arXiv subjects
Publications and source records attributed to Nobuyuki Sakai.
We investigate the properties of global monopoles with an event horizon. We find that there is an unstable circular orbit even if a particle does not have an angular momentum when the core mass is negative. We also obtain the asymptotic form of solutions when the event horizon is much larger than the core radius of the monopole, and discuss if they could be a model of galactic halos.
We reconstruct a 3+1 formalism of general relativistic electromagnetism, and derive the equations of motion of charged particles in the pulsar magnetosphere, taking account of the inclination between the rotation axis and the magnetic axis. Apart from the previous works where space charge is evaluated by assuming the flow velocity being the light speed, we analyze particle motion in the polar cap, finding that gravity changes significantly its dynamics and the condition for acceleration.
We consider a modified ``Swiss cheese'' model in Brans-Dicke theory, and use it to discuss the evolution of black holes in an expanding universe. We define the black hole radius by the Misner-Sharp mass and find their exact time evolutions for dust and vacuum universes of all curvatures.
We consider a modified ``Swiss cheese'' model in the Brans-Dicke theory, and discuss the evolution of black holes in the expanding universe. We define the black hole radius by the Misner-Sharp mass and find the time evolution for dust and vacuum universes.
We investigate a topological inflation model in supergravity. By means of numerical simulations, it is confirmed that topological inflation can take place in supergravity. We also show that the condition for successful inflation depends not only on the vacuum-expectation value (VEV) of inflaton field but also on the form of its Kähler potential. In fact, it is found that the required VEV of the inflaton $ϕ$ can be as small as $<ϕ> \simeq 1 \times M_G$, where $M_{G}$ is the gravitational scale.
We clarify the causal structure of an inflating magnetic monopole. The spacetime diagram shows explicitly that this model is free from ``graceful exit'' problem, while the monopole itself undergoes ``eternal inflation''. We also discuss general nature of inflationary spacetimes.
We address some issues of topological defect inflation. (1) We clarify the causal structure of an inflating magnetic monopole. The spacetime diagram shows explicitly that this model is free from the ``graceful exit'' problem, while the monopole itself undergoes ``eternal inflation''. (2) We extend the study of inflating topological defects to Brans-Dicke gravity. Contrary to the case of Einstein gravity, any inflating monopole eventually shrinks and takes a stable configuration. (3) We reanalyze chaotic inflation with a non-minimally coupled massive scalar field. We find a new solution of domain wall inflation, which relaxes constraints on the coupling constant for successful inflation.
Propagation of light in a clumpy universe is examined. As an inhomogeneous matter distribution, we take a spherical void surrounded by a dust shell where the ``lost mass'' in the void is compensated by the shell. We study how the angular-diameter distance behaves when such a structure exists. The angular-diameter distance is calculated by integrating the Raychaudhuri equation including the shear. An explicit expression for the junction condition for the massive thin shell is calculated. We apply these results to a dust shell embedded in a Friedmann universe and determine how the distance-redshift relation is modified compared with that in the purely Friedmann universe. We also study the distribution of distances in a universe filled with voids. We show that the void-filled universe gives a larger distance than the FRW universe by $\sim 5%$ at $z \sim 1$ if the size of the void is $\sim 5%$ of the Horizon radius.
As a study of peculiar velocities of nonlinear structure, we analyze the model of a relativistic thin-shell void in the expanding universe. (1) Adopting McVittie (MV) spacetime as a background universe, we investigate the dynamics of an uncompensated void with negative MV mass. Although the motion itself is quite different from that of a compensated void, as shown by Haines & Harris (1993), the present peculiar velocities are not affected by MV mass. (2) We discuss how precisely the formula in the linear perturbation theory applies to nonlinear relativistic voids, using the results in (1) as well as the previous results for the homogeneous background (Sakai, Maeda, & Sato 1993). (3) We re-examine the effect of the cosmic microwave background radiation. Contrary to the results of Pim & Lake (1986, 1988), we find that the effect is negligible. We show that their results are due to inappropriate initial conditions. Our results (1)-(3) suggest that the formula in the linear perturbation theory is approximately valid even for nonlinear voids.
We reanalyze cosmology of a non-minimally coupled massive scalar field which was originally discussed by Futamase and Maeda in the context of chaotic inflation scenario. We find a new type of inflationary solution where inflation occurs inside a domain wall. This new solution relaxes constraints on the coupling constant for successful inflation.
The decay of a metastable false vacuum by bubble nucleation is studied in the high temperature limit of the gauge theory in which an SO(3) gauge symmetry is spontaneously broken to an SO(2). The effects of internal symmetry are so drastic that, in addition to the known Euclidean bounce solution, there exists a new bubble solution involving a 't Hooft-Polyakov monopole at its center the moment it is nucleated. The decay rate and evolution are analyzed.
According to previous work, topological defects expand exponentially without an end if the vacuum expectation value of the Higgs field is of the order of the Planck mass. We extend the study of inflating topological defects to the Brans-Dicke gravity. With the help of numerical simulation we investigate the dynamics and spacetime structure of a global monopole. Contrary to the case of the Einstein gravity, any inflating monopole eventually shrinks and takes a stable configuration. We also discuss cosmological constraints on the model parameters.
We study the effect of a void network on the CMB anisotropy in the Einstein-de Sitter background using Thompson &Vishniac's model. We consider comprehensively the Sacks-Wolfe effect, the Rees-Sciama effect and the gravitational lensing effect. Our analysis includes the model of primordial voids existing at recombination, which is realized in some inflationary models associated with a first-order phase transition. If there exist primordial voids whose comoving radius is larger than $\sim10h^{-1}$Mpc at recombination, not only the Sachs-Wolfe effect but also the Rees-Sciama effect is appreciable even for multipoles $l\lsim1000$ of the anisotropy spectrum. The gravitational lensing effect, on the other hand, slightly smoothes the primary anisotropy; quantitatively, our results for the void model are similar to the previous results for a CDM model. All the effects, together, would give some constraints on the configuration or origin of voids with high-resolution data of the CMB anisotropy.
First-order cosmological phase transitions are considered in the models with an $O(3)$-symmetric scalar field, in the high temperature limit. It is shown that a global monopole can be produced at the center of a bubble when the bubble is nucleated.
According to previous work on magnetic monopoles, static regular solutions are nonexistent if the vacuum expectation value of the Higgs field $η$ is larger than a critical value $η_{\rm cr}$, which is of the order of the Planck mass. In order to understand the properties of monopoles for $η>η_{\rm cr}$, we investigate their dynamics numerically. If $η$ is large enough ($\ggη_{\rm cr}$), a monopole expands exponentially and a wormhole structure appears around it, regardless of coupling constants and initial configuration. If $η$ is around $η_{\rm cr}$, there are three types of solutions, depending on coupling constants and initial configuration: a monopole either expands as stated above, collapses into a black hole, or comes to take a stable configuration.
We consider the classical fluctuations of the gravitational constant generated by bubbles in the inflationary universe. For extended inflation, we demonstrate numerically how and how large fluctuations are produced during bubble expansion. The amplitude of the fluctuations depends on the Brans-Dicke parameter $ω$: if $ω$ is of the order of unity, the amplitude becomes of the order of unity within one Hubble expansion time; if $ω$ is large (say, $ω=1000$), the growth rate of the fluctuations is small, but it keeps growing without freezing during inflation. We also discuss some astrophysical implications of our results.
The nucleation and evolution of bubbles are investigated in the model of an $O(3)$-symmetric scalar field coupled to gravity in the high temperature limit. It is shown that, in addition to the well-known bubble of which the inside region is true vacuum, there exists another decay channel at high temperature which is described by a new solution such that a false vacuum region like a global monopole remains at the center of a bubble. The value of the Euclidean action of this bubble is higher than that of the ordinary bubble; however, the production rate of it can be considerable for a certain range of scalar potentials.
We review how the continuous symmetry can support a soliton inside a high-temperature bubble at the time of its nucleation. This solitonic island in disoriented phase remains stable during the growth of bubbles before their collision.