Searcharxiv⌕ Search

arXiv subjects

Masahiro Morikawa

Publications and source records attributed to Masahiro Morikawa.

At least 37 records · Page 2Linked to original sources
arXiv2008

Gravitational Fugacity as the seed of self-organized violent relaxaton process toward Local Virial relation

We propose the self-organized relaxation process which drives a collisionless self-gravitating system (SGS) to the equilibrium state satisfying local virial (LV) relation. During the violent relaxation process, particles can move widely within the time interval as short as a few free fall times, because of the effective potential oscillations. Since such particle movement causes further potential oscillations, it is expected that the system approaches the critical state where such particle activities, which we call gravitational fugacity, is independent of the local position as much as possible. Here we demonstrate that gravitational fugacity can be described as the functional of the LV ratio, which means that the LV ratio is a key ingredient estimating the particle activities against gravitational potential. We also demonstrate that LV relation is attained if the LV ratio exceeds the crticaial value $b=1$ everywhere in the bound region during the violent relaxation process. The local region which does not meet this criterion can be trapped into the pre-saturated state. However, small phase-space perturbation can bring the inactive part into the LV critical state.

astro-ph↗
arXiv2007

Cosmic structures via Bose Einstein condensation and its collapse

We develop our novel model of cosmology based on the Bose-Einstein condensation. This model unifies the Dark Energy and the Dark Matter, and predicts multiple collapse of condensation, followed by the final acceleration regime of cosmic expansion. We first explore the generality of this model, especially the constraints on the boson mass and condensation conditions. We further argue the robustness of this model over the wide range of parameters of mass, self coupling constant and the condensation rate. Then the dynamics of BEC collapse and the preferred scale of the collapse are studied. Finally, we describe possible observational tests of our model, especially, the periodicity of the collapses and the gravitational wave associated with them.

astro-ph↗
arXiv2006

An Interpretation of Flat Density Cores of Clusters of Galaxies by Degeneracy Pressure of Fermionic Dark Matter: A Case Study of Abell 1689

Flat density cores have been obtained for a limited number of clusters of galaxies by strong gravitational lensing. This paper explores the possibility that the degeneracy pressure of fermionic dark matter accounts for the flat top density profiles. This is a case study of A1689 for which the density profile has been obtained from the inner region out to 1Mpc by the combination of strong and weak lensing. In the case that the dark matter consists of the mixture of degenerate relic neutrinos and collisionless cold dark matter particles, the acceptable mass range for relic neutrinos is between 1 and 2 eV, if the ratio of the two kinds of dark matter particles is fixed to its cosmic value.

astro-ph↗
arXiv2006

Relativistic Gross-Pitaevskii equation and the cosmological Bose Einstein Condensation

We do not know 96% of the total matter in the universe at present. In this paper, a cosmological model is proposed in which Dark Energy (DE) is identified as Bose-Einstein Condensation (BEC) of some boson field. Global cosmic acceleration caused by this BEC and multiple rapid collapses of BEC into black holes etc. (=Dark Matter (DM)) are examined based on the relativistic version of the Gross-Pitaevskii equation. We propose (a) a novel mechanism of inflation free from the slow-rolling condition, (b) a natural solution for the cosmic coincidence ('Why Now?') problem through the transition from DE into DM, (c) very early formation of highly non-linear objects such as black holes, which might trigger the first light as a form of quasars, and (d) log-z periodicity in the subsequent BEC collapsing time. All of these are based on the steady slow BEC process.

astro-ph↗
arXiv2005

Local virial relation for self-gravitating system

We demonstrate that the quasi-equilibrium state in self-gravitating $N$-body system after cold collapse are uniquely characterized by the local virial relation using numerical simulations. Conversely assuming the constant local virial ratio and Jeans equation for spherically steady state system, we investigate the full solution space of the problem under the constant anisotropy parameter and obtain some relevant solutions. Especially, the local virial relation always provides a solution which has a power law density profile in both the asymptotic regions $r\to 0$ and $\infty$. This type of solutions observed commonly in many numerical simulations. Only the anisotropic velocity dispersion controls this asymptotic behavior of density profile.

astro-ph↗
arXiv2005

Fully Degenerate Self-Gravitating Fermionic Dark Matter: Implications to the Density Profile of the Cluster of Galaxies A1689, and the Mass Hierarchy of Black Holes

Equilibrium configurations of weakly interacting fully degenerate fermionic dark matter are considered at various scales in the Universe. We treat the general situations for the gravity from Newtonian to general relativity and the degeneracy from nonrelativistic to relativistic. A dimensionless equilibrium configuration is specified by a single parameter regardless of particle properties, the Fermi velocity at the center, and the scalings of mass and length are specified by the rest mass and statistical weight of the dark matter particle. We focus our attention to the flat-top nature of the mass column density profile of the cluster of galaxies, A1689, recently reported by Broadhurst et al. using gravitational lensing. We convert the column density profile to a volume density profile assuming spherical symmetry and derive a 3D encircled mass profile of A1689, which is compared with the model profiles of degenerate fermion structures. The flat-top profile is reproduced. The corresponding fermion mass ranges from 2 eV to 30 eV depending on the actual scale of the degenerate structure. If massive neutrinos are the dominant dark matter, the rest mass will be about 4.7 or 2.3 eV respectively for Majorana or Dirac neutrinos. The mass and size of the degenerate structure are $10^{14}M_\odot$ and 100 kpc for Majorana neutrinos, and 5$\times10^{14}M_\odot$ and 300 kpc for Dirac neutrinos. If we identify the fermions as heavier sterile neutrinos, they yield the characteristic mass hierarchy of black holes; giant black hole at the center of a galaxy and the intermediate mass black holes. Thus we propose the possibility that the mass hierarchy of fermions determines that of black holes in the Universe.

astro-ph↗
arXiv2005

Quantum measurement driven by spontaneous symmetry breaking

The measurement process in quantum mechanics is usually described by the von Neumann projection postulate, which forms a basic constituent of the laws of quantum mechanics. Since this postulate requires the outside observer of the system, it is hard to apply quantum mechanics to the whole Universe. Therefore we propose that the quantum measurement process is actually a physical process associated with the ubiquitous mechanism of spontaneous symmetry breaking. Based on this proposal, we construct a quantum measurement model in which the von Neumann projection is described as the dynamical pro-coherence process. Furthermore, the classically distinguishable pointer parameter emerges as the c-number order parameter in the formalism of closed time-path quantum filed theory. We also discuss the precision of the measurement and the possible deduction of the Born probability postulate.

quant-ph↗
arXiv2004

Bose Einstein Condensation as Dark Energy and Dark Matter

We study a cosmological model in which the boson dark matter gradually condensates into dark energy. Negative pressure associated with the condensate yields the accelerated expansion of the Universe and the rapid collapse of the smallest scale fluctuations into many black holes, which become the seeds of the first galaxies. The cycle of gradual sedimentation and rapid collapse of condensate repeats many times and self-regularizes the ratio of dark energy and dark matter to be order one.

astro-ph↗
arXiv2003

Is Galaxy Distribution Non-extensive and Non-Gaussian?

Self gravitating systems (SGS) in the Universe are generally thought to be non-extensive, and often show long-tails in various distribution functions. In principle, these non-Boltzmann properties are naturally expected from the peculiar property of gravity, long-range and unshielded. Therefore the ordinary Boltzmann statistical mechanics would not be applicable for these self gravitating systems (SGS) in its naive form. In order to step further, we quantitatively investigate the above two properties, non-extensivity and long-tails, by explicitly introducing various models of statistical mechanics. We use the data of CfA II South redshift survey and apply the count-in-cell method. We study four statistical mechanics, (1) Boltzmann, (2) Fractal, (3) Rényi, and (4) Tsallis, and use Akaike information criteria (AIC) for the fair comparison.

astro-ph↗
arXiv2001

Origin of scaling structure and non-gaussian velocity distribution in self-gravitating ring model

Fractal structures and non-Gaussian velocity distributions are characteristic properties commonly observed in virialized self-gravitating systems such as galaxies or interstellar molecular clouds. We study the origin of these properties using the one-dimensional ring model which we newly propose in this paper. In this simple model, $N$ particles are moving, on a circular ring fixed in the three dimensional space, with mutual interaction of gravity. This model is suitable for accurate symplectic integration method by which we find the phase transition in this system from extended-phase to collapsed-phase through an interesting phase (\halo-phase) which has negative specific heat. In this intermediate energy scale, there appear scaling properties, non-thermal and non-Gaussian velocity distributions. In contrast, these peculiar properties are never observed in other \gas and \core phases. Particles in each phase have typical time scales of motion determined by the cutoff length $ξ$, the ring radius $R$ and the total energy $E$. Thus all relaxation patterns of the system are determined by these three time scales.

astro-ph↗
arXiv2001

On the Power Spectrum Density of Gamma Ray Bursts

Gamma ray bursts (GRBs) are known to have short-time variability and power-law behavior with the index -1.67 in the power spectrum density. Reanalyzing the expanded data, we have found a) the power-law comes from the global profile of the burst and not from the self-similar shots nor rapid fluctuations in the luminosity profile. b) The power indices vary from burst to burst and the value -1.67 is given simply as the mean value of the distribution; there is no systematic correlation among GRBs to yield the power law.

astro-ph↗
arXiv2000

Galaxy Distributions and Tsallis Statistical Mechanics

Large-scale astrophysical systems are non-extensive due to their long-range force of gravity. Here we show an approach toward the statistical mechanics of such self-gravitating systems (SGS). This is a generalization of the standard statistical mechanics based on the new definition of entropy; Tsallis statistical mechanics. Developing the composition of entropy and the generalized Euler relation, we investigate the galaxy distributions in count-in-cell method. This is applied to the data of CfA II South redshift survey.

astro-ph↗
arXiv1998

Statistical Mechanics of Self--Gravitating System : Cluster Expansion Method

We study statistical mechanics of the self--gravitating system applying the cluster expansion method developed in solid state physics. By summing infinite series of diagrams, we derive a complex free energy whose imaginary part is related to the relaxation time of the system. Summation of another series yields two--point correlation function whose correlation length is essentially given by the Jeans wavelength of the system.

astro-ph↗
arXiv1997

Beyond Zel'dovich-Type Approximations in Gravitational Instability Theory --- Padé Prescription in Spheroidal Collapse ---

Among several analytic approximations for the growth of density fluctuations in the expanding Universe, Zel'dovich approximation in Lagrangian coordinate scheme is known to be unusually accurate even in mildly non-linear regime. This approximation is very similar to the Padé approximation in appearance. We first establish, however, that these two are actually different and independent approximations with each other by using a model of spheroidal mass collapse. Then we propose Padé-prescribed Zel'dovich-type approximations and demonstrate, within this model, that they are much accurate than any other known nonlinear approximations.

astro-ph↗
arXiv1997

Accuracy of Nonlinear Approximations in Spheroidal Collapse --- Why are Zel'dovich-type approximations so good? ---

Among various analytic approximations for the growth of density fluctuations in the expanding Universe, Zel'dovich approximation and its extensions in Lagrangian scheme are known to be accurate even in mildly non-linear regime. The aim of this paper is to investigate the reason why these Zel'dovich-type approximations work accurately beyond the linear regime from the following two points of view: (1) Dimensionality of the system and (2) the Lagrangian scheme on which the Zel'dovich approximation is grounded. In order to examine the dimensionality, we introduce a model with spheroidal mass distribution. In order to examine the Lagrangian scheme, we introduce the Padé approximation in Eulerian scheme. We clarify which of these aspects supports the unusual accuracy of the Zel'dovich-type approximations. We also give an implication for more accurate approximation method beyond the Zel'dovich-type approximations.

astro-ph↗
arXiv1996

Late-Time Mild Inflation --- a possible solution of dilemma: cosmic age and the Hubble parameter ---

We explore the cosmological model in which a late-time mild inflation is realized after the star formation epoch. Non-vanishing curvature coupling of a classical boson field yields this mild inflation without a cosmological constant. Accordingly the lifetime of the present Universe is remarkably increased in our model. Thus we show that the present observed high value of the Hubble parameter $H_0 \approx 70-80{\rm km/sec/Mpc}$ is compatible with the age of the oldest stars $14{\rm Gyr}$ without introducing the cosmological constant or the open Universe model. Moreover in our model, the local Hubble parameter becomes larger than the global one. Thus we show that the present observed local Hubble parameter measured by using the Cepheid variables is compatible with the global Hubble parameter measured by using the Sunyaev-Zeldovich effect. We also examine the energy conditions, evolution of the effective gravitational constant and the nucleosynthesis process.

astro-ph↗
arXiv1996

Quantum Subcritical Bubbles

We quantize subcritical bubbles which are formed in the weakly first order phase transition. We find that the typical size of the thermal fluctuation reduces in the quantum-statistical physics. We estimate the typical size and the amplitude of thermal fluctuations near the critical temperature in the electroweak phase transition using quantum statistical average. Furthermore based on our study, we give implication on the dynamics of phase transition.

hep-ph↗