Hideo Iguchi

Is Dark Energy the Only Solution to the Apparent Acceleration of the Present Universe

Even for the observed luminosity distance D L (z), which suggests the existence of dark energy, we show that an inhomogeneous dust universe solution without dark energy is possible in general. Future observation of D L (z) for 1 ≤ z < 1.7 may confirm or refute this possibility.

Physical processes in naked singularity formation

Gravitational collapse is one of the most fruitful subjects in gravitational physics. It is well known that singularity formation is inevitable in complete gravitational collapse. It was conjectured that such a singularity should be hidden by horizons if it is formed from generic initial data with physically reasonable matter fields. Many possible counterexamples to this conjecture have been proposed over the past three decades, although none of them has proved to be sufficiently generic. In these examples, there appears a singularity that is not hidden by horizons. This singularity is called a `naked singularity. The appearance of a naked singularity represents the formation of an observable high-curvature, strong-gravity region. In this paper we review examples of naked singularity formation and recent progress in research of observable physical processes - gravitational radiation and quantum particle creation - from a forming naked singularity.

Gravitational Radiation from a Naked Singularity Odd-Parity Perturbation

A naked singularity occurs in the generic collapse of an inhomogeneous dust ball. We study the even-parity mode of gravitational waves from a naked singularity of the LemaˆotreTolman-Bondi spacetime. The wave equations for gravitational waves are solved by numerical integration using the single null coordinate. The result implies that the metric perturbation grows when it approaches the Cauchy horizon and diverges there, although the naked singularity is not a strong source of even-parity gravitational radiation. Therefore, the Cauchy horizon in this spacetime should be unstable with respect to linear even-parity perturbations.

A Classification of Spherically Symmetric Kinematic Self-Similar Perfect-Fluid Solutions

We classify all spherically symmetric spacetimes admitting a kinematic self-similar vector of the second, zeroth or infinite kind. We assume that the perfect fluid obeys either a polytropic equation of state or an equation of state of the form p = Kµ, where p and µ are the pressure and the energy density, respectively, and K is a constant. We study the cases in which the kinematic self-similar vector is not only “tilted” but also parallel or orthogonal to the fluid flow. We find that, in contrast to Newtonian gravity, the polytropic perfect-fluid solutions compatible with kinematic self-similarity are the Friedmann-Robertson-Walker solution and general static solutions. We find three new exact solutions, which we call the dynamical solutions (A) and (B) and the Λ-cylinder solution.

Naked singularity formation in the collapse of a spherical cloud of counterrotating particles

We investigate collapse of a spherical cloud of counter-rotating particles. An explicit solution is given using an elliptic integral. If the specific angular momentum

Computing gravitational waves from slightly nonspherical stellar collapse to a black hole: Odd-parity perturbation

L(r)=O(r^2)

Naked singularities and quantum gravity

at

Birth of a timelike naked singularity

rto 0

Nakedness and curvature strength of a shell-focusing singularity in spherically symmetric spacetime with vanishing radial pressure

, no central singularity occurs. With

No go theorem for kinematic self-similarity with a polytropic equation of state

L(r)