arXiv subjects

Kenji Tomita

Publications and source records attributed to Kenji Tomita.

33 records · Page 2Linked to original sources

arXiv2005

Junction conditions of cosmological perturbations

The behavior of perturbations is studied in cosmological models which consist of two different homogeneous regions connected in a spherical shell boundary. The junction conditions for the metric perturbations and the displacements of the shell boundary are analyzed and the surface densities of the perturbed energy and momentum in the shell are derived, using Mukohyama's gauge-invariant formalism and the Israel discontinuity condition. In both homogeneous regions the perturbations of scalar, vector and tensor types are expanded using the 3-dimensional harmonic functions, but the model coupling among them is caused in the shell by the inhomogeneity. By treating the perturbations with odd and even parities separately, it is found, however, that we can have consistent displacements and surface densities for given metric parturbations

astro-ph↗

arXiv2004

Cluster abundance evolution and the Sunyaev-Zeldovich effect in various cosmological models

The redshift dependence of the observed cluster abundance due to the Sunyaev-Zeldovich Effect (SZE) is studied in various cosmological models including flat and open homogeneous (CDM) models and an inhomogeneous model with a large-scale local void. The Press-Schechter formalism is used to derive the abundance at epochs 0 < z < 2 and the cluster mass limit Mlim is given from a flux limit for SZE. It is shown that SZE is useful for constraining the cosmological model parameters, and the abundance in the inhomogeneous model may be comparable with that in the low-density homogeneous models. The significance of relative difference of abundances in the various models is discussed.

astro-ph↗

arXiv2002

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.

astro-ph↗

arXiv2002

Luminous Matter Distribution, Bulk Flows and Baryon Contents in Cosmological Models with a Local Void

First, we consider galaxy formation from the viewpoint of hierarchical clustering theory and discuss the possibility that inhomogeneous models with a local void may be compatible with the observed homogeneity of galactic distributions found by recent redshift surveys, because their inhomogeneity can be weakened by the difference in the feedback system of galaxy formation between the inner and outer regions. Next, it is shown with the results of numerical simulations that the observed inhomogeneity of two-point correlations of galaxies can be accounted for by these models. Also, the natural appearance of bulk flows for an off-central observer is shown. Finally the inhomogeneity of baryon contents is discussed from the viewpoint of our inhomogeneous models.

astro-ph↗

arXiv2001

Analyses of Type Ia Supernova Data in Cosmological Models with a Local Void

The data of type Ia supernovae observed by the High-z SN Search Team and Supernova Cosmology Project are analyzed in inhomogeneous cosmological models with a local void on scales of about 200 Mpc, to derive the best-fit values of model parameters and the confidence contours. The $χ^2$ fitting is found to be slightly better than that in homogeneous models. It is shown that (1) the best-fit values are most sensitive to the ratio $R$ of the outer Hubble constant ($H_0^{\rm II}$) to the inner Hubble constant ($H_0^{\rm I}$), (2) the best-fit outer density parameter ($Ω_0^{\rm II}$) (and cosmological constant parameter ($λ_0^{\rm II}$)) increases (and decreases) with the increase of $R$, respectively, and (3) $(Ω_0^{\rm II}, λ_0^{\rm II})$ can be $(1, 0)$ for $R \approx 0.8$. Moreover it is shown that these models may be naturally consistent with the new supernova data (SN1997ff) for $z = 1.7$.

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arXiv2001

A Local Void and the Accelerating Universe

Corresponding to the recent observational claims that we are in a local void (an underdense region) on scales of 200 - 300 Mpc, the magnitude-redshift relation in a cosmological model with a local void is investigated. It is already evident that the accelerating behavior of high-z supernovae can be explained in this model, because the local void plays a role similar to the positive cosmological constant. In this paper the dependence of the behavior on the gaps of cosmological parameters in the inner (low-density) region and the outer (high-density) region, the radius of the local void, and the clumpiness parameter is studied and its implication is discussed.

astro-ph↗

arXiv2001

Anisotropy of the Hubble Constant in a Cosmological Model with a Local Void on Scales of ~ 200 Mpc

A spherical cosmological model with a local void on scales of $\sim 200$ Mpc and with an inhomogeneous Hubble constant was proposed in recent two papers. This model explains consistently the observed properties of the cosmic bulk flow, the accelerating behavior of type Ia supernovae and the CMB dipole anisotropy without invoking a cosmological constant. As we are in a position deviated from the center in the model, the anisotropy of the Hubble constant appears owing to the directional difference of the distance from the observer to the boundary of the void region. It is found that the anisotropy is maximally about 6% of the constant in the region of $200 \sim 500$ Mpc from us. This inhomogeneity and anisotropy of the Hubble constant are not so large as to be inconsistent with the present observation. The detection of this anisotropy in the future will be useful to clarify the implication of the inhomogeneity of the Hubble constant.

astro-ph↗

arXiv2000

Temperature correlations in a compact hyperbolic universe

The effect of a non-trivial topology on the temperature correlations on the cosmic microwave background (CMB) in a small compact hyperbolic universe with volume comparable to the cube of the curvature radius is investigated. Because the bulk of large-angular CMB fluctuations is produced at the late epoch in low $Ω_0$ models, the effect of a long wavelength cut-off due to the periodic structure does not lead to the significant suppression of large-angular power as in compact flat models. The angular power spectra are consistent with the COBE data for $Ω_0 \ge 0.1$.

astro-ph↗

arXiv1999

Bulk flows and CMB dipole anisotropy in cosmological void models

The observational behavior of spherically symmetric inhomogeneous cosmological models is studied, which consist of inner and outer homogeneous regions connected by a shell or an intermediate self-similar region. It is assumed that the present matter density parameter in the inner region is smaller than that in the outer region, and the present Hubble parameter in the inner region is larger than that in the outer region. Then galaxies in the inner void-like region can be seen to have a bulk motion relative to matter in the outer region, when we observe them at a point O deviated from the center C of the inner region. Their velocity $v_p$ in the CD direction is equal to the difference of two Hubble parameters multiplied by the distance between C and O. It is found also that the velocity $v_d$ corresponding to CMB dipole anisotropy observed at O is by a factor $\approx 10$ small compared with $v_p$. This behavior of $v_d$ and $v_p$ may explain the puzzling situation of the cosmic flow of cluster galaxies, when the radius of the inner region and the distance CD are about 200 $h^{-1}$ Mpc and 40 $h^{-1}$ Mpc, respectively ($H_0 = 100 h^{-1} km/sec Mpc^{-1}$), and when the gaps of density and Hubble parameters are $\approx 0.5$ and 18%, respectively.

astro-ph↗

arXiv1999

Distances and lensing in cosmological void models

We study the distances and gravitational lensing in spherically symmetric inhomogeneous cosmological models consisting of inner and outer homogeneous regions which are connected by a single shell or double shells at the redshift $z_1 \sim 0.067$. The density and Hubble parameters in the inner region are assumed to be smaller and larger, respectively, than those in the outer region. It is found that at the stage $z_1 < z < 1.5$ the distances from an observer in the inner void-like region are larger than the counterparts (with equal $z$) in the corresponding homogeneous Friedmann models, and hence the magnitudes for the sources at this stage are larger. This effect of the void-like low-density region may explain the deviations of the observed [magnitude-redshift] relation of SNIa from the relation in homogeneous models, independently of the cosmological constant. When the position of the observer deviates from the center, moreover, it is shown that the distances are anisotropic and the images of remote sources are systematically deformed. The above relation at $z \geq 1.0$ and this anisotropy will observationally distinguish the role of the above void-like region from that of the positive cosmological constant. The influence on the time-delay measurement is also discussed.

astro-ph↗

arXiv1999

Distances in Inhomogeneous Cosmological Models

Distances play important roles in cosmological observations, especially in gravitational lens systems, but there is a problem in determining distances because they are defined in terms of light propagation, which is influenced gravitationally by the inhomogeneities in the universe. In this paper we first give the basic optical relations and the definitions of different distances in inhomogeneous universes. Next we show how the observational relations depend quantitatively on the distances. Finally, we give results for the frequency distribution of different distances and the shear effect on distances obtained using various methods of numerical simulation.

astro-ph↗

arXiv1999

Various Approaches to Cosmological Gravitational Lensing in Inhomogeneous Models

Gravitational lensing of distant objects caused by gravitational tidal forces from inhomogeneities in the universe is weak in most cases, but it is noticed that it gives a great deal of information about the universe, especially regarding the distribution of dark matter. The statistical values of optical quantities such as convergence, amplification and shear have been derived by many people using various approaches, which include the linear perturbational treatment in the weak limit and the nonlinear treatment considering small-scale matter distribution. In this review paper we compare the following three main approaches: (a) the approach in the multi-lens-plane theory; (b) the approach due to the direct integration method; and (c) the perturbational approach. In the former two approaches inhomogeneous matter distributions are produced in the CDM model using $N$-body simulations (the P$^3$M code and the tree-code, respectively). In (c) the power spectrum corresponding to the CDM model is used for the large-scale matter distribution.

astro-ph↗

arXiv1998

Plane-Symmetric Vacuum Solutions with Null Singularities for Inhomogeneous Models and Colliding Gravitational Waves

New exact vacuum solutions with various singularities in the plane-symmetric spacetime are shown, and they are applied to the analysis of inhomogeneous cosmological models and colliding gravitational waves. One of the singularities can be true null singularities, whose existence was locally clarified by Ori. These solutions may be interesting from the viewpoint of the variety of cosmological singularities and the instability problem of Cauchy horizons inside black holes.

gr-qc↗

arXiv1998

Angular Diameter Distances in Clumpy Friedmann Universes

Solving null-geodesic equations, behavior of angular diameter distances is studied in inhomogeneous cosmological models, which are given by performing N-body simulations with the CDM spectrum. The distances depend on the separation angle of ray pairs, the mass and the radius of particles cosisting of galaxies and dark matter balls, and cosmological model parameters. The calculated distances are compared with the Dyer- Roeder distance, and after many ray-shooting, the average, dispersion and distribution of the clumpiness parameter are derived.

astro-ph↗

arXiv1998

Cosmological Simulation of Gravitational Weak Lensing in Open Models

Cosmological simulation of gravitational weak lensing in open models is treated using the direct integration method, which is independent of the mult-lens-plane method. Statistical averages of optical quantuties (the convergencen shear and amplification) are derived for ray bundles passing through the clumpy universe, in which the initial state has the CDM spectrum with the power n = 1.

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