Dipole anisotropies of IRAS galaxies and the contribution of a large-scale local void

Recent observations of dipole anisotropies show that the velocity of the Local Group (\Vec v_{\rm G}) induced by the clustering of IRAS galax ies has an amplitude and direction similar to those of the velocity of Cosmic Microwave Background dipole anisotropy (\Vec v_{\rm CMB}), but the difference | \Vec v_{\rm G} - \Vec v_{\rm CMB} | is still $\sim 170$ km/s, which is about 28% of |\Vec v_{\rm CMB} |. Here we consider the possibility that the origin of this difference comes from a hypothetical large-scale local void, with which we can account for the accelerating behavior of type Ia supernovae due to the spatial inhomogeneity of the Hubble constant without dark energies and derive the constraint to the model parameters of the local void. It is found as a result that the distance between the Local Group and the center of the void must be $(10 -- 20) h^{-1}$ Mpc, whose accurate value depends on the background model parameters.Comment: 13 pages, 1 figure, to be published in ApJ 584, No.2 (2003

Gravitational Lens Statistics and The Density Profile of Dark Halos

We investigate the influence of the inner profile of lens objects on gravitational lens statistics taking into account of the effect of magnification bias and both the evolution and the scatter of halo profiles. We take the dark halos as the lens objects and consider the following three models for the density profile of dark halos; SIS (singular isothermal sphere), the NFW (Navarro Frenk White) profile, and the generalized NFW profile which has a different slope at smaller radii. The mass function of dark halos is assumed to be given by the Press-Schechter function. We find that magnification bias for the NFW profile is order of magnitude larger than that for SIS. We estimate the sensitivity of the lensing probability of distant sources to the inner profile of lenses and to the cosmological parameters. It turns out that the lensing probability is strongly dependent on the inner density profile as well as on the cosmological constant. We compare the predictions with the largest observational sample, the Cosmic Lens All-Sky Survey. The absence or presence of large splitting events in larger surveys currently underway such as the 2dF and SDSS could set constraints on the inner density profile of dark halos.Comment: 22 pages, minor changes and references added, accepted for publication in Ap

Spin melting and refreezing driven by uniaxial compression on a dipolar hexagonal plate

We investigate freezing characteristics of a finite dipolar hexagonal plate by the Monte Carlo simulation. The hexagonal plate is cut out from a piled triangular lattice of three layers with FCC-like (ABCABC) stacking structure. In the present study an annealing simulation is performed for the dipolar plate uniaxially compressed in the direction of layer-piling. We find spin melting and refreezing driven by the uniaxial compression. Each of the melting and refreezing corresponds one-to-one with a change of the ground states induced by compression. The freezing temperatures of the ground-state orders differ significantly from each other, which gives rise to the spin melting and refreezing of the present interest. We argue that these phenomena are originated by a finite size effect combined with peculiar anisotropic nature of the dipole-dipole interaction.Comment: Proceedings of the Highly Frustrated Magnetism (HFM2006) conference. To appear in a special issue of J. Phys. Condens. Matte

The Resonant Cavity Radiator (RCR)

The design of the resonant cavity radiator (RCR) is compared to that of the slotted waveguide array in terms of efficiency, weight, and structural integrity. It is shown that the RCR design has three significant potentials over the slotted waveguide array: (1) improvement in efficiency; (2) lighter weight; and (3) simpler structure which allows the RCR to be integrated with the RF tube to alleviate thermal interface problems

Finite-size Scaling of Correlation Ratio and Generalized Scheme for the Probability-Changing Cluster Algorithm

We study the finite-size scaling (FSS) property of the correlation ratio, the ratio of the correlation functions with different distances. It is shown that the correlation ratio is a good estimator to determine the critical point of the second-order transition using the FSS analysis. The correlation ratio is especially useful for the analysis of the Kosterlitz-Thouless (KT) transition. We also present a generalized scheme of the probability-changing cluster algorithm, which has been recently developed by the present authors, based on the FSS property of the correlation ratio. We investigate the two-dimensional quantum XY model of spin 1/2 with this generalized scheme, obtaining the precise estimate of the KT transition temperature with less numerical effort.Comment: 4 pages, RevTeX4, to appear in Phys. Rev. B, Rapid Communication

Probability-Changing Cluster Algorithm: Study of Three-Dimensional Ising Model and Percolation Problem

We present a detailed description of the idea and procedure for the newly proposed Monte Carlo algorithm of tuning the critical point automatically, which is called the probability-changing cluster (PCC) algorithm [Y. Tomita and Y. Okabe, Phys. Rev. Lett. {\bf 86} (2001) 572]. Using the PCC algorithm, we investigate the three-dimensional Ising model and the bond percolation problem. We employ a refined finite-size scaling analysis to make estimates of critical point and exponents. With much less efforts, we obtain the results which are consistent with the previous calculations. We argue several directions for the application of the PCC algorithm.Comment: 6 pages including 8 eps figures, to appear in J. Phys. Soc. Jp

Renormalization Group Approach to Einstein Equation in Cosmology

The renormalization group method has been adapted to the analysis of the long-time behavior of non-linear partial differential equation and has demonstrated its power in the study of critical phenomena of gravitational collapse. In the present work we apply the renormalization group to the Einstein equation in cosmology and carry out detailed analysis of renormalization group flow in the vicinity of the scale invariant fixed point in the spherically symmetric and inhomogeneous dust filled universe model.Comment: 16 pages including 2 eps figures, RevTe

Distance-Redshift in Inhomogeneous Omega0=1Omega_0=1 Friedmann-Lemaitre-Robertson-Walker Cosmology

Distance--redshift relations are given in terms of associated Legendre functions for partially filled beam observations inspatially flat Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies. These models are dynamically pressure-free, flat FLRW on large scales but, due to mass inhomogeneities, differ in their optical properties. The partially filled beam area-redshift equation is a Lame$^{\prime}$ equation for arbitrary FLRW and is shown to simplify to the associated Legendre equation for the spatially flat, i.e. $\Omega_0=1$ case. We fit these new analytic Hubble curves to recent supernovae (SNe) data in an attempt to determine both the mass parameter $\Omega_m$ and the beam filling parameter $\nu$. We find that current data are inadequate to limit $\nu$. However, we are able to estimate what limits are possible when the number of observed SNe is increased by factor of 10 or 100, sample sizes achievable in the near future with the proposed SuperNova Acceleration Probe satellite.Comment: 9 pages, 3 figure

Low Energy Effective Action for Horava-Witten Cosmology

As a supersymmetric extension of the Randall-Sundrum model, we consider a 5-dimensional Horava-Witten type theory, and derive its low energy effective action. The model we consider is a two-brane system with a bulk scalar field satisfying the BPS condition. We solve the bulk equations of motion using a gradient expansion method, and substitute the solution into the original action to get the 4-dimensional effective action. The resultant effective theory can be casted into the form of Einstein gravity coupled with two scalar fields, one arising from the radion, the degree of freedom of the inter-brane distance, and the other from the bulk scalar field. We also clarify the relation between our analysis and the moduli approximation.Comment: 11 page