arXiv subjects

Kiyotomo Ichiki

Publications and source records attributed to Kiyotomo Ichiki.

136 records · Page 8Linked to original sources

arXiv2006

Cosmological Magnetic Field: a fossil of density perturbations in the early universe

The origin of the substantial magnetic fields that are found in galaxies and on even larger scales, such as in clusters of galaxies, is yet unclear. If the second-order couplings between photons and electrons are considered, then cosmological density fluctuations, which explain the large scale structure of the universe, can also produce magnetic fields on cosmological scales before the epoch of recombination. By evaluating the power spectrum of these cosmological magnetic fields on a range of scales, we show here that magnetic fields of 10^{-18.1} gauss are generated at a 1 megaparsec scale and can be even stronger at smaller scales (10^{-14.1} gauss at 10 kiloparsecpc). These fields are large enough to seed magnetic fields in galaxies and may therefore have affected primordial star formation in the early universe.

astro-ph↗

arXiv2006

Biermann Mechanism in Primordial Supernova Remnant and Seed Fields

We have studied the generation of magnetic fields by the Biermann mechanism in the pair-instability supernovae explosions of the first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We have performed a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the produced magnetic fields. We find that magnetic fields with amplitude $10^{-14}-10^{-17}$ G are generated inside the bubble, though the amount of magnetic fields generated depend on specific values of initial conditions. This corresponds to magnetic fields of $10^{28}-10^{31}$ ergs per each supernova remnant, which is strong enough to be the seed magnetic field for a galactic and/or interstellar dynamo.

astro-ph↗

arXiv2006

Generation of magnetic field from cosmological perturbations

We discuss generation of magnetic field from cosmological perturbations. We consider the evolution of three component plasma (electron, proton and photon) evaluating the collision term between elecrons and photons up to the second order. The collision term is shown to induce electric current, which then generate magnetic field. There are three contributions, two of which can be evaluated from the first-order quantities, while the other one is fluid vorticity which is purely second order. We compute numerically the magnitudes of the former contributions and shows that the amplitude of the produced magnetic field is about $\sim 10^{-19} {\rm G}$ at 10kpc comoving scale at present. Compared to astrophysical and inflationary mechanisms for seed-field generation, our study suffers from much less ambiguities concerning unknown physics and/or processes.

astro-ph↗

arXiv2005

Magnetic field generation from cosmological perturbations

In this letter, we discuss generation of magnetic field from cosmological perturbations. We consider the evolution of three component plasma (electron, proton and photon) evaluating the collision term between elecrons and photons up to the second order. The collision term is shown to induce electric current, which then generate magnetic field. There are three contributions, two of which can be evaluated from the first-order quantities, while the other one is fluid vorticity which is purely second order. We estimate the magnitudes of the former contributions and shows that the amplitude of the produced magnetic field is about $\sim 10^{-19} {\rm G}$ at 10Mpc comoving scale at the recombination. Compared to astrophysical and inflationary mechanisms for seed-field generation, our study suffers from much less ambiguities concerning unknown physics and/or processes.

astro-ph↗

arXiv2005

Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields

We study generation of magnetic fields by the Biermann mechanism in the pair-instability supernovae explosions of first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We perform a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the produced magnetic fields. We find that magnetic fields with amplitude $10^{-14}-10^{-17}$ G are generated inside the bubble, though the amount of magnetic fields generated depend on specific values of initial conditions. This corresponds to magnetic fields of $10^{28}-10^{31}$ erg per each supernova remnant, which is strong enough to be the seed magnetic field for galactic and/or interstellar dynamo.

astro-ph↗

arXiv2004

An Analytic Model of Superclusters

We construct an analytic model for the mass distributions of superclusters and clusters in each supercluster. Our model is a modification of the Press-Schechter theory, and defines superclusters as the regions that have some overdensity smaller than those for usual virialized objects. We compare the mass functions with a catalog of superclusters in the Sloan Digital Sky Survey Early Data Release and found that they are in reasonable agreement with each other.

astro-ph↗

arXiv2004

Causal Structure and Gravitational Waves in Brane World Cosmology

The causal structure of the flat brane universe of RSII type is re-investigated to clarify the boundary conditions for stochastic gravitational waves. In terms of the Gaussian normal coordinate of the brane, a singularity of the equation for gravitational waves appears in the bulk. We show that this singularity corresponds to the ``seam singularity'' which is a singular subspace on the brane universe. Based upon the causal structure, we discuss the boundary conditions for gravitational waves in the bulk. Introducing a null coordinate, we propose a numerical procedure to solve gravitational waves with appropriate boundary conditions and show some examples of our numerical results. This procedure could be also applied in scalar type perturbations. The problem in the choice of the initial condition for gravitational waves is briefly discussed.

hep-th↗

arXiv2004

Stochastic Gravitational Wave Background in Brane World Cosmology

We investigate the cosmological evolution of gravitational waves in Friedman-Robertson-Walker brane world models embedded in a five dimensional anti de-Sitter spacetime. To predict the spectrum of stochastic gravitational background at present generated in the inflationary phase, we numerically calculate the evolution of gravitational waves according to the method developed in our previous paper [K. Ichiki and K. Nakamura, arXiv:hep-th/0310282]. The resulting logarithmic energy spectrum is significantly altered from that of the standard four dimensional inflationary models in the frequency range f > f_{AdS} ~ 10^{-4} Hz, which corresponds to the curvature scale we set of anti de-Sitter spacetime, $l ~ 1$mm.

astro-ph↗

arXiv2004

WMAP Constraints on Decaying Cold Dark Matter

We re-formulate cosmological perturbations in the decaying cold dark matter model, and calculate cosmological microwave background (CMB) anisotropies. By comparing our theoretical predictions with recent observational data from the Wilkinson Microwave Anisotropy Probe(WMAP), we derive a new bound on the abundance and lifetime of decaying dark matter particles. We show that the data of WMAP alone do not prefer the decaying cold dark matter model: the lifetime is constrained to Γ^{-1}> 123 Gyr at 68% C.L. (52 Gyr at 95.4% C.L.) when cold dark matter consists only of such decaying particles. We also consider a more general case in which cold dark matter consists of both stable and decaying particles, and show that the constraint generalizes down to Ω_{DDM}h^2 < -0.5 (Gamma^{-1}/1 Gyr)^{-1}+0.12 for Γ^{-1} > 5 Gyr at 95.4% C.L.. These bounds are robust and widely applicable, because they are derived only from gravitational effects and therefore do not rely on the details of decay channels or decay products.

astro-ph↗

arXiv2003

Sunyaev-Zel'dovich power spectrum with decaying cold dark matter

Recent studies of structures of galaxies and clusters imply that dark matter might be unstable and decay with lifetime $Γ^{-1}$ about the age of universe. We study the effects of the decay of cold dark matter on the Sunyaev-Zel'dovich (SZ) power spectrum. We analytically calculate the SZ power spectrum taking finite lifetime of cold dark matter into account. We find the finite lifetime of dark matter decreases the power at large scale ($l < 4000$) and increases at small scale ($l > 4000$). This is in marked contrast with the dependence of other cosmological parameters such as the amplitude of mass fluctuations $σ_{8}$ and the cosmological constant $Ω_λ$ (under the assumption of a flat universe) which mainly change the normalization of the angular power spectrum. This difference allows one to determine the lifetime and other cosmological parameters rather separately. We also investigate sensitivity of a future SZ survey to the cosmological parameters including the life time, assuming a fiducial model $Γ^{-1} = 10 h^{-1} {\rm Gyr}$, $σ_{8} = 1.0$, and $Ω_λ = 0.7$. We show that future SZ surveys such as ACT, AMIBA, and BOLOCAM can determine the lifetime within factor of two even if $σ_{8}$ and $Ω_λ$ are marginalized.

astro-ph↗