, we studied the inhomogeneous universe including the gravitational wave(GW). For a plane-symmetric cosmological model, we could...
Application of Gradient Expansion to Inflationary Universe (1994)
Nambu, Yasusada, Taruya, Atsushi
Using the long wave perturbation scheme(gradient expansion), the effect of inhomogeneity on the inflationary phase is investigated. We solved the perturbation equation of which source term comes from...
Fate of Inhomogeneity in Schwarzschild-deSitter Space-time (1993)
We investigate the global structure of the space time with a spherically symmetric inhomogeneity using a metric junction, and classify all possible types. We found that a motion with a negative...
Local Structure of Numerically Generated Worm Hole Spacetime (1993)
Nambu, Yasusada, Siino, Masaru
We investigate the evolution of the apparent horizons in a numerically gererated worm hole spacetime. The behavior of the apparent horizons is affected by the dynamics of the matter field. By using...