The Equation of State of Tracker Fields (2009)
We derive the equation of state of tracker fields, which are typical examples of freezing quintessence (quintessence with the equation of state approaching toward -1), taking into account of the...
Chiba, Takeshi, Dutta, Sourish, Scherrer, Robert J.
We derive slow-roll conditions for thawing k-essence with a separable Lagrangian $p(X,\phi)=F(X)V(\phi)$. We examine the evolution of the equation of state parameter, $w$, as a function of the scale...
Ando, Masaki, Kawamura, Seiji, Sato, Shuichi, Nakamura, Takashi, Tsubono, Kimio, Araya, Akito, ...
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...
Slow-Roll Thawing Quintessence (2009)
We derive slow-roll conditions for thawing quintessence. We solve the equation of motion of $\phi$ for a Taylor expanded potential (up to the quadratic order) in the limit where the equation of state...
Chiba, Takeshi, Yamaguchi, Masahide
As an extension of our previous study, we derive slow-roll conditions for multiple scalar fields which are non-minimally coupled with gravity and for generalized gravity theories of the form...
Search for a Stochastic Background of 100-MHz GravitationalWaves with Laser Interferometers (2008)
Akutsu, Tomotada, Kawamura, Seiji, Nishizawa, Atsushi, Arai, Koji, Yamamoto, Kazuhiro, Tatsumi, Daisuke, ...
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...
Ando, Masaki, Kawamura, Seiji, Nakamura, Takashi, Tsubono, Kimio, Tanaka, Takahiro, Funaki, Ikkoh, ...
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...
The Japanese space gravitational wave antenna; DECIGO (2008)
Kawamura, Seiji, Ando, Masaki, Nakamura, Takashi, Tsubono, Kimio, Tanaka, Takahiro, Funaki, Ikkoh, ...
DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the future Japanese space gravitational wave antenna. DECIGO is expected to open a new window of observation for gravitational...
Extended Slow-Roll Conditions and Rapid-Roll Conditions (2008)
Chiba, Takeshi, Yamaguchi, Masahide
We derive slow-roll conditions for a scalar field which is non-minimally coupled with gravity in a consistent manner and express spectral indices of scalar/tensor perturbations in terms of the...
Initial Conditions for Vector Inflation (2008)
Recently, a model of inflation using non-minimally coupled massive vector fields has been proposed. For a particular choice of non-minimal coupling parameter and for a flat FRW model, the model is...
Precision calculations of the gravitational wave background spectrum from inflation (2008)
Kuroyanagi, Sachiko, Chiba, Takeshi, Sugiyama, Naoshi
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...
Search for a stochastic background of 100-MHz gravitational waves with laser interferometers (2008)
Akutsu, Tomotada, Kawamura, Seiji, Nishizawa, Atsushi, Arai, Koji, Yamamoto, Kazuhiro, Tatsumi, Daisuke, ...
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...
Nishizawa, Atsushi, Kawamura, Seiji, Akutsu, Tomotada, Arai, Koji, Yamamoto, Kazuhiro, Tatsumi, Daisuke, ...
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...
Nishizawa, Atsushi, Kawamura, Seiji, Akutsu, Tomotada, Arai, Koji, Yamamoto, Kazuhiro, Tatsumi, Daisuke, ...
Recently, observational searches for gravitational wave background (GWB) have been developed and given direct and indirect constraints on the energy density of GWB in a broad range of frequencies....
Takeshi Chiba, Naoshi Sugiyama, Jun'ichi Yokoyama
Spectra of density perturbations produced during chaotic inflation in scalartensor theories of gravity are calculated, taking both adiabatic and isocurvature modes into account. Comparing the...
Nishizawa, Atsushi, Kawamura, Seiji, Akutsu, Tomotada, Arai, Koji, Yamamoto, Kazuhiro, Tatsumi, Daisuke, ...
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...
A Null Test of the Cosmological Constant (2007)
Chiba, Takeshi, Nakamura, Takashi
We provide a consistency relation between cosmological observables in general relativity with the cosmological constant. Breaking of this relation at any redshift would imply the breakdown of the...
Solar System constraints to general f(R) gravity (2007)
Chiba, Takeshi, Smith, Tristan L., Erickcek, Adrienne L.
It has been proposed that cosmic acceleration or inflation can be driven by replacing the Einstein-Hilbert action of general relativity with a function f(R) of the Ricci scalar R. Such f(R) gravity...
Effective Search Templates for a Primordial Stochastic Gravitational Wave Background (2007)
Chiba, Takeshi, Himemoto, Yoshiaki, Yamaguchi, Masahide, Yokoyama, Jun'ichi
We calculate the signal-to-noise ratio (SNR) of the stochastic gravitational-wave background in an extreme case that its spectrum has a sharp falloff with its amplitude close to the detection...
A Consistency Relation in Cosmology (2007)
Chiba, Takeshi, Takahashi, Ryuichi
We provide a consistency relation between cosmological observables in general relativity without relying on the equation of state of dark energy. The consistency relation should be satisfied if...
Weak Lensing of Galaxy Clusters in MOND (2007)
Takahashi, Ryuichi, Chiba, Takeshi
We study weak gravitational lensing of galaxy clusters in terms of the MOND (MOdified Newtonian Dynamics) theory. We calculate shears and convergences of background galaxies for three clusters...
Solar System constraints to general f(R) gravity (2006)
Chiba, Takeshi, Smith, Tristan L., Erickcek, Adrienne L.
It has been proposed that cosmic acceleration or inflation can be driven by replacing the Einstein-Hilbert action of general relativity with a function f(R) of the Ricci scalar R. Such f(R) gravity...
Time variation of proton-electron mass ratio and fine structure constant with runaway dilaton (2006)
Chiba, Takeshi, Kobayashi, Tatsuo, Yamaguchi, Masahide, Yokoyama, Jun'ichi
Recent astrophysical observations indicate that the proton-electron mass ratio and the fine structure constant have gone through nontrivial time evolution. We discuss their time variation in the...
w and w' of Scalar Field Models of Dark Energy (2005)
Important observables to reveal the nature of dark energy are the equation of state $w$ and its time derivative in units of the Hubble time $w'$. Recently, it is shown that the simplest scalar field...
Numerical Solutions of Inflating Higher Dimensional Global Defects (2005)
Shimono, Satsuki, Chiba, Takeshi
We find numerical solutions of Einstein equations and scalar field equation for a global defect in higher dimensional spacetimes ($\geq 6$). We examine in detail the relation among the expansion rate...
Generalized Gravity and a Ghost (2005)
We show that generalized gravity theories involving the curvature invariants of the Ricci tensor and the Riemann tensor as well as the Ricci scalar are equivalent to multi- scalar-tensor gravities...
Classifying the Future of Universes with Dark Energy (2005)
Chiba, Takeshi, Takahashi, Ryuichi, Sugiyama, Naoshi
We classify the future of the universe for general cosmological models including matter and dark energy. If the equation of state of dark energy is less then -1, the age of the universe becomes...
Does Positronium Form in the Universe ? (2004)
Chiba, Takeshi, Sugiyama, Naoshi
Positronium (the bound state of electron and positron) has been thought to be formed after proton decay ($>10^{34}$yr) through collisional recombination and then decays by pair annihilation, thereby...
WMAP constraints on scalar-tensor cosmology and the variation of the gravitational constant (2003)
Nagata, Ryo, Chiba, Takeshi, Sugiyama, Naoshi
We present observational constraints on a scalar-tensor gravity theory by $\chi^2$ test for CMB anisotropy spectrum. We compare the WMAP temperature power spectrum with the harmonic attractor model,...
1/R gravity and Scalar-Tensor Gravity (2003)
We point out that extended gravity theories, the Lagrangian of which is an arbitrary function of scalar curvature $R$, are equivalent to a class of the scalar tensor theories of gravity. The...
Reheating After Quintessential Inflation and Gravitational Waves (2003)
Tashiro, Hiroyuki, Chiba, Takeshi, Sasaki, Misao
We investigate the dependence of the gravitational wave spectrum from quintessential inflation on the reheating process. We consider two extreme reheating processes. One is the gravitational...
Supernova Cosmology and the Fine Structure Constant (2003)
Chiba, Takeshi, Kohri, Kazunori
We study the dependence of the peak luminosity of Type Ia supernovae on the fine structure constant $\alpha$. We find that decreasing (increasing) $\alpha$ enhances (reduces) the luminosity. Future...
Baryogenesis in a flat direction with neither baryon nor lepton charge (2003)
Chiba, Takeshi, Takahashi, Fuminobu, Yamaguchi, Masahide
We present a new mechanism of spontaneous baryogenesis. Usually such mechanisms require a derivative coupling between a scalar field and baryon current. In our model, the scalar field instead...
Observational Consequences of Evolution of Primordial Fluctuations in Scalar-Tensor Cosmology (2002)
Nagata, Ryo, Chiba, Takeshi, Sugiyama, Naoshi
Evolution of primordial fluctuations in a Brans-Dicke type scalar-tensor gravity theory is comprehensively investigated. The harmonic attractor model, in which the scalar field has its harmonic...
We derive a condition for converging a common evolutionary track for k-essence (a scalar field dark energy with non-canonical kinetic terms). For the Lagrangian density V(phi)W(X) with...
Quintessence Cosmology and Varying $\alpha$ (2001)
Chiba, Takeshi, Kohri, Kazunori
If the reported measurements of the time variation of the fine structure constant from observations of distant QSOs are correct, combined with the Oklo limit they would strongly constrain the class...
Constancy of the Constants of Nature (2001)
The current observational and experimental bounds on time variation of the constants of Nature are briefly reviewed.
Extended Quintessence and its Late-time Domination (2001)
Various astronomical observations point towards the evidence for dark energy. One of the most mysterious problem is the coincidence problem: why dark energy becomes dominant only recently. We present...
Lens Model Degeneracy and Cosmological Tests by Strong Gravitational Lensing (2001)
Chiba, Takeshi, Takahashi, Ryuichi
We estimate the sensitivity of lensing observables to the parameters in the lens model (isothermal sphere/Navarro-Frenk-White profile) parameters and to cosmological parameters. We find that the...
Gravitational Lens Statistics and The Density Profile of Dark Halos (2001)
Takahashi, Ryuichi, Chiba, Takeshi
We investigate the influence of the inner profile of lens objects on gravitational lens statistics taking into account of the effect of magnification bias and both the evolution and the scatter of...
Determining the Equation of State of the Expanding Universe Using a New Independent Variable (2000)
Nakamura, Takashi, Chiba, Takeshi
To determine the equation of state of the universe, we propose to use a new independent variable $R\equiv (H_0/c)(d_L(z)/(1+z))$, where $H_0$ and $d_L(z)$ are the present Hubble parameter and the...
Feasibility of Reconstructing the Quintessential Potential Using SNIa Data (2000)
Chiba, Takeshi, Nakamura, Takashi
We investigate the feasibility of the method for reconstructing the equation of state and the effective potential of the quintessence field from SNIa data. We introduce a useful functional form to...
Scalar-Tensor Gravity in Two 3-brane System (2000)
We derive the low-energy effective action of four-dimensional gravity in the Randall-Sundrum scenario in which two 3-branes of opposite tension reside in a five-dimensional spacetime. The dimensional...
Kinetically Driven Quintessence (1999)
Chiba, Takeshi, Okabe, Takahiro, Yamaguchi, Masahide
Recently, a novel class of models for inflation has been found in which the inflationary dynamics is driven solely by (non-canonical) kinetic terms rather than by potential terms. As an obvious...
Extended Open Inflation (1999)
Chiba, Takeshi, Yamaguchi, Masahide
We present a new type of one-field models for open inflation utilizing a nonminimally coupled scalar field with polynomial potentials in which the Coleman-de Luccia instanton does exist and slow-roll...
Apparent Horizon Formation and Hoop Conjecture in Non-axisymmetric Spaces (1999)
We investigate the validity of Thorne's hoop conjecture in non-axisymmetric spacetimes by examining the formation of apparent horizons numerically. If spaces have a discrete symmetry about one axis,...
Quintessence, the Gravitational Constant, and Gravity (1999)
Dynamical vacuum energy or quintessence, a slowly varying and spatially inhomogeneous component of the energy density with negative pressure, is currently consistent with the observational data. One...
Numerical Study of Inhomogeneous Pre-Big-Bang Inflationary Cosmology (1999)
We study numerically the inhomogeneous pre-big-bang inflation in a spherically symmetric space-time. We find that a large initial inhomogeneity suppresses the onset of the pre-big-bang inflation. We...
Determining the Equation of State of the Expanding Universe: Inverse Problem in Cosmology (1998)
Nakamura, Takashi, Chiba, Takeshi
Even if the luminosity distance as a function of redshift is obtained accurately using, for example, Type Ia supernovae, the equation of state of the Universe cannot be determined uniquely but...
Resolving the singularity of the Hawking-Turok type instanton (1998)
We point out that the singular instanton of Hawking-Turok type, in which the singularity occurs due to the divergence of a massless scalar field, can be generated by Euclideanized regular $p$-brane...
The Luminosity Distance, the Equation of State, and the Geometry of the Universe (1998)
Chiba, Takeshi, Nakamura, Takashi
The second derivative of the luminosity distance with respect to the redshift is written in terms of the deceleration parameter $q_0$. We point out that the third derivative contains the information...
Ioka, Kunihito, Chiba, Takeshi, Tanaka, Takahiro, Nakamura, Takashi
We study black hole MACHO binary formation through three-body interactions in the early universe at $t\sim 10^{-5}$s. The probability distribution functions of the eccentricity and the semimajor axis...
Observational tests of x-matter models (1998)
Chiba, Takeshi, Sugiyama, Naoshi, Nakamura, Takashi
We study gravitational lensing statistics, matter power spectra and the angular power spectra of the cosmic microwave background (CMB) radiation in x-matter models. We adopt an equation-of-state of...
Chiba, Takeshi, Maeda, Kei-ichi, Nakao, Ken-ichi, Tsukamoto, Taro
We study the apparent horizon for two boosted black holes in the asymptotically de Sitter space-time by solving the initial data on a space with punctures. We show that the apparent horizon enclosing...
Imprints of the Metrically-coupled Dilaton on Density Perturbations in Inflationary Cosmology (1997)
Chiba, Takeshi, Sugiyama, Naoshi, Yokoyama, Jun'ichi
Spectra of density perturbations produced during chaotic inflation are calculated, taking both adiabatic and isocurvature modes into account in a class of scalar-tensor theories of gravity in which...
Chiba, Takeshi, Mukohyama, Shinji, Nakamura, Takashi
We consider the horizon problem in a homogeneous but anisotropic universe (Bianchi type I). We show that the problem cannot be solved if (1) the matter obeys the strong energy condition with the...
Cosmology with x-matter (1997)
Chiba, Takeshi, Sugiyama, Naoshi, Nakamura, Takashi
Motivated by the possibility of $H_0t_0 > 1$ where $H_0$ and $t_0$ are the Hubble parameter and the age of the universe, respectively, we investigate the cosmology including x-matter. x-matter is...
The Minimum Mass of the First Stars and the Anthropic Principle (1996)
Nakamura, Takashi, Uehara, Hideya, Chiba, Takeshi
The lower limit of the mass of the first stars suggested recently may imply the formation of massive stars of mass greater than 8 solar mass irrespective of the details of the initial mass function....
Harada, Tomohiro, Chiba, Takeshi, Nakao, Ken-ichi, Nakamura, Takashi
Unlike general relativity, scalar-tensor theories of gravity predict scalar gravitational waves even from a spherically symmetric gravitational collapse. We solve numerically the generation and...
Critical Behavior in the Brans-Dicke Theory of Gravitation (1996)
The collapse of a massless scalar field in the Brans-Dicke theory of gravitation is studied in the analysis of both analytical solution and numerical one. By conformally transforming the Roberts's...
Volume Expansion Rate and The Age of The Universe (1995)
Nakamura, Takashi, Nakao, Ken-ichi, Chiba, Takeshi, Shiromizu, Tetsuya
Under four assumptions such that 1) Einstein's theory of gravity is correct, 2) existence of foliation by geodesic slicing, 3) the trace of the extrinsic curvature, $K$, is negative at the present...
Hubble Parameter in Void Universe (1995)
Nakao, Ken-ichi, Gouda, Naoteru, Chiba, Takeshi, Ikeuchi, Satoru, Nakamura, Takashi, Shibata, Masaru
We investigate the distance-redshift relation in the simple void model. As discussed by Moffat and Tatarski, if the observer stays at the center of the void, the observed Hubble parameter is not so...