Yasusada Nambu

Black hole formation in the grazing collision of high-energy particles

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 find that the formation of an apparent horizon occurs when the distance between the colliding particles is less than 1.5 times the effective gravitational radius of each particles. Our numerical result indicates that although both the one-dimensional hoop and the (D-3)-dimensional volume corresponding to the typical scale of the system give a fairly good condition for the horizon formation in the higher-dimensional gravity, the (D-3)-dimensional volume provide a better condition to judge the existence of the horizon.

Can Inhomogeneities Accelerate the Cosmic Volume Expansion

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 speed of the cosmic volume expansion is accelerated for finite periods. These models are constructed by removing spherical domains from the Einstein-de Sitter �

Instability of Massive Scalar Fields in Kerr-Newman Spacetime

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, and its amplitude grows in time. By solving the Klein-Gordon equation of the scalar field as an eigenvalue problem, we numerically obtain the growth rate of the amplitude of the scalar field. Although the dependence of the scalar field mass and the scalar field charge on this growth rate agrees with the result of the analytic approximation, the maximum value of the growth rate is three times larger than that of the analytic approximation. We also discuss the effect of the electric charge on the instability of the scalar field.

High-energy head-on collisions of particles and the hoop conjecture

We investigate the apparent horizon formation for high-energy head-on collisions of particles in D-dimensional spacetime. The apparent horizons formed before the instant of collision are obtained analytically. We apply the D-dimensional hoop conjecture to these solutions and show that the horizon formation becomes more difficult for larger D.

Cosmological perturbation with two scalar fields in reheating after inflation

Abstract We investigate the cosmological perturbation of two-scalar field model during the reheating phase after inflation. Using the exact solution of the perturbation in long-wavelength limit, which is expressed in terms of the background quantities, we analyze the behavior of the metric perturbation. The oscillating inflaton field gives rise to the parametric resonance of the massless scalar field and this leads to the amplification of the iso-curvature mode of the metric perturbations.We investigate the cosmological perturbation of two-scalar field model during the reheating phase after inflation. Using the exact solution of the perturbation in long-wavelength limit, which is expressed in terms of the background quantities, we analyze the behavior of the metric perturbation. The oscillating inflaton field gives rise to the parametric resonance of the massless scalar field and this leads to the amplification of the iso-curvature mode of the metric perturbations.

Evolution of cosmological perturbation in reheating phase of the universe

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 Mukhanovs gauge invariant variable is reduced to the Mathieu equation and the density perturbation grows by the parametric resonance.

Luminosity distance–redshift relation for the LTB solution near the centre

Motivated by the inverse problem for the Lemaitre-Tolman-Bondi dust solution, in which problem the luminosity distance function

Classical and quantum correlations of scalar field in the inflationary universe

D_L(z)

Entanglement of quantum fluctuations in the inflationary universe

is taken as an input to select a specific model, we compute the function

The Back reaction and the effective Einstein's equation for the universe with ideal fluid cosmological perturbations

D_L(z)