29643076
Nonexistence of self-similar solutions containing a black hole in a universe with a stiff fluid or scalar field or quintessence
We consider the possible existence of self-similar solutions containing black holes in a Friedmann background with a stiff fluid or a scalar field. We carefully study the relationship between the self-similar equations in these two cases and emphasize the crucial role of the similarity horizon. We show that there is no self-similar black hole solution surrounded by an exact or asymptotically flat Friedmann background containing a massless scalar field. This result also applies for a scalar field with a potential, providing the universe is decelerating. However, if there is a potential and the universe is accelerating (as in the quintessence scenario), the result only applies for an exact Friedmann background. This extends the result previously found in the stiff fluid case and strongly suggests that accretion onto primordial black holes is ineffective even during scalar field domination. It also contradicts recent claims that such black holes can grow appreciably by accreting quintessence. Appreciable growth might be possible with very special matter fields but this requires ad hoc and probably unphysical conditions.
28063362
Convergence to a selfsimilar solution in general relativistic gravitational collapse
We study the spherical collapse of a perfect fluid with an equation of state $P=k\rho$ by full general relativistic numerical simulations. For $0<k\alt 0.036$, it has been known that there exists a general relativistic counterpart of the Larson-Penston self-similar Newtonian solution. The numerical simulations strongly suggest that, in the neighborhood of the center, generic collapse converges to this solution in an approach to a singularity and that self-similar solutions other than this solution, including a ``critical solution'' in the black hole critical behavior, are relevant only when the parameters which parametrize initial data are fine-tuned. This result is supported by a mode analysis on the pertinent self-similar solutions. Since a naked singularity forms in the general relativistic Larson-Penston solution for $0<k\alt0.0105$, this will be the most serious known counterexample against cosmic censorship. It also provides strong evidence for the self-similarity hypothesis in general relativistic gravitational collapse. The direct consequence is that critical phenomena will be observed in the collapse of isothermal gas in Newton gravity, and the critical exponent $\gamma$ will be given by $\gamma\approx 0.11$, though the order parameter cannot be the black hole mass.
46580419
Final fate of the spherically symmetric collapse of a perfect fluid
The final fate of the spherically symmetric collapse of a perfect fluid which follows the \ensuremath{\gamma}-law equation of state and adiabatic condition is investigated. Full general relativistic hydrodynamics is solved numerically using a retarded time coordinate, the so-called observer time coordinate. Thanks to this coordinate, the causal structure of the resultant space-time is automatically constructed. Then, it is found that a globally naked, shell-focusing singularity can occur at the center from relativistically high-density, isentropic, and time symmetric initial data if $\ensuremath{\gamma}\ensuremath{\lesssim}1.01$ within the numerical accuracy. The result is free from the assumption of self-similarity. The upper limit of \ensuremath{\gamma} with which a naked singularity can occur from generic initial data is consistent with the result of Ori and Piran based on the assumption of self-similarity.
207379213
Kink instability and stabilization of the Friedmann universe with scalar fields
The evolution of weak discontinuity is investigated in the flat FRW universe with a single scalar field and with multiple scalar fields. We consider both massless scalar fields and scalar fields with exponential potentials. Then we find that a new type of instability, i.e., kink instability develops in the flat FRW universe with massless scalar fields. The kink instability develops with scalar fields with considerably steep exponential potentials, while less steep exponential potentials do not suffer from kink instability. In particular, assisted inflation with multiple scalar fields does not suffer from kink instability. The stability of general spherically symmetric self-similar solutions is also discussed.
118497303
Visible borders of spacetime generated by high-energy collisions
Several years ago, two of the present authors proposed the concept of the border of spacetime as a generalization of spacetime singularities. Visible borders of spacetime, which replace naked singularities of classical theory, are not only necessary for the mathematical completeness of general relativity but they also provide a window into new physics of strongly curved spacetime, which is observable in principle. By employing simple geometrical and dimensional arguments, we show that not only black holes but also visible borders of spacetime will be generated at, for example, the CERN Large Hadron Collider, provided that the energy scale of quantum gravity is near 1 TeV in the framework of the large extra-dimension scenario.
119742208
Horizon Instability: a Local Analysis
The evolution of weak discontinuity is investigated on horizons in the n-dimensional static solutions in the Einstein-Maxwell-scalar-\Lambda system, including 
the Reissner-Nordstr"om-(anti) de Sitter black hole. The analysis is essentially local and nonlinear. We find that the Cauchy horizon is unstable, whereas both 
the black-hole event horizon and the cosmological event horizon are stable. This new instability, the so-called kink instability, of the Cauchy horizon is completely different from the well-known "infinite-blueshift" instability. The kink instability makes the analytic continuation beyond the Cauchy horizon unstable.
123245410
Neutron-rich Hypernuclei
Abstract Production and spectroscopy of neutron-rich Λ hypernuclei by double-charge exchange reactions ( π − , K + ) on nuclear targets are investigated in a theoretical viewpoint. The Λ–Σ coupling in nuclei induces a Σ admixture, which is enhanced in a neutron-excess environment, and plays an important role in production by a one-step mechanism π − p → K + Σ − via Σ − doorways in these reactions. A feasibility of extracting the Σ − admixture probability in the neutron-rich 10ΛLi hypernucleus by the 10B( π − , K + ) reaction is examined within a distorted-wave impulse approximation.
118332512
Isospin Properties of (K-, N) Reactions for the Formation of Deeply-bound Antikaonic Nuclei
Abstract The formation of deeply-bound antikaonic K − / K ¯ 0 nuclear states in nuclear ( K − , N ) reactions is investigated theoretically within a distorted-wave impulse approximation (DWIA), considering the isospin properties of the Fermi-averaged K − + N → N + K ¯ elementary amplitudes. We calculate the formation cross sections of the deeply-bound K ¯ states by the ( K − , N ) reactions on the nuclear targets, 12C and 28Si, at incident K − lab momentum p K − = 1.0 GeV / c and θ lab = 0 ° , introducing a complex effective nucleon number N eff for unstable bound states in the DWIA. The results show that the deeply-bound K ¯ states can be populated dominantly by the ( K − , n ) reaction via the total isoscalar Δ T = 0 transition owing to the isospin nature of the K − + N → N + K ¯ amplitudes, and that the cross sections described by Re N eff and Arg N eff enable to deduce the structure of the K ¯ nuclear states; the calculated inclusive nucleon spectra for a deep K ¯ -nucleus potential do not show distinct peak structure in the bound region. The few-body K ¯ ⊗ [ N N ] and K ¯ ⊗ [ N N N ] states formed in ( K − , N ) reactions on s-shell nuclear targets, 3He, 3H and 4He, are also discussed.
95195670
Structures and productions of typical sd-shell hypernuclei in shell-model calculations
As one of the typical sd-shell hypernuclei, the positive- and negative-parity energy levels of 20ΛNe are calculated to investigate structures of low-lying states within the shell model calculations. In addition to the conventional NN effective interactions, the ΛN G-matrix interactions derived from the Nijmegen NSC97f potentials are employed. The production cross sections of (π+,K+) and (K−,π−) reactions are estimated using the calculated shell-model wave functions.
119341375
Calculation of the 3He (in-flight K−,n) reaction for searching the deeply-bound K−pp state
Abstract The formation of a deeply-bound K − p p state by the 3 He (in-flight K − , n ) reaction is investigated theoretically in the distorted-wave impulse approximation using the Green's function method. The expected inclusive and semi-exclusive spectra at p K − = 1.0 GeV / c and θ n = 0 ° are calculated for the forthcoming J-PARC E15 experiment. We employ optical potentials between the K − and “ pp ” core-nucleus, and demonstrate systematically the dependence of the spectral shape on V 0 and W 0 , which are the real and imaginary parts of the strength for the optical potential, respectively. The necessary condition to observe a distinct peak of the K − p p bound state with I = 1 / 2 , J π = 0 − in the spectrum turns out to be that the value of V 0 is deeper than ∼ − 100 MeV and W 0 shallower than ∼ − 100  MeV , of which the strength parameters come up to recent theoretical predictions.