arXiv subjects

Naoshi Sugiyama

Publications and source records attributed to Naoshi Sugiyama.

At least 73 records · Page 4Linked to original sources

arXiv2010

Ghost Dark Matter

We revisit ghost dark matter, the possibility that ghost condensation may serve as an alternative to dark matter. In particular, we investigate the Friedmann-Robertson-Walker (FRW) background evolution and the large-scale structure (LSS) in the $Λ$GDM universe, i.e. a late-time universe dominated by a cosmological constant and ghost dark matter. The FRW background of the $Λ$GDM universe is indistinguishable from that of the standard $Λ$CDM universe if $M\gtrsim 1 {\rm eV}$, where $M$ is the scale of spontaneous Lorentz breaking. From the LSS we find a stronger bound: $M\gtrsim 10 {\rm eV}$. For smaller $M$, ghost dark matter would have non-negligible sound speed after the matter-radiation equality, and thus the matter power spectrum would significantly differ from observation. These bounds are compatible with the phenomenological upper bound $M\lesssim 100 {\rm GeV}$ known in the literature.

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arXiv2010

A numerical study of primordial magnetic field amplification by inflation-produced gravitational waves

We numerically study the interaction of inflation-produced magnetic fields with gravitational waves, both of which originate from quantum fluctuations during inflation. The resonance between the magnetic field perturbations and the gravitational waves has been suggested as a possible mechanism for magnetic field amplification. However, some analytical studies suggest that the effect of the inflationary gravitational waves is too small to provide significant amplification. Our numerical study shows more clearly how the interaction affects the magnetic fields and confirms the weakness of the influence of the gravitational waves. We present an investigation based on the magnetohydrodynamic approximation and take into account the differences of the Alfven speed.

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arXiv2009

Constraints on neutrino masses from WMAP5 and BBN in the lepton asymmetric universe

In this paper, we put constraints on neutrino properties such as mass $m_ν$ and degeneracy parameters $ξ_i$ from WMAP5 data and light element abundances by using a Markov chain Monte Carlo (MCMC) approach. In order to take consistently into account the effects of the degeneracy parameters, we run the Big Bang Nucleosynthesis code for each value of $ξ_i$ and the other cosmological parameters to estimate the Helium abundance, which is then used to calculate CMB anisotropy spectra instead of treating it as a free parameter. We find that the constraint on $m_ν$ is fairly robust and does not vary very much even if the lepton asymmetry is allowed, and is given by $\sum m_ν< 1.3 \rm eV$ ($95 % \rm C.L.$).

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arXiv2009

Precision calculations of the gravitational wave background spectrum from inflation

The spectrum of the gravitational wave background originating from quantum fluctuations during inflation is calculated numerically for various inflation models over a wide range of frequencies. We take into account four ingredients : the scalar field dynamics during inflation making no use of the slow-roll approximation, the fermionic decay of the scalar field with a small coupling constant during the reheating process, the change of the effective number of degrees of freedom g_* in the radiation-dominated era, and the anisotropic stress of free-streaming neutrinos. By numerically solving the evolution of gravitational waves during and after inflation up to the present, all of these effects can be examined comprehensively and accurately over a broad spectrum, even at very high frequencies. We find that the spectrum shows (i) a large deviation from the spectrum less accurate obtained by Taylor expanding around the CMB scale using the slow-roll approximation (ii) a characteristic frequency dependence due to the reheating effect, and (iii) damping due to the g_* changes and the neutrino anisotropic stress. We suggest that future analysis of the gravitational wave background should take into consideration the fact that analytical estimates using the Taylor expansion overestimate the amplitude of the spectrum.

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arXiv2009

Simulations of Baryon Acoustic Oscillations II: Covariance matrix of the matter power spectrum

We use 5000 cosmological N-body simulations of 1(Gpc/h)^3 box for the concordance LCDM model in order to study the sampling variances of nonlinear matter power spectrum. We show that the non-Gaussian errors can be important even on large length scales relevant for baryon acoustic oscillations (BAO). Our findings are (1) the non-Gaussian errors degrade the cumulative signal-to-noise ratios (S/N) for the power spectrum amplitude by up to a factor of 2 and 4 for redshifts z=1 and 0, respectively. (2) There is little information on the power spectrum amplitudes in the quasi-nonlinear regime, confirming the previous results. (3) The distribution of power spectrum estimators at BAO scales, among the realizations, is well approximated by a Gaussian distribution with variance that is given by the diagonal covariance component. (4) For the redshift-space power spectrum, the degradation in S/N by non-Gaussian errors is mitigated due to nonlinear redshift distortions. (5) For an actual galaxy survey, the additional shot noise contamination compromises the cosmological information inherent in the galaxy power spectrum, but also mitigates the impact of non-Gaussian errors. The S/N is degraded by up to 30% for a WFMOS-type survey. (6) The finite survey volume causes additional non-Gaussian errors via the correlations of long-wavelength fluctuations with the fluctuations we want to measure, further degrading the S/N values by about 30% even at high redshift z=3.

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arXiv2009

The 21cm Signature of Early Relic \HII Regions

We calculate the spin temperature and 21 cm brightness of early \HII regions around the first stars. We use outputs from cosmological radiation-hydrodynamics simulations of the formation and evolution of early \HII regions. In the pre-reionization era, \HII regions around massive primordial stars have diameters of a few kpc. The gas within the \HII regions is almost fully ionized, but begins recombining after the central stars die off. The relic \HII regions are then seen as bright {\it emission} sources in hydrogen 21 cm. We make brightness temperature maps of the \HII regions, accounting for radiative coupling with Lyman-$α$ photons in a simplified manner. The spin temperature in the relic \HII region is close to the gas kinetic temperature, generally several hundred to several thousand degrees. We show that the relic \HII region can be as bright as $δT_{\rm b} \sim 100$ mK in differential temperature against the cosmic microwave background for an angular resolution of sub-arcseconds. While individual early \HII patches will not be identified by currently planned radio telescopes, the collective fluctuations from early \HII regions might imprint signatures in the 21 cm background.

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arXiv2008

Modeling Nonlinear Evolution of Baryon Acoustic Oscillations: Convergence Regime of N-body Simulations and Analytic Models

We use a series of cosmological N-body simulations and various analytic models to study the evolution of the matter power spectrum in real space in a ΛCold Dark Matter universe. We compare the results of N-body simulations against three analytical model predictions; standard perturbation theory, renormalized perturbation theory, and the closure approximation. We include the effects from finite simulation box size in the comparison. We determine the values of the maximum wavenumbers, k^{lim}_{1%} and k^{lim}_{3%}, below which the analytic models and the simulation results agree to within 1 and 3 percent, respectively. We then provide a simple empirical function which describes the convergence regime determined by comparison between our simulations and the analytical models. We find that if we use the Fourier modes within the convergence regime alone, the characteristic scale of baryon acoustic oscillations can be determined within 1% accuracy from future surveys with a volume of a few h^{-3}Gpc^3 at z\sim1 or z\sim3 in the absence of any systematic distortion of the power spectrum.

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arXiv2008

Search for a stochastic background of 100-MHz gravitational waves with laser interferometers

This letter reports the results of a search for a stochastic background of gravitational waves (GW) at 100 MHz by laser interferometry. We have developed a GW detector, which is a pair of 75-cm baseline synchronous recycling (resonant recycling) interferometers. Each interferometer has a strain sensitivity of ~ 10^{-16} Hz^{-1/2} at 100 MHz. By cross-correlating the outputs of the two interferometers within 1000 seconds, we found h_{100}^2 Omega_{gw} < 6 times 10^{25} to be an upper limit on the energy density spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat spectrum is assumed.

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arXiv2008

The Sunyaev-Zel'dovich effect and Faraday rotation contributions of galaxy groups to the CMB angular power spectrum

The S-Z effect and Faraday rotation from halos are examined over a wide mass range, an including gas condensation and magnetic field evolution. Contributions to the CMB angular power spectrum are evaluated for galaxy clusters, galaxy groups and galaxies. Smaller mass halos are found to play a more important role than massive halos for the B-mode polarisation associated with the S-Z CMB anisotropies. The B-modes from Faraday rotation dominate the secondary B-modes caused by gravitational lensing at l > 3000. Measurement of B-mode polarisation in combination with the S-Z power spectrum can potentially provide important constraints on intracluster magnetic field and gas evolution at early epochs.

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arXiv2008

Constraints on Primordial Black Holes by Distortions of Cosmic Microwave Background

Possible influence of primordial black hole (PBH) evaporations on cosmic microwave background (CMB) is investigated. The spectrum distortions of CMB from the black-body spectrum are described by the chemical potential $μ$ and the Compton parameter $y$. From COBE/FIRAS limits on $μ$ and $y$, the power law index $n$ of primordial density fluctuations and the mass fraction of PBHs $β$ are constrained by employing the peak theory for the formation process of PBHs. Constraints set here are $n < 1.304$ and $n<1.333$ in the thresholds of peaks $ζ_{\rm th} =0.7$ and $ζ_{\rm th} =1.2$, respectively, for the PBH mass range between $2.7\times 10^{11}$g and $1.6 \times 10^{12}$g, and $n < 1.312$ and $n<1.343$ in the thresholds of peaks $ζ_{\rm th} =0.7$ and $ζ_{\rm th} =1.2$, respectively, for the PBH mass range between $1.6 \times 10^{12} {\rm g}$ and $3.5\times 10^{13}$g, which correspond to the comoving scales between $3 \times 10^{-18}$Mpc and $ 4\times 10^{-17}$Mpc. The constraint on the PBH fraction, which is the direct probe of the amplitude of density fluctuations in these scales, stays an almost same value as $β<10^{-21}$ in these mass ranges. It is also found that, with these constraints, UV photons injected by PBH evaporations are unlikely ionize vast hydrogen atoms after recombination.

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arXiv2008

Simulations of Baryon Acoustic Oscillations I: Growth of Large-Scale Density Fluctuations

We critically examine how well the evolution of large-scale density perturbations is followed in cosmological $N$-body simulations. We first run a large volume simulation and perform a mode-by-mode analysis in three-dimensional Fourier space. We show that the growth of large-scale fluctuations significantly deviates from linear theory predictions. The deviations are caused by {\it nonlinear} coupling with a small number of modes at largest scales owing to finiteness of the simulation volume. We then develop an analytic model based on second-order perturbation theory to quantify the effect. Our model accurately reproduces the simulation results. For a single realization, the second-order effect appears typically as ``zig-zag'' patterns around the linear-theory prediction, which imprints artificial ``oscillations'' that lie on the real baryon-acoustic oscillations. Although an ensemble average of a number of realizations approaches the linear theory prediction, the dispersions of the realizations remain large even for a large simulation volume of several hundred megaparsecs on a side. For the standard $Λ$CDM model, the deviations from linear growth rate are as large as 10 percent for a simulation volume with $L = 500h^{-1}$Mpc and for a bin width in wavenumber of $Δk = 0.005h$Mpc$^{-1}$, which are comparable to the intrinsic variance of Gaussian random realizations. We find that the dispersions scales as $\propto L^{-3/2} Δk^{-1/2}$ and that the mean dispersion amplitude can be made smaller than a percent only if we use a very large volume of $L > 2h^{-1}$Gpc. The finite box size effect needs to be appropriately taken into account when interpreting results from large-scale structure simulations for future dark energy surveys using baryon acoustic oscillations.

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arXiv2008

Electromagnetic Properties of the Early Universe

Detailed physical processes of magnetic field generation from density fluctuations in the pre-recombination era are studied. Solving Maxwell equations and the generalized Ohm's law, the evolutions of the net charge density, the electric current and the electromagnetic field are solved. Unlike most of previous works, we treat electrons and photons as separate components under the assumption of tight coupling. We find that generation of the magnetic field due to density fluctuations takes place only from the second order of both perturbation theory and the tight coupling approximation.

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arXiv2008

Laser-interferometric Detectors for Gravitational Wave Background at 100 MHz : Detector Design and Sensitivity

Recently, observational searches for gravitational wave background (GWB) have developed and given direct and indirect constraints on the energy density of GWB in a broad range of frequencies. These constraints have already rejected some theoretical models of large GWB spectra. However, at 100 MHz, there is no strict upper limit from direct observation, though the indirect limit by He4 abundance due to big-bang nucleosynthesis exists. In this paper, we propose an experiment with laser interferometers searching GWB at 100 MHz. We considered three detector designs and evaluated the GW response functions of a single detector. As a result, we found that, at 100 MHz, the most sensitive detector is the design, a so-called synchronous recycling interferometer, which has better sensitivity than an ordinary Fabry-Perot Michelson interferometer by a factor of 3.3 at 100 MHz. We also give the best sensitivity achievable at 100 MHz with realistic experimental parameters.

gr-qc↗

arXiv2008

Optimal Location of Two Laser-interferometric Detectors for Gravitational Wave Backgrounds at 100 MHz

Recently, observational searches for gravitational wave background (GWB) have been developed and given constraints on the energy density of GWB in a broad range of frequencies. These constraints have already resulted in the rejection of some theoretical models of relatively large GWB spectra. However, at 100 MHz, there is no strict upper limit from direct observation, though an indirect limit exists due to He4 abundance due to big-bang nucleosynthesis. In our previous paper, we investigated the detector designs that can effectively respond to GW at high frequencies, where the wavelength of GW is comparable to the size of a detector, and found that the configuration, a so-called synchronous-recycling interferometer is best at these sensitivity. In this paper, we investigated the optimal location of two synchronous-recycling interferometers and derived their cross-correlation sensitivity to GWB. We found that the sensitivity is nearly optimized and hardly changed if two coaligned detectors are located within a range 0.2 m, and that the sensitivity achievable in an experiment is far below compared with the constraint previously obtained in experiments.

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arXiv2007

Cosmic Microwave Background--Weak Lensing Correlation: Analytical and Numerical Study of Non-linearity and Implications for Dark Energy

Non-linear evolution of density fluctuations yields secondary anisotropies in the cosmic microwave background (CMB), which are correlated with the same density fluctuations that can be measured by weak lensing (WL) surveys. We study the CMB-WL correlation using analytical models as well as $N$-body simulations. We show that an analytical model based upon the time derivative of non-linear matter power spectrum agrees with simulations. All-sky cosmic-variance limited CMB and WL surveys allow us to measure the correlation from non-linearity with high significance ($50σ$) for $l_{\rm max}=10^4$, whereas the forthcoming missions such as Planck and LSST are expected to yield only marginal detections. The CMB-WL correlation is sensitive to the time derivative of structure growth. We study how this property may be used to constrain the nature of dark energy. While the expected constraints are not very strong, they may provide a cross check of results from other observations.

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arXiv2007

Analyzing WMAP Observation by Quantum Gravity

The angular power spectra of cosmic microwave background are analyzed under the light of the evolutional scenario of the universe based on the renormalizable quantum theory of gravity in four dimensions. The equation of evolution is solved numerically fixing the power law spectrum predicted by the conformal gravity for the initial condition. The equation requires to introduce a dynamical energy scale about 10^{17}GeV, where the inflationary space-time evolution makes a transition to the big-bang of the conventional Friedmann universe. The quality of fit to the three-year data of WMAP implies the possibility to understand the observation by quantum gravity.

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arXiv2007

Magnetic Field Spectrum at Cosmological Recombination

A generation of magnetic fields from cosmological density perturbations is investigated. In the primordial plasma before cosmological recombination, all of the materials except dark matter in the universe exist in the form of photons, electrons, and protons (and a small number of light elements). Due to the different scattering nature of photons off electrons and protons, electric currents and electric fields are inevitably induced, and thus magnetic fields are generated. We present a detailed formalism of the generation of cosmological magnetic fields based on the framework of the well-established cosmological perturbation theory following our previous works. We numerically obtain the power spectrum of magnetic fields for a wide range of scales, from $k\sim 10^{-5}$ Mpc$^{-1}$ up to $k\sim 2\times 10^{4}$ Mpc$^{-1}$ and provide its analytic interpretation. Implications of these cosmologically generated magnetic fields are discussed.

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arXiv2006

Heating of the Intergalactic Medium by Primordial Miniquasars

A simple analytical model is used to calculate the X-ray heating of the IGM for a range of black hole masses. This process is efficient enough to decouple the spin temperature of the intergalactic medium from the cosmic microwave background (CMB) temperature and produce a differential brightness temperature of the order of $\sim 5-20 \mathrm{mK}$ out to distances as large as a few co-moving Mpc, depending on the redshift, black hole mass and lifetime. We explore the influence of two types of black holes, those with and without ionising UV radiation. The results of the simple analytical model are compared to those of a full spherically symmetric radiative transfer code. Two simple scenarios are proposed for the formation and evolution of black hole mass density in the Universe. The first considers an intermediate mass black hole that form as an end-product of Population III stars, whereas the second considers super-massive black holes that form directly through the collapse of massive halos with low spin parameter. These scenarios are shown not to violate any of the observational constraints, yet produce enough X-ray photons to decouple the spin-temperature from that of the CMB. This is an important issue for future high redshift 21 cm observations.

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