Kazuyuki Yamashita

Testing the Domain Wall Dominated Scenario for the Evolution of the Large Scale Structure of the Universe

A network of domain walls would accelerate the expansion of the universe, but it would also exert a repulsive force expected to help the formation of large-scale structures. We used three-dimensional (3D) N-body simulations of the dynamical evolution of the formation of the structure under the assumption of various configurations of the domain wall network and obtained some cosmological observables: the cosmic Mach number, the velocity distribution, and the two-point correlation. We conclude by comparing the simulation-derived values with values derived from observational data that an era of the domain wall dominated universe can exist. Recent observations of distant Type Ia supernovae 1) suggest that the expansion of the universe is accelerating.Some exotic form of the energy density is required for this accelerated expansion of the universe.More specifically, an equation of state for such energy density is α ≡ p/ρ < 0 with pressure p and energy density ρ, and this is not possible for ordinary matter.Candidates for such an exotic energy density are a non-Abelian cosmic string network, a domain wall network, and a cosmological constant.For example, if the cosmic string settles down to an equilibrium configuration, and if its energy density dominates the universe, an equation of state results with α = −1/3.Similarly, the equation of state for the domain wall dominated universe is α = −2/3, and that for the cosmological constant dominated universe is α = −1.Bucher and Spergel 2) derived an evolution equation for the cosmological perturbations in a flat universe with cold dark matter (CDM) and matter satisfying α< 0, and computed the resulting large-scale cosmic microwave background (CMB) anisotropy. In this paper, we report 3-dimensional (3D) numerical simulations testing the scenario of the domain wall dominated universe by investigating the possibility of the formation of large scale structure.The domain wall discussed here is a relic of the phase transition at late time and moves with only Hubble flow in the universe. According to the current understanding of particle physics, a high degree of symmetry among elementary fields is recovered at high energy scales.In the early universe, this symmetry is expected to be recovered also because of high temperature.The symmetry has been broken as the universe has cooled during its expansion.If the way in which the symmetry is broken is peculiar, topological defects, which are the

Discovery of X-Ray Emission from the Distant Lensing Cluster of Galaxies Cl 2236–04 at z = 0.552

X-ray emission from the distant lensing cluster Cl 2236-04 at z = 0.552 was discovered by ASCA and ROSAT/HRI observations. If the spherical symmetric mass distribution model of the cluster is assumed, the lensing estimate of the cluster mass is a factor of 2 higher than that obtained from X-ray observations as reported for many distant clusters. However, the elliptical and clumpy lens model proposed by Kneib and coworkers is surprisingly consistent with the X-ray observations, assuming that the X-ray-emitting hot gas is isothermal and in a hydrostatic equilibrium state. The existence of the cooling flow in the central region of the cluster is indicated by the short central cooling time and the excess flux detected by ROSAT/HRI compared to the ASCA flux. However, it is shown that, even if the Cl 2236-04 has a cooling flow in the central region, the temperature measured by ASCA, which is the mean emission-weighted cluster temperature in this case, should not be different from the virial temperature of the cluster. Therefore, we conclude that the effects of the clumpiness and nonzero ellipticity in the mass distribution of the cluster are essential to explain the observed feature of the giant luminous arc, and in this cluster there is no discrepancy between strong lensing and X-ray estimation of the cluster mass.

Development of Simulation Code Connecting Particle-in-Cell and Magnetohydrodynamics on Hierarchical Mesh

We develop a simulation code for multi-hierarchical phenomena. The simulation domain is divided to a Particle-in-Cell (PIC) domain and a magnetohydrodynamics (MHD) domain [1]. The MHD domain has a regular-intervals mesh in the center region and a hierarchical mesh in the peripheral region, and the latter is managed by Adaptive Mesh Refinement (AMR) method [2]. It is able to extend the MHD domain outward with low computational costs. This approach makes it possible to treat microscopic physics and macroscale structure simultaneously. Test models are implemented and computational costs in memory and CPU time are estimated.

Performance of Vector/Parallel Orientated Hydrodynamic Code

We have developed a hydrodynamic 3 dimensional AMR code which is suitable for vectorization and parallelization based on Fully Threaded Tree method. For the case of 3D FTT-AMR simulation, the number of higher resolution cells is eight times of that of mother low resolution cells. Therefore if the region where the highest resolution is required has certain volume, it is better to use single time step for every levels of resolution because we can avoid artificial flow at the surface of resolution interface. This level-independent time step also makes possible to simplify the evaluation of refinement indicator. As a result, we can overcome the demerit of consuming CPU time by computing at level-independent time step less than CFL limit, by boosting vector acceleration ratio.

Performance of Adaptive Mesh Refinement Scheme for Hydrodynamics on Simulations of Expanding Supernova Envelope

We present results of performance measurement of our astrophysical fluid dynamics code with Adaptive Mesh Refinement (AMR) scheme. As an example of its application for astrophysical phenomena, we show the results of 3D simulation of Rayleigh-Taylor instability in expanding supernova envelope and the possibility of reconciliation of the present model with observations. The efficiency of memory and CPU-time savings is discussed and measured with this simulation. Its result shows that our code has the ability to simulate phenomena with wide dynamic ranges even in limited computing resources.

Hydrodynamical simulations using a fully threaded tree

Abstract We developed a 3D code of Adaptive Mesh Refinement (AMR) hydrodynamical simulations that is useful for propagations of an interplanetary shock wave. In this code, we adopted the Fully Threaded Tree (FTT; Khokhlov 1998) for AMR part, the 3rd order MUSCL of the Roe method for hydrodynamical part, and the two step method for the 2nd order time development. In this article, we explain our 3D AMR code and the FTT, and report some results of test calculation.

Increase in high energy particle flux affected by an interplanetary shock wave

Abstract An energetic proton event was observed at a geosynchronous orbit at nearly the same time as an interplanetary shock wave reached the Earths magnetosphere, indicating that these two events interacted with each other during their passage to Earth. The variation of the energetic proton flux is not explained by the shock acceleration mechanism because there was no diffusive behavior upwind of the shock wave. These protons could have been ejected by a solar flare, because an energetic flare occurred at a well-connected position before the proton event, however, the delay time for the propagation of the energetic particles was too long if this flare was the origin of those particles. In the previous paper, we showed that the peculiar behavior of high-energy particles could be explained by the following scenario: the protons produced during a proton flare, which occurred after the CME, catch up with it and enter the turbulent region behind the shock wave, they are then scattered by the irregular magnetic field, that is, the Fermi acceleration, and are captured in the turbulent region behind the shock wave. In this paper, we obtain a scale of the irregularity of the turbulent magnetic field using Fourier transformation, compare it with a mean free path consistent with the one required from the observational data and the Larmor radius, and show that the proposed acceleration mechanism explains these events.

New Data Compression for Multi-Dimensional Numerical Simulation

With an increase in the processing speed, we can perform multi-dimensional numerical simulations. On the other hand, we are facing with problems that we have to handle huge amount of output data. We can attain higher time resolution in the analysis of simulation results by storing evolutional sequence more frequently. However, that interval is limited by the disk space and the speed of I/O. To overcome this dilemma, we propose a new real time data compression algorithm.

The Angular Correlation Function of Faint Galaxies during the Decreasing Correlation Period

During the period of galaxy formation, many small-length-scale perturbations superimposed on high-amplitude large-length-scale perturbations should have their overdensities boosted into the non-linear regime, while small-scale perturbations in other regions remain linear. The formation of the first galaxies in discrete, high overdensity regions may lead to an initial amplitude of the spatial correiatio? function of galaxies, ξ 0 , much higher than that expected from linear fluctuation theory. This initially high “bias” would consequently decrease to the near-unity values expected from local observations. Such a Decreasing Correlation Period (DCP) is detected in N-body simulations under certain conditions by several authors; Roukema (1993), Brainerd & Villumsen (1994), Roukema et al. (1997) and unpublished simulations by one of us (Yamashita).