Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We provide a cosmological test of modified gravity with two tensorial degrees of freedom and no extra propagating scalar mode. The theory of gravity we consider admits a cosmological model that is indistinguishable from the ΛCDM model at the level of the background evolution. The model has a single modified-gravity parameterβ, the effect of which can be seen in linear perturbations, though no extra scalar mode is propagating. Using the Boltzmann code modified to incorporate the present model, we derive the constraints -0.047 <β< -0.028 at 68% confidence from Planck CMB data. Since our modified gravity model can hardly be constrained by the Solar System tests and gravitational-wave propagation, our result offers the first observational test on the model.

DOI： 10.1088/1475-7516/2022/07/040

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Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2022/07/040/pdf

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

We discuss how the topological defects in the dark sector affect the Standard Model sector when the dark photon has a kinetic mixing with the QED photon. In particular, we consider the dark photon appearing in the successive gauge symmetry breaking, SU(2)→U(1)→ ℤ₂, where the remainingℤ₂is the center of SU(2). In this model, the monopole is trapped into the cosmic strings and forms the so-called bead solution. As we will discuss, the dark cosmic string induces the QED magnetic flux inside the dark string through the kinetic mixing. The dark monopole, on the other hand, does not induce the QED magnetic flux in the U(1) symmetric phase, even in the presence of the kinetic mixing. Finally, we show that the dark bead solution induces a spherically symmetric QED magnetic flux through the kinetic mixing. The induced flux looks like the QED magnetic monopole viewed from a distance, although QED satisfies the Bianchi identity everywhere, which we call a pseudo magnetic monopole.

DOI： 10.1007/jhep12(2021)122

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

We systematically investigate the preheating behavior of single field inflation with an oscillon-supporting potential. We compute both the properties of the emitted gravitational waves as well as the number density and characteristics of the produced oscillons. By performing numerical simulations for a variety of potential types, we divide the analyzed potentials in two families, each of them containing potentials with varying large- or small-field dependence. We find that the shape of the spectrum and the amplitude of emitted gravitational waves have a universal feature with the peak around the physical wavenumber k/a ∼ m at the inflaton oscillation starting period, irrespective of the exact potential shape. This can be used as a smoking-gun for deducing the existence of a violent preheating phase and possible oscillon formation after inflation. Despite this apparent universality, we also find differences in the shape of the spectrum of emitted gravitational waves between the two families of potentials, leading to discriminating features between them. In particular, all potentials show the emergence of a two-peak structure in the gravitational wave spectrum, arising at the time of oscillon formation. However, potentials that exhibit efficient parametric resonance tend to smear out this structure and by the end of the simulation the two-peak structure is replaced by one broad peak in the GW spectrum. We further compute the number density and properties of the produced oscillons for each potential choice, finding differences in the number density and size distribution of stable oscillons and transient overdensities. We also perform a linear fluctuation analysis and use the corresponding Floquet charts to relate the results of our simulations to the structure of parametric resonance for the various potential types. We find that the growth rate of the scalar perturbations and the associated oscillon formation time are sensitive to the small-field shape of a potential while the macroscopic physical properties of oscillons such as the total number depend on the large-field shape of a potential.

DOI： 10.1007/jhep03(2021)021

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Other Link： http://link.springer.com/article/10.1007/JHEP03(2021)021/fulltext.html

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We develop a possibility of generating tensor non-Gaussianity in a kind of anisotropic inflation, where a (1) gauge field is kinetically coupled to a spectator scalar field.Owing to this coupling, the coherent mode of the electric field appears and softly breaks the isotropy of the Universe.We compute the bispectrum of linearly-polarized tensor perturbations sourced by the gauge field and find that it is strongly red-tiltedand has distinctive statistical anisotropies including higher-order multipole moments.Interestingly, the tensor bispectra with the specific combinations of linear polarization modes are dominant, and their amplitudes depend on the different sets of multipole moments.This new type of statistically-anisotropic tensor non-Gaussianity can be potentially testable with the upcoming cosmic microwave background B-mode polarization experiments.

DOI： 10.1088/1475-7516/2021/03/047

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Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2021/03/047/pdf

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevd.102.083525

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.102.083525/fulltext

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>A<sc>bstract</sc>
</title>
In our previous work, we found new types of the cosmic string solutions in the Abelian-Higgs model with an enhanced U(1) global symmetry. We dubbed those solutions as the compensated/uncompensated strings. The compensated string is similar to the conventional cosmic string in the Abrikosov-Nielsen-Olesen (ANO) string, around which only the would-be Nambu-Goldstone (NG) boson winds. Around the uncompensated string, on the other hand, the physical NG boson also winds, where the physical NG boson is associated with the spontaneous breaking of the enhanced symmetry. Our previous simulation in the 2+1 dimensional spacetime confirmed that both the compensated/uncompensated strings are formed at the phase transition of the symmetry breaking. Non-trivial winding of the physical NG boson around the strings potentially causes the so-called axion domain- wall problem when the model is applied to the axion model. In this paper, we perform simulation in the 3+1 dimensional spacetime to discuss the fate of the uncompensated strings. We observe that the evolution of the string-network is highly complicated in the 3+1 dimensional simulation compared with that seen in the previous simulation. Despite such complications, we find that the number of the uncompensated strings which could cause can be highly suppressed at late times. Our observation suggests that the present setup can be applied to the axion model without suffering from the axion domain-wall problem.

DOI： 10.1007/jhep09(2020)054

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Other Link： http://link.springer.com/article/10.1007/JHEP09(2020)054/fulltext.html

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)  

We present the possibility that the seesaw mechanism with thermal
leptogenesis can be tested using the stochastic gravitational background.
Achieving neutrino masses consistent with atmospheric and solar neutrino data,
while avoiding non-perturbative couplings, requires right-neutrinos lighter
than the typical scale of grand unification. This scale separation suggests a
symmetry protecting the right handed neutrinos from getting a mass. Thermal
leptogenesis would then require that such a symmetry be broken below the
reheating temperature. We enumerate all such possible symmetries consistent
with these minimal assumptions and their corresponding defects, finding that in
many cases, gravitational waves from the network of cosmic strings should be
detectable. Estimating the predicted gravitational wave background we find that
future space-borne missions could probe the entire range relevant for thermal
leptogenesis.

DOI： 10.1103/PhysRevLett.124.041804

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We investigate a possibility of constraining statistical anisotropies of the<br />
primordial tensor perturbations by using future observations for the Cosmic<br />
Microwave Background (CMB) B-mode polarization. By parameterizing a<br />
statistically-anisotropic tensor power spectrum as $P_h ({\boldsymbol{k } }) =<br />
P_h (k) \sum_n g_n \cos^n \theta_{\boldsymbol{k } }$, where<br />
$\theta_{\boldsymbol{k } }$ is an angle of the direction of<br />
$\hat{k}={\boldsymbol{k } }/k$ from a preferred direction, we find that it would<br />
be possible for future B-mode observations such as CMB-S4 to detect the tensor<br />
statistical anisotropy at the level of $g_n \sim {\mathcal O} (0.1)$.

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Given observations of B-mode polarization power spectrum of the cosmic microwave background (CMB), we can reconstruct power spectra of primordial tensor modes from the early Universe without assuming their functional form such as a power-law spectrum. Shape of the reconstructed spectra can then be used to probe the origin of tensor modes in a model-independent manner. We use the Fisher matrix to calculate the covariance matrix of tensor power spectra reconstructed in bins. We find that the power spectra are best reconstructed at wavenumbers in the vicinity of $k\approx 6\times 10^{-4}$ and $5\times 10^{-3}~{\rm Mpc}^{-1}$, which correspond to the &quot;reionization bump&quot; at $\ell\lesssim 6$ and &quot;recombination bump&quot; at $\ell\approx 80$ of the CMB B-mode power spectrum, respectively. The error bar between these two wavenumbers is larger because of lack of the signal between the reionization and recombination bumps. The error bars increase sharply towards smaller (larger) wavenumbers because of the cosmic variance (CMB lensing and instrumental noise). To demonstrate utility of the reconstructed power spectra we investigate whether we can distinguish between various sources of tensor modes including those from the vacuum metric fluctuation and SU(2) gauge fields during single-field slow-roll inflation, open inflation and massive gravity inflation. The results depend on the model parameters, but we find that future CMB experiments are sensitive to differences in these models. We make our calculation tool available on-line.

DOI： 10.1103/PhysRevD.97.123511

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We study relativistic stars in degenerate higher-order scalar-tensor theories that evade the constraint on the speed of gravitational waves imposed by GW170817. It is shown that the exterior metric is given by the usual Schwarzschild solution if the lower order Horndeski terms are ignored in the Lagrangian and a shift symmetry is assumed. However, this class of theories exhibits partial breaking of Vainshtein screening in the stellar interior and thus modifies the structure of a star. Employing a simple concrete model, we show that for high-density stars the mass-radius relation is altered significantly even if the parameters are chosen so that only a tiny correction is expected in the Newtonian regime. We also find that, depending on the parameters, there is a maximum central density above which solutions cease to exist.

DOI： 10.1103/PhysRevD.97.104012

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Language：English   Publishing type：Research paper (scientific journal)  

The Vainshtein mechanism is known as an efficient way of screening the fifth force around a matter source in modified gravity. This has been verified mainly in highly symmetric matter configurations. To study how the Vainshtein mechanism works in a less symmetric setup, we numerically solve the scalar field equation around a disk with a hole at its center in the cubic Galileon theory. We find, surprisingly, that the Galileon force is enhanced, rather than suppressed, in the vicinity of the hole. This anti-screening effect is larger for a thinner, less massive disk with a smaller hole. At this stage our setup is only of academic interest and its astrophysical consequences are unclear, but this result implies that the Vainshtein screening mechanism around less symmetric matter configurations is quite nontrivial.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

Under certain conditions the collision and intercommutation of two cosmic strings can result in the formation of a third string, with the three strings then remaining connected at Y-junctions. The kinematics and dynamics of collisions of this type have been the subject of analytical and numerical analyses in the special case in which the strings are Nambu-Goto. Cosmic strings, however, may well carry currents, in which case their dynamics is not given by the Nambu-Goto action. Our aim is to extend the kinematic analysis to more general kinds of string model. We focus in particular on the collision of strings described by conservative elastic string models, characteristic of current carrying strings, and which are expected to form in a cosmological context. As opposed to Nambu-Goto strings collisions, we show that in this case the collision cannot lead to the formation of a third elastic string: if dynamically such a string forms then the joining string must be described by a more general equation of state. This process will be studied numerically in a forthcoming publication.

DOI： 10.1103/PhysRevD.97.023507

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Rikkyo University  

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© 2018 World Scientific Publishing Company DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO) is a future Japanese space gravitational-wave antenna. The most important objective of DECIGO, among various sciences to be aimed at, is to detect gravitational waves coming from the inflation of the universe. DECIGO consists of four clusters of spacecraft, and each cluster consists of three spacecraft with three Fabry–Perot Michelson interferometers. As a pathfinder mission of DECIGO, B-DECIGO will be launched, hopefully in the 2020s, to demonstrate technologies necessary for DECIGO as well as to lead to fruitful multimessenger astronomy. B-DECIGO is a small-scale or simpler version of DECIGO with the sensitivity slightly worse than that of DECIGO, yet good enough to provide frequent detection of gravitational waves.

DOI： 10.1142/S0218271818450013

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Language：English   Publishing type：Research paper (international conference proceedings)  

© Published under licence by IOP Publishing Ltd. DECIGO (DECi-hertz Interferometer Gravitational wave Observatory) is the planned Japanese space gravitational wave antenna, aiming to detect gravitational waves from astrophysically and cosmologically significant sources mainly between 0.1 Hz and 10 Hz and thus to open a new window for gravitational wave astronomy and for the universe. DECIGO will consists of three drag-free spacecraft arranged in an equilateral triangle with 1000 km arm lengths whose relative displacements are measured by a differential Fabry-Perot interferometer, and four units of triangular Fabry-Perot interferometers are arranged on heliocentric orbit around the sun. DECIGO is vary ambitious mission, we plan to launch DECIGO in era of 2030s after precursor satellite mission, B-DECIGO. B-DECIGO is essentially smaller version of DECIGO: B-DECIGO consists of three spacecraft arranged in an triangle with 100 km arm lengths orbiting 2000 km above the surface of the earth. It is hoped that the launch date will be late 2020s for the present..

DOI： 10.1088/1742-6596/840/1/012010

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Language：English   Publishing type：Research paper (scientific journal)  

We consider a cosmological model in which the tensor mode becomes massive during inflation, and study the Cosmic Microwave Background (CMB) temperature and polarization bispectra arising from the mixing between the scalar mode and the massive tensor mode during inflation. The model assumes the existence of a preferred spatial frame during inflation. The local Lorentz invariance is already broken in cosmology due to the existence of a preferred rest frame. The existence of a preferred spatial frame further breaks the remaining local SO(3) invariance and in particular gives rise to a mass in the tensor mode. At linear perturbation level, we minimize our model so that the vector mode remains non-dynamical, while the scalar mode is the same as the one in single-field slow-roll inflation. At non-linear perturbation level, this inflationary massive graviton phase leads to a sizeable scalar-scalar-tensor coupling, much greater than the scalar-scalar-scalar one, as opposed to the conventional case. This scalar-scalar-tensor interaction imprints a scale dependent feature in the CMB temperature and polarization bispectra. Very intriguingly, we find a surprizing similarity between the predicted scale dependence and the scale-dependent non-Gaussianities at low multipoles hinted in the WMAP and Planck results.

DOI： 10.1088/1475-7516/2017/05/034

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Hiroshima University  

How significantly can we distinguish theoretical models from the fiducial power spectrum ? Computed the Fisher matrix for a primordial tensor power spectrum parametrised as bandpowers within bins. (but, with a simple detector noise model + no other foreground noises) If only a theoretical prediction of GW spectrum is prepared, we can instantly know its detectability without running a Boltzmann solver. (+ simulator is to be appeared).

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

Although they have not yet been detected, axions and axion-like particles (ALPs) continue to maintain the interest (even increasingly so) of the rare-event searches community as viable candidates for the Dark Matter of the Universe but also as a solution for several other puzzles of astrophysics. Their property of coupling to photons has inspired different experimental methods for their detection, one of which is the helioscope technique. The CERN Axion Solar Telescope (CAST) is the most sensitive helioscope built up to date and has recently published part of the latest data taken with the magnet bores gradually filled with 3He, probing the mass range up to 1.17 eV. The International AXion Observatory (IAXO) is being proposed as a facility where different axion studies can be performed, with the primary goal to study axions coming from the Sun. Designed to maximize sensitivity, it will improve the levels reached by CAST by almost 5 orders of magnitude in signal detection, that is more than one order of magnitude in terms of ga γ. Here we will summarize the most important aspects of the helioscopes, and focus mainly on IAXO, based on the recent papers [1, 2].

DOI： 10.1016/j.nuclphysbps.2015.09.033

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We investigate the imprint of reheating on the gravitational wave spectrum produced by self-ordering of multi-component scalar fields after a global phase transition. The equation of state of the Universe during reheating, which usually has different behaviour from that of a radiation-dominated Universe, affects the evolution of gravitational waves through the Hubble expansion term in the equations of motion. This gives rise to a different power-law behavior of frequency in the gravitational wave spectrum. The reheating history is therefore imprinted in the shape of the spectrum. We perform 512(3) lattice simulations to investigate how the ordering scalar field reacts to the change of the Hubble expansion and how the reheating effect arises in the spectrum. We also compare the result with inflation produced gravitational waves, which has a similar spectral shape, and discuss whether it is possible to distinguish the origin between inflation and global phase transition by detecting the shape with future direct detection gravitational wave experiments such as DECIGO.

DOI： 10.1088/1475-7516/2016/02/023

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

The IAXO (International Axion Experiment) is a fourth generation helioscope with a sensitivity, in terms of detectable signal counts, at least 104 better than CAST phase-I, resulting in sensitivity on gaγ one order of magnitude better. To achieve this performance IAXO will count on a 8-coil toroidal magnet with 60 cm diameter bores and equipped with X-ray focusing optics into 0.20 cm2 spots coupled to ultra-low background Micromegas X-ray detectors. The magnet will be on a platform that will allow solar tracking for 12 hours per day. The next short term objectives are to prepare a Technical Design Report and to construct the first prototypes of the hardware main ingredients: demonstration coil, X-ray optics and low background detector while refining the physics case and studying the feasibility studies for Dark Matter axions.

DOI： 10.1088/1742-6596/650/1/012009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The mechanism of thermal inflation, a relatively short period of accelerated expansion after primordial inflation, is a desirable ingredient for a certain class of particle physics models if they are not to be in contention with the cosmology of the early Universe. Though thermal inflation is most simply described in terms of a thermal effective potential, a thermal environment also gives rise to thermal fluctuations that must be taken into account. We numerically study the effects of these thermal fluctuations using lattice simulations. We conclude that though they do not ruin the thermal inflation scenario, the phase transition at the end of thermal inflation proceeds through phase mixing and is therefore not accompanied by the formations of bubbles nor appreciable amplitude of gravitational waves.

DOI： 10.1088/1475-7516/2015/03/024

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

The International Axion Observatory (IAXO) is a proposed 4th-generation axion helioscope with the primary physics research goal to search for solar axions via their Primakoff conversion into photons of 1 - 10 keV energies in a strong magnetic field. IAXO will achieve a sensitivity to the axion-photon coupling ga. down to a few x10(-12) GeV-1 for a wide range of axion masses up to similar to 0.25 eV. This is an improvement over the currently best (3rd generation) axion helioscope, the CERN Axion Solar Telescope (CAST), of about 5 orders of magnitude in signal strength, corresponding to a factor similar to 20 in the axion photon coupling. IAXO's sensitivity relies on the construction of a large superconducting 8-coil toroidal magnet of 20 m length optimized for axion research. Each of the eight 60 cm diameter magnet bores is equipped with x-ray optics focusing the signal photons into similar to 0.2 cm(2) spots that are imaged by very low background x-ray detectors. The magnet will be built into a structure with elevation and azimuth drives that will allow solar tracking for 12 hours each day. This contribution is a summary of our papers [1, 2, 3] and we refer to these for further details. (C) 2015 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2014.12.031

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Based on the multipoint propagator expansion, we present resummed perturbative calculations for cosmological power spectra and correlation functions in the context of modified gravity. In a wide class of modified gravity models that have a screening mechanism to recover general relativity (GR) on small scales, we apply the eikonal approximation to derive the governing equation for resummed propagator that partly includes the nonperturbative effect in the high-k limit. The resultant propagator in the high-k limit contains the new corrections arising from the screening mechanism as well as the standard exponential damping. We explicitly derive the expression for new high-k contributions in specific modified gravity models, and find that in the case of f(R) gravity for a currently constrained model parameter, the corrections are basically of the subleading order and can be neglected. Thus, in f(R) gravity, similarly to the GR case, we can analytically construct the regularized propagator that reproduces both the resummed high-k behavior and the low-k results computed with standard perturbation theory, consistently taking account of the nonlinear modification of gravity valid at large scales. With the regularized multipoint propagators, we give predictions for power spectrum and correlation function at one-loop order, and compare those with N-body simulations in f(R) gravity model. As an important application, we also discuss the redshift-space distortions and compute the anisotropic power spectra and correlation functions.

DOI： 10.1103/PhysRevD.90.123515

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

In this paper, we introduce a new approach to a treatment of the gravitational effects (redshift, time delay and lensing) on the observed cosmic microwave background (CMB) anisotropies based on the Boltzmann equation. From the Liouville's theorem in curved space-time, the intensity of photons is conserved along a photon geodesic when non-gravitational scatterings are absent. Motivated by this fact, we derive a second-order line-of-sight formula by integrating the Boltzmann equation along a perturbed geodesic (curve) instead of a background geodesic (line). In this approach, the separation of the gravitational and intrinsic effects are manifest. This approach can be considered as a generalization of the remapping approach of CMB lensing, where all the gravitational effects can be treated on the same footing.

DOI： 10.1088/1475-7516/2014/10/051

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The International Axion Observatory (IAXO) will be a forth generation axion helioscope. As its primary physics goal, TAX will look for axions or axion-like particles (ALPs) originating in the Sun via the Primakoff conversion of the solar plasma photons. In terms of signalto-noise ratio, TAX will be about 4-5 orders of magnitude more sensitive than CAST, currently the most powerful axion helioscope, reaching sensitivity to axion-photon couplings down to a few x 10-12 GeV-1 and thus probing a large fraction of the currently unexplored axion and ALP parameter space. TAX will also be sensitive to solar axions produced by mechanisms mediated by the axion-electron coupling ga, with sensitivity for the first time to values of ga, not previously excluded by astrophysics. With several other possible physics cases, TAX has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade. In this paper we present the conceptual design of IAXO, which follows the layout of an enhanced axion helioscope, based on a purpose-built 20 m-long 8-coils toroidal superconducting magnet. All the eight 60cm-diameter magnet bores are equipped with focusing x-ray optics, able to focus the signal photons into similar to 0.2 cm(2) spots that are imaged by ultra-low-background Micromegas x-ray detectors. The magnet is built into a structure with elevation and azimuth drives that will allow for solar tracking for similar to 12 h each day.

DOI： 10.1088/1748-0221/9/05/T05002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We revisit the production of gravitational waves from unstable domain walls analyzing their spectrum by the use of field theoretic lattice simulations with grid size 10243, which is larger than the previous study. We have recognized that there exists an error in the code used in the previous study, and the correction of the error leads to the suppression of the spectrum of gravitational waves at high frequencies. The peak of the spectrum is located at the scale corresponding to the Hubble radius at the time of the decay of domain walls, and its amplitude is consistent with the naive estimation based On the quadrupole formula. Using the numerical results, the magnitude and the peak frequency of gravitational waves at the present time are estimated. It is shown that for some choices of parameters the signal of gravitational waves is strong enough to be probed in the future gravitational wave experiments.

DOI： 10.1088/1475-7516/2014/02/031

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Redshift-space distortions (RSDs) offer an attractive method to measure the growth of cosmic structure on large scales, and combining with the measurement of the cosmic expansion history, they can be used as cosmological tests of gravity. With the advent of future galaxy redshift surveys aiming at precisely measuring the RSD, an accurate modeling of RSD going beyond linear theory is a critical issue in order to detect or disprove small deviations from general relativity (GR). While several improved models of RSD have been recently proposed based on the perturbation theory (PT), the framework of these models heavily relies on GR. Here, we put forward a new PT prescription for RSD in general modified gravity models. As a specific application, we present theoretical predictions of the redshift-space power spectra in the f(R) gravity model, and compare them with N-body simulations. Using the PT template that takes into account the effects of both modifications of gravity and RSD properly, we successfully recover the fiducial model parameter in N-body simulations in an unbiased way. On the other hand, we found it difficult to detect the scale dependence of the growth rate in a model-independent way based on GR templates.

DOI： 10.1103/PhysRevD.89.043509

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The breaking of U(1) R symmetry plays a crucial role in modeling the breaking of supersymmetry (SUSY). In the models that possess both SUSY preserving and SUSY breaking vacua, tube-like cosmic strings called R-tubes, whose surfaces are constituted by domain walls interpolating a false and a true vacuum with some winding numbers, can exist. Their (in) stability can strongly constrain SUSY breaking models theirselves. In the present study, we investigate the dynamical (in) stability of two colliding metastable tube-like strings by field-theoretic simulations. From them, we find that the strings become unstable, depending on the relative collision angle and speed of two strings, and the false vacuum is eventually filled out by the true vacuum owing to rapid expansion of the strings or unstable bubbles created as remnants of the collision.

DOI： 10.1007/JHEP01(2014)165

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：University of Tokyo, Kavli Institute for Physics and Mathematics of the Universe, KIPMU  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the network of Type-I cosmic strings using the field-theoretic numerical simulations in the Abelian-Higgs model. For Type-I strings, the gauge field plays an important role, and thus we find that the correlation length of the strings is strongly dependent upon the parameter beta the ratio between the masses of the scalar field and the gauge field, namely, beta = m(phi)(2)/m(A)(2). In particular, if we take the cosmic expansion into account, the network becomes densest in the comoving box for a specific value of beta for beta &lt; 1.

DOI： 10.1103/PhysRevD.88.085021

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Language：English   Publishing type：Research paper (scientific journal)  

In massive gravity, galileon, and braneworld explanations of cosmic acceleration, force modifications are screened by nonlinear derivative self-interactions of the scalar field mediating that force. Interactions between the field of a central body ("A") and an orbiting body ("B") imply that body B does not move as a test body in the field of body A if the orbit is smaller than the Vainshtein radius of body B. We find through numerical solutions of the joint field at the position of B that the A-field Laplacian is nearly perfectly screened by the B self-field, whereas first derivative or net forces are reduced in a manner that scales with the mass ratio of the bodies as (MB/MA)3/5. The latter causes mass-dependent reductions in the universal perihelion precession rate due to the fifth force, with deviations for the Earth-Moon system at the ∼4% level. In spite of universal coupling, which preserves the microscopic equivalence principle, the motion of macroscopic screened bodies depends on their mass providing in principle a means for testing the Vainshtein mechanism. © 2013 American Physical Society.

DOI： 10.1103/PhysRevD.87.063525

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

In massive gravity, galileon, and braneworld explanations of cosmic acceleration, force modifications are screened by nonlinear derivative self-interactions of the scalar field mediating that force. Interactions between the field of a central body ("A'') and an orbiting body ("B'') imply that body B does not move as a test body in the field of body A if the orbit is smaller than the Vainshtein radius of body B. We find through numerical solutions of the joint field at the position of B that the A-field Laplacian is nearly perfectly screened by the B self-field, whereas first derivative or net forces are reduced in a manner that scales with the mass ratio of the bodies as (M-B/M-A)(3/5). The latter causes mass-dependent reductions in the universal perihelion precession rate due to the fifth force, with deviations for the Earth-Moon system at the similar to 4% level. In spite of universal coupling, which preserves the microscopic equivalence principle, the motion of macroscopic screened bodies depends on their mass providing in principle a means for testing the Vainshtein mechanism. DOI: 10.1103/PhysRevD.87.063525

DOI： 10.1103/PhysRevD.87.063525

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We investigate the cosmological constraints on axion models where the domain wall number is greater than one. In these models, multiple domain walls attached to strings are formed, and they survive for a long time. Their annihilation occurs due to the effects of explicit symmetry breaking term which might be raised by Planck-scale physics. We perform three-dimensional lattice simulations and compute the spectra of axions and gravitational waves produced by long-lived domain walls. Using the numerical results, we estimated relic density of axions and gravitational waves. We find that the existence of long-lived domain walls leads to the overproduction of cold dark matter axions, while the density of gravitational waves is too small to observe at the present time. Combining the results with other observational constraints, we find that the whole parameter region of models are excluded unless an unacceptable fine-tuning exists.

DOI： 10.1088/1475-7516/2013/01/001

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Hirosaki University  

In order to study the number fraction of dynamically formed bound states in Abelian-Higgs model, we performed string 'network' simulations in the expanding 2D space. The important parameters characterising string networks are and. Varying them, we obtain number of strings - The number of strings depends on them. - Fixing λ, there is a specific value of β realising the largest number of strings. - The 'critical β' becomes smaller when λ is smaller, and the number becomes also smaller. number fraction of bound states - The fraction is ~ 3.5% even for the best case. - The fraction seems to be limited even if we set smaller. In the 3D case, it is expected that two strings are required to collide head-on, where the collision angle is as small as possible, to form a bound state, and thus the formation rate would be reduced.

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Publishing type：Research paper (international conference proceedings)  

Sample: SDSS LRG (P0,P2) @ z = 0.3 Model: Perturbation Theory fR0 < 1.5*10-4 Cosmic Expansion & fR0 With More General Gravity Model With More Refined Galaxy Samples; SDSS-III BOSS, Subaru PFS (Upcoming)... etc.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

The International Axion Observatory (IAXO) is a new generation axion helioscope aiming at a sensitivity to the axion-photon coupling of g(alpha gamma) greater than or similar to few x 10(-12) GeV-1, i.e. 1-1.5 orders of magnitude beyond the one achieved by CAST, currently the most sensitive axion helioscope. The main elements of IAXO are an increased magnetic field volume together with extensive use of x-ray focusing optics and low background detectors, innovations already successfully tested in CAST. Additional physics cases of IAXO could include the detection of electron-coupled axions invoked to explain the white dwarf cooling, relic axions, and a large variety of more generic axion-like particles (ALPs) and other novel excitations at the low-energy frontier of elementary particle physics.

DOI： 10.1088/1742-6596/460/1/012002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We analyze the spectrum of axions radiated from collapse of domain walls, which have received less attention in the literature. The evolution of topological defects related to the axion models is investigated by performing field-theoretic lattice simulations. We simulate the whole process of evolution of the defects, including the formation of global strings, the formation of domain walls and the annihilation of the defects due to the tension of walls. The spectrum of radiated axions has a peak at the low frequency, which implies that axions produced by the collapse of domain walls are not highly relativistic. We revisit the relic abundance of cold dark matter axions and find that the contribution from the decay of defects can be comparable with the contribution from strings. This result leads to a more severe upper bound on the axion decay constant.

DOI： 10.1103/PhysRevD.86.089902

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We analyze the spectrum of axions radiated from collapse of domain walls, which have received less attention in the literature. The evolution of topological defects related to the axion models is investigated by performing field-theoretic lattice simulations. We simulate the whole process of evolution of the defects, including the formation of global strings, the formation of domain walls, and the annihilation of the defects due to the tension of walls. The spectrum of radiated axions has a peak at the low frequency, which implies that axions produced by the collapse of domain walls are not highly relativistic. We revisit the relic abundance of cold dark matter axions and find that the contribution from the decay of defects can be comparable with the contribution from strings. This result leads to a more severe upper bound on the axion decay constant.

DOI： 10.1103/PhysRevD.85.105020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We study the cosmological evolution of domain walls bounded by strings which arise naturally in axion models. If we introduce a bias in the potential, walls become metastable and finally disappear. We perform two dimensional lattice simulations of domain wall networks and estimate the decay rate of domain walls. By using the numerical results, we give a constraint for the bias parameter and the Peccei-Quinn scale. We also discuss the possibility to probe axion models by direct detection of gravitational waves produced by domain walls.

DOI： 10.1088/1475-7516/2011/08/030

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Cosmological evolution of axionic string network is analyzed in terms of field-theoretic simulations in a box of 512(3) grids, which are the largest ever, using a new and more efficient identification scheme of global strings. The scaling parameter is found to be xi = 0.87 +/- 0.14 in agreement with previous results. The energy spectrum is calculated precisely using a pseudo power spectrum estimator which significantly reduces the error in the mean reciprocal comoving momentum. The resultant constraint on the axion decay constant leads to f(a) &lt;= 3 x 10(11) GeV. We also discuss implications for the early Universe.

DOI： 10.1103/PhysRevD.83.123531

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The objectives of the DECi-hertz Interferometer Gravitational Wave Observatory (DECIGO) are to open a new window of observation for gravitational wave astronomy and to obtain insight into significant areas of science, such as verifying and characterizing inflation, determining the thermal history of the universe, characterizing dark energy, describing the formation mechanism of supermassive black holes in the center of galaxies, testing alternative theories of gravity, seeking black hole dark matter, understanding the physics of neutron stars and searching for planets around double neutron stars. DECIGO consists of four clusters of spacecraft in heliocentric orbits; each cluster employs three drag-free spacecraft, 1000 km apart from each other, whose relative displacements are measured by three pairs of differential Fabry-Perot Michelson interferometers. Two milestone missions, DECIGO pathfinder and Pre-DECIGO, will be launched to demonstrate required technologies and possibly to detect gravitational waves.

DOI： 10.1088/0264-9381/28/9/094011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

We study Q-ball formation in the expanding universe on 3D lattice simulations. We obtain detailed Q-ball charge distributions, and find that the distribution is peaked at Q(peak)(3D) similar or equal to 1.9 x 10(-2)(vertical bar Phi(in)vertical bar/m)(2), which is greater than the existing result by about 60%. Based on the numerical simulations, we discuss how the Q-ball formation proceeds. Also we make a comment on possible deviation of the charge distributions from what was conjectured in the past.

DOI： 10.1088/1475-7516/2010/06/008

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Publishing type：Research paper (international conference proceedings)  

We present results of our simulation of axionic strings and domain walls generated in the early Universe. We developed new method for identification of strings and energy spectrum estimation. By estimating the abundance of axions radiated from the strings and domain walls, we present a constraint on the axion decay constant.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We study Q-ball formation in the expanding universe on 1D, 2D and 3D lattice simulations. We obtain detailed Q-ball charge distributions, and find that the distribution is peaked at Q(peak)(3D) similar or equal to 1.9 x 10(-2) (vertical bar Phi(in)vertical bar/m)(2), which is greater than the existing result by about 60%. Based on the numerical simulations, we discuss how the Q-ball formation proceeds. Also we make a comment on possible deviation of the charge distributions from what was conjectured in the past.

DOI： 10.1088/1475-7516/2010/06/008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We study the production of gravitational waves from cosmic domain walls created during phase transition in the early universe. We investigate the process of formation and evolution of domain walls by running three dimensional lattice simulations. If we introduce an approximate discrete symmetry, walls become metastable and finally disappear. This process might occur by a pressure difference between two vacua if a quantum tunneling is neglected. We calculate the spectrum of gravitational waves produced by collapsing metastable domain walls. Extrapolating the numerical results, we find that the signal of gravitational waves produced by domain walls whose energy scale is around 10(10)-10(12)GeV will be observable in the next generation gravitational wave interferometers.

DOI： 10.1088/1475-7516/2010/05/032

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A space gravitational-wave antenna, DECIGO (DECI-hertz interferometer Gravitational wave Observatory), will provide fruitful insights into the universe, particularly on the formation mechanism of supermassive black holes, dark energy and the inflation of the universe. In the current pre-conceptual design, DECIGO will be comprising four interferometer units; each interferometer unit will be formed by three drag-free spacecraft with 1000 km separation. Since DECIGO will be an extremely challenging mission with high-precision formation flight with long baseline, it is important to increase the technical feasibility before its planned launch in 2027. Thus, we are planning to launch two milestone missions. DECIGO pathfinder (DPF) is the first milestone mission, and key components for DPF are being tested on ground and in orbit. In this paper, we review the conceptual design and current status of DECIGO and DPF.

DOI： 10.1088/0264-9381/27/8/084010

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Language：English   Publishing type：Research paper (international conference proceedings)  

We estimate gravitational waves produced during the formation of Q-balls associated with the Affleck-Dine mechanism in the gravity mediation on 3D lattice simulations. As a preliminary result, we find that the peak amplitude at the Q-ball formation epoch comes up to ΩGW ∼ 10 -3(Φin/ Mpl)4 at the scale of physical size of Q-balls, k≈ 0.06m.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the nonlinear evolution of baryon acoustic oscillations in the matter power spectrum and correlation function from the improved perturbation theory (PT). Based on the framework of renormalized PT, which provides a nonperturbative way to treat the gravitational clustering of large-scale structure, we apply the closure approximation that truncates the infinite series of loop contributions at one-loop order, and obtain a closed set of integral equations for power spectrum and nonlinear propagator. The resultant integral expressions are basically equivalent to those previously derived in the form of evolution equations, and they keep important nonperturbative properties which can dramatically improve the prediction of nonlinear power spectrum. Employing the Born approximation, we then derive the analytic expressions for nonlinear power spectrum and the predictions are made for nonlinear evolution of baryon acoustic oscillations in power spectrum and correlation function. We find that the improved PT possesses a better convergence property compared with standard PT calculation. A detailed comparison between improved PT results and N-body simulations shows that a percent-level agreement is achieved in a certain range in power spectrum and in a rather wider range in correlation function. Combining a model of nonlinear redshift-space distortion, we also evaluate the power spectrum and correlation function in redshift space. In contrast to the results in real space, the agreement between N-body simulations and improved PT predictions tends to be worse, and a more elaborate modeling for redshift-space distortion needs to be developed. Nevertheless, with the currently existing model, we find that the prediction of correlation function has a sufficient accuracy compared with the cosmic-variance errors for future galaxy surveys with volume of a few h(-3) Gpc(3) at z greater than or similar to 0.5.

DOI： 10.1103/PhysRevD.80.123503

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Language：English   Publishing type：Research paper (scientific journal)  

We present a formalism to calculate the nonlinear matter power spectrum in modified gravity models that explain the late-time acceleration of the Universe without dark energy. Any successful modified gravity models should contain a mechanism to recover general relativity (GR) on small scales in order to avoid the stringent constrains on deviations from GR at solar system scales. Based on our formalism, the quasi-nonlinear power spectrum in the Dvali-Gabadadze- Porratti braneworld models and f(R) gravity models are derived by taking into account the mechanism to recover GR properly. We also extrapolate our predictions to fully nonlinear scales using the parametrized post-Friedmann framework. In the Dvali-Gabadadze-Porratti and f(R) gravity models, the predicted nonlinear power spectrum is shown to reproduce N-body results. We find that the mechanism to recover GR suppresses the difference between the modified gravity models and dark energy models with the same expansion history, but the difference remains large at the weakly nonlinear regime in these models. Our formalism is applicable to a wide variety of modified gravity models and it is ready to use once consistent models for modified gravity are developed. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevD.79.123512

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a formalism to calculate the nonlinear matter power spectrum in modified gravity models that explain the late-time acceleration of the Universe without dark energy. Any successful modified gravity models should contain a mechanism to recover general relativity (GR) on small scales in order to avoid the stringent constrains on deviations from GR at solar system scales. Based on our formalism, the quasi-nonlinear power spectrum in the Dvali-Gabadadze-Porratti braneworld models and f(R) gravity models are derived by taking into account the mechanism to recover GR properly. We also extrapolate our predictions to fully nonlinear scales using the parametrized post-Friedmann framework. In the Dvali-Gabadadze-Porratti and f(R) gravity models, the predicted nonlinear power spectrum is shown to reproduce N-body results. We find that the mechanism to recover GR suppresses the difference between the modified gravity models and dark energy models with the same expansion history, but the difference remains large at the weakly nonlinear regime in these models. Our formalism is applicable to a wide variety of modified gravity models and it is ready to use once consistent models for modified gravity are developed.

DOI： 10.1103/PhysRevD.79.123512

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a new numerical scheme to treat the nonlinear evolution of cosmological power spectra. Governing equations for matter power spectra have been previously derived by a nonperturbative technique with closure approximation. Solutions of the resultant closure equations just correspond to the resummation of an infinite class of perturbation corrections, and they consistently reproduce the one-loop results of standard perturbation theory. We develop a numerical algorithm to solve closure evolutions in both perturbative and nonperturbative regimes. The present numerical scheme is particularly suited for examining nonlinear matter power spectrum in general cosmological models, including modified theory of gravity. As a demonstration, we study weakly nonlinear evolution of power spectrum in a class of modified gravity models, as well as various dark energy models.

DOI： 10.1103/PhysRevD.79.103526

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Language：English   Publishing type：Research paper (scientific journal)  

We present a new numerical scheme to treat the nonlinear evolution of cosmological power spectra. Governing equations for matter power spectra have been previously derived by a nonperturbative technique with closure approximation. Solutions of the resultant closure equations just correspond to the resummation of an infinite class of perturbation corrections, and they consistently reproduce the one-loop results of standard perturbation theory. We develop a numerical algorithm to solve closure evolutions in both perturbative and nonperturbative regimes. The present numerical scheme is particularly suited for examining nonlinear matter power spectrum in general cosmological models, including modified theory of gravity. As a demonstration, we study weakly nonlinear evolution of power spectrum in a class of modified gravity models, as well as various dark energy models. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevD.79.103526

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

DECIGO pathfinder (DPF) is a milestone satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory), which is a future space gravitational wave antenna. DECIGO is expected to provide fruitful insights into the universe, particularly about dark energy, the formation mechanism of supermassive black holes and the inflation of the universe. Since DECIGO will be an extremely challenging mission, which will be formed by three drag-free spacecraft with 1000 km separation, it is important to increase the technical feasibility of DECIGO before its planned launch in 2024. Thus, we are planning to launch two milestone missions: DPF and pre-DECIGO. In this paper, we review the conceptual design and current status of the first milestone mission, DPF.

DOI： 10.1088/0264-9381/26/9/094019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

We apply a nonlinear statistical method in turbulence to the cosmological perturbation theory and derive a closed set of evolution equations for matter power spectra. The resultant closure equations consistently recover the one-loop results of standard perturbation theory, and beyond that, it is still capable of treating the nonlinear evolution of matter power spectra. We find the exact integral expressions for the solutions of closure equations. These analytic expressions coincide with the renormalized one-loop results presented by Crocce and Scoccimarro apart from the vertex renormalization. By constructing the nonlinear propagator, we analytically evaluate the nonlinear matter power spectra based on the first-order Born approximation of the integral expressions and compare it with those of the renormalized perturbation theory. © 2008. The American Astronomical Society. All rights reserved.

DOI： 10.1086/526515

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Language：English   Publishing type：Research paper (international conference proceedings)  

We study the non-linear evolution of cosmological power spectra in a closure theory. Governing equations for matter power spectra have been previously derived by a non-perturbative technique with closure approximation. Solutions of the resultant closure equations just correspond to the resummation of an infinite class of perturbation corrections, and they consistently reproduce the one-loop results of standard perturbation theory. We develop a numerical algorithm to solve closure evolutions in both perturbative and non-perturbative regimes. The present numerical scheme is particularly suited for examining non-linear matter power spectrum in general cosmological models, including modified theory of gravity. As a demonstration, we apply our numerical scheme to the Dvali-Gabadadze-Porrati braneworld model.

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Language：English  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We study the behaviour of scalar perturbations in the radiation-dominated era of Randall-Sundrum braneworld cosmology by numerically solving the coupled bulk and brane master wave equations. We find that density perturbations with wavelengths less than a critical value ( set by the bulk curvature length) are amplified during horizon re-entry. This means that the radiation-era matter power spectrum will be at least an order of magnitude larger than the predictions of general relativity (GR) on small scales. Conversely, we explicitly confirm from simulations that the spectrum is identical to GR on large scales. Although this magnification is not relevant for the cosmic microwave background or measurements of large scale structure, it will have some bearing on the formation of primordial black holes in Randall-Sundrum models.

DOI： 10.1088/1475-7516/2007/07/008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

In this paper we quantize scalar perturbations in a Randall Sundrum- type model of inflation where the inflaton field is confined to a single brane embedded in five-dimensional anti-de Sitter space-time. In the high energy regime, small-scale in inflaton fluctuations are strongly coupled to metric perturbations in the bulk and gravitational back-reaction has a dramatic effect on the behaviour of inflaton perturbations on sub-horizon scales. This is in contrast to the standard four-dimensional result where gravitational back-reaction can be neglected on small scales. Nevertheless, this does not give rise to signifacant particle production, and the correction to the power spectrum of the curvature perturbations on super-horizon scales is shown to be suppressed by a slow-roll parameter. We calculate the complete first-order slow-roll corrections to the spectrum of primordial curvature perturbations.

DOI： 10.1088/1475-7516/2007/04/001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We discuss a robust data analysis method to detect a stochastic background of gravitational waves in the presence of non-Gaussian noise. In contrast to the standard cross-correlation (SCC) statistic frequently used in the stochastic background searches, we consider a generalized cross-correlation (GCC) statistic, which is nearly optimal even in the presence of non-Gaussian noise. The detection efficiency of the GCC statistic is investigated analytically, particularly focusing on the statistical relation between the false-alarm and the false-dismissal probabilities, and the minimum detectable amplitude of gravitational-wave signals. We derive simple analytic formulas for these statistical quantities. The robustness of the GCC statistic is clarified based on these formulas, and one finds that the detection efficiency of the GCC statistic roughly corresponds to the one of the SCC statistic neglecting the contribution of non-Gaussian tails. This remarkable property is checked by performing the Monte Carlo simulations and successful agreement between analytic and simulation results was found.

DOI： 10.1103/PhysRevD.75.022003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We study the evolution of scalar curvature perturbations in a brane-world inflation model in a 5D anti-de Sitter space-time. The inflaton perturbations are confined to a 4D brane but they are coupled to the 5D bulk metric perturbations. We numerically solve full coupled equations for the inflaton perturbations and the 5D metric perturbations using the Hawkins-Lidsey inflationary model. At an initial time, we assume that the bulk is unperturbed. We find that the inflaton perturbations at high energies are strongly coupled to the bulk metric perturbations even on subhorizon scales, leading to the suppression of the amplitude of the comoving curvature perturbations at a horizon crossing. This indicates that the linear perturbations of the inflaton field do not obey the usual 4D Klein-Gordon equation due to the coupling to 5D gravitational fields on small scales, and it is necessary to quantize the coupled brane-bulk system in a consistent way in order to calculate the spectrum of the scalar perturbations in a brane-world inflation.

DOI： 10.1088/1475-7516/2006/12/009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We discuss the cosmological evolution of the inflationary gravitational wave background in the Randall-Sundrum single-brane model. In braneworld cosmology, in which the three-dimensional spacelike hypersurface that we live in is embedded in five-dimensional anti-de-Sitter (AdS(5)) spacetime, the evolution of gravitational wave (GW) modes is affected by the nonstandard expansion of the universe and the excitation of the Kaluza-Klein modes. These are significant in the high-energy regime of the universe. We numerically evaluate these two effects by solving the evolution equation for GWs propagating through the AdS(5) spacetime. Using a plausible initial condition from inflation, we find that the excitation of Kaluza-Klein modes can be characterized by a simple scaling relation above the critical frequency f(crit) determined from the length scale of the fifth dimension l. The remarkable point is that this relation generally holds as long as the matter content of the universe is described by the perfect fluid with the equation of state p=w rho for 0 &lt;= w &lt;= 1. The resultant scaling relation is translated into the energy spectrum of the inflationary gravitational wave background as Omega(GW)proportional to f((3w-1)/(3w+2)) for f &gt; f(crit). This indicates that in the radiation dominant case (w=1/3), the two high-energy effects accidentally compensate each other and the spectrum becomes almost the same as the one predicted in the four-dimensional theory, i.e., Omega(GW)proportional to f(0).

DOI： 10.1103/PhysRevD.73.084008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Future missions of gravitational-wave astronomy will be operated by space-based interferometers, covering a very wide range of frequencies. Search for stochastic gravitational-wave backgrounds (GWBs) is one of the main targets for such missions, and we here discuss the prospects for direct measurement of isotropic and anisotropic components of (primordial) GWBs around the frequency 0.1-10 Hz. After extending the theoretical basis for correlation analysis, we evaluate the sensitivity and the signal-to-noise ratio for the proposed future space interferometer missions, like Big-Bang Observer (BBO), Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), and the recently proposed Fabry-Perot type DECIGO. The astrophysical foregrounds which are expected at low frequency may be a big obstacle and may significantly reduce the signal-to-noise ratio of GWBs. As a result, the minimum detectable amplitude may reach h(2)Omega(gw) = 10(-15)similar to 10(-16), as long as foreground point sources are properly subtracted. Based on correlation analysis, we also discuss measurement of anisotropies of GWBs. As an example, the sensitivity level required for detecting the dipole moment of GWB induced by the proper motion of our local system is closely examined.

DOI： 10.1103/PhysRevD.73.064006

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Language：English   Publishing type：Research paper (scientific journal)  

Future missions of gravitational-wave astronomy will be operated by space-based interferometers, covering a very wide range of frequencies. Search for stochastic gravitational-wave backgrounds (GWBs) is one of the main targets for such missions, and we here discuss the prospects for direct measurement of isotropic and anisotropic components of (primordial) GWBs around the frequency 0.1-10 Hz. After extending the theoretical basis for correlation analysis, we evaluate the sensitivity and the signal-to-noise ratio for the proposed future space interferometer missions, like Big-Bang Observer (BBO), Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), and the recently proposed Fabry-Perot type DECIGO. The astrophysical foregrounds which are expected at low frequency may be a big obstacle and may significantly reduce the signal-to-noise ratio of GWBs. As a result, the minimum detectable amplitude may reach h2Ωgw=10-15∼10-16, as long as foreground point sources are properly subtracted. Based on correlation analysis, we also discuss measurement of anisotropies of GWBs. As an example, the sensitivity level required for detecting the dipole moment of GWB induced by the proper motion of our local system is closely examined. © 2006 The American Physical Society.

DOI： 10.1103/PhysRevD.73.064006

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

We discuss the robust data analysis method to detect a stochastic background of gravitational waves in the presence of non-Gaussian noise. Usually, the so-called standard cross-correlation (SCC) statistic is used for the data analysis of gravitational-wave background search, which is optimal only if the detector noise obeys a stationary Gaussian process. Here, we consider a generalized cross-correlation (GCC) statistic, which is nearly optimal even in the presence of non-Gaussian noise. The detection efficiency and the general tendency of the GCC statistic are investigated analytically, particularly focusing on the statistical quantities of the false-alarm and the false-dismissal probabilities and the minimum detectable amplitude of gravitational-wave signals. We find that the GCC statistics is robust against the non-Gaussian tail of detector noises.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL PUBLISHING  

We discuss the gravitational wave background (GWB) from a cosmological population of gamma-ray bursts (GRBs). Among the various emission mechanisms for the gravitational waves (GWs), we pay particular attention to the vast anisotropic neutrino emissions from the accretion discs around black holes formed after so-called failed supernova explosions. The GWs produced by such mechanisms are known as 'burst with memory', and could dominate in the low-frequency regime below similar to 10 Hz. To estimate their amplitudes, we derive general analytic formulae for gravitational waveforms from the axisymmetric jets. Based on the formulae, we first quantify the spectrum of GWs from a single GRB. Then, summing the cosmological population, we find that the resultant value of the density parameter becomes roughly Omega(GW) approximate to 10(-20) over the wide-band of the low-frequency region, f similar to 10(-4) Hz. The amplitude of the GWB is significantly smaller than that of the primordial GWB originating from the inflationary epoch, and is far below the detection limit.

DOI： 10.1111/j.1365-2966.2005.09643.x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We discuss the evolution of gravitational waves (GWs) after inflation in a brane-world cosmology embedded in five-dimensional anti-de Sitter (AdS(5)) bulk spacetime. In a brane-world scenario, the evolution of GWs is affected by the nonstandard cosmological expansion and the excitation of the Kaluza-Klein (KK) modes, which are significant in the high-energy regime of the universe. We numerically solve the wave equation of GWs in the Poincare coordinates of the AdS5 spacetime. Using a plausible initial condition from inflation, we find that, while the behavior of GWs in the bulk is sensitive to the transition time from inflation to the radiation-dominated epoch, the amplitude of GWs on the brane is insensitive to this time if the transition occurs early enough before horizon reentry. As a result, the amplitude of GWs is suppressed by the excitation of KK-modes and the effect may compensate the enhancement of the GWs by the non-standard cosmological expansion. Based on this, the influence of the high-energy effects on the GW backgrounds is discussed. (C) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physletb.2005.01.024

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Language：English   Publishing type：Research paper (international conference proceedings)  

We discuss tbe detection of stochastic background of gravitational waves in presence of non-Gaussian noise. In this paper we apply two-component Gaussian noise as non- Gaussian noise model. Tben using the generalized cross correlation (GCC) statistic that reduces to the Standard cross correlation statistic (SCC) in which the probability distribution function of the detector noise behavcs as Gaussian, we analytically and numerically show that the GCC performs better than the SCC in terms of the false alarm versus false dismissal curves and the minimum detectable gravitational Signal amplitude.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We study the evolution of gravitational waves (GWs) after inflation in a brane-world cosmology embedded in five-dimensional anti-de Sitter space-time. Contrary to the standard four-dimensional results, the GWs at the high-energy regime in brane-world model suffer from the effects of the non-standard cosmological expansion and the excitation of the Kaluza-Klein modes (KK-modes), which can affect the amplitude of stochastic gravitational wave background significantly. To investigate these two high-energy effects quantitatively, we numerically solve the wave equation of the GWs in the radiation dominated epoch at relatively low-energy scales. We show that the resultant GWs are suppressed by the excitation of the KK modes. The created KK modes are rather soft and escape away from the brane to the bulk gravitational field. The results are also compared to the semi-analytic prediction from the low-energy approximation and the evolved amplitude of GWs on the brane reasonably matches the numerical simulations. (C) 2003 Published by Elsevier B.V.

DOI： 10.1016/j.physletb.2003.10.111

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Event date： 12 3 2021 - 15 3 2021

Language：Japanese   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Language：Japanese   Presentation type：Oral presentation (invited, special)  

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Event date： 14 9 2020 - 17 9 2020

Language：Japanese   Presentation type：Oral presentation (general)  

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Language：Japanese   Presentation type：Oral presentation (invited, special)  

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Language：Japanese  

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Language：English   Presentation type：Poster presentation  

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Language：English   Presentation type：Oral presentation (general)  

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Language：Japanese   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Language：Japanese   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Poster presentation  

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Language：English   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Oral presentation (general)  

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Language：Japanese   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Oral presentation (general)  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Yamaguchi University  

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Language：English   Presentation type：Poster presentation  

Venue：Rikkyo University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Shinshu University  

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Language：English   Presentation type：Poster presentation  

Venue：Roma University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Tohoku Univ.  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Kyoto University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Yamaguchi University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Kyoto University  

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Language：English   Presentation type：Poster presentation  

Venue：Kyoto University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Tohoku Univ.  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Kyoto University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Hiroshima University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Utsunomiya University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Kyoto University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：UPMC, Paris  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Tohoku University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Yamaguchi University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Osaka University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：KEK  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Osaka City Univ.  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Miyazaki Univ.  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Hirosaki Univ.  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：/Tohoku/Univ.  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：TAP, Kyoto Univ.  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Uniwersytet Warszawski  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Kinugawa  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Waseda Univ.  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Nagoya University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Kavli IPMU  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Saga University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：YITP  

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Language：English   Presentation type：Poster presentation  

Venue：Jimoto-ya  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Tokai University  

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Language：English   Presentation type：Poster presentation  

Venue：Hirosaki University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Kouchi University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Hiroshima University  

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Language：English   Presentation type：Poster presentation  

Venue：YITP  

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Language：English   Presentation type：Poster presentation  

Venue：YITP  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Kyoto Sangyo University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Stockholm University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Hirosaki University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：da Universidade do Porto  

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Language：English   Presentation type：Poster presentation  

Venue：San-ai Kougen Hotel  

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Language：Japanese   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Oral presentation (general)  

Venue：University of Tokyo  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Kyoto University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Kyushu Institute of Technology  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Fuji Calm  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Yukawa Institute for Theoretical Physics  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Yukawa Institute for Theoretical Physics  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Okayama University  

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Language：English   Presentation type：Poster presentation  

Venue：CERN  

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Language：English   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Yukawa Institute for Theoretical Physics  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Rikkyo University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：University of Hiroshima  

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Language：English   Presentation type：Poster presentation  

Venue：University of Tokyo  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：University of Yamagata  

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Language：English   Presentation type：Poster presentation  

Venue：Yanagi-no-Yu  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：University of Kinki  

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Language：English   Presentation type：Poster presentation  

Venue：Waikoloa Beach Marriott  

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Language：English   Presentation type：Poster presentation  

Venue：University of Nagoya  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：University of Hokkaido  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：University of Tokyo  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Hotel Izu-Kougen  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Institute for Cosmology and Gravity  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Kinugawa Onsen Hotel  

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Language：English   Presentation type：Oral presentation (general)  

Venue：Freie Universtaet Berlin  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Matsuyama University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：/Osaka/University  

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Language：English   Presentation type：Oral presentation (general)  

Venue：University of Bonn  

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Language：Japanese   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Poster presentation  

Venue：Yukawa Institute for Theoretical Physics  

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Language：English   Presentation type：Poster presentation  

Venue：University of Tokyo  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Tokyo University of Science  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：/Osaka/University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：/Osaka/University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Yukawa Institute for Theoretical Physics  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Kochi University  

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Language：English   Presentation type：Poster presentation  

Venue：Dublin City University  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Kyuushu University  

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Language：English   Presentation type：Poster presentation  

Venue：Yukawa Institute for Theoretical Physics  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：/Osaka/University  

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Language：Japanese   Presentation type：Poster presentation  

Venue：University of Tokyo  

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Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Miyazaki World Convention Center Summit  

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Language：Japanese   Presentation type：Oral presentation (general)  

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