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Takeshi Chiba

Publications and source records attributed to Takeshi Chiba.

At least 73 records · Page 4Linked to original sources
arXiv2004

Reheating After Quintessential Inflation and Gravitational Waves

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 reheating by particle creation in the expanding universe in which the beginning of the radiation dominated epoch is delayed due to the presence of the epoch of domination of the kinetic energy of the inflaton (kination). The other is the instant preheating considered by Felder et al. in which the Universe becomes radiation dominated soon after the end of inflation. We find that the spectrum of the gravitational waves at $\sim 100$ MHz is quite sensitive to the reheating process. This result is not limited to quintessential inflation but applicable to various inflation models. Conversely, the detection or non-detection of primordial gravitational waves at $\sim$100 MHz would provide useful information regarding the reheating process in inflation.

gr-qc↗
arXiv2004

Baryogenesis in a flat direction with neither baryon nor lepton charge

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 derivatively couples to a current associated with some global symmetry $U(1)_Q$. In this case, baryogenesis is still possible provided that an interaction exists, which violates the baryon and $U(1)_Q$ symmetries simultaneously. As a concrete example, we discuss baryogenesis in a flat direction with neither baryon nor lepton charge.

hep-ph↗
arXiv2003

WMAP constraints on scalar-tensor cosmology and the variation of the gravitational constant

We present observational constraints on a scalar-tensor gravity theory by $χ^2$ test for CMB anisotropy spectrum. We compare the WMAP temperature power spectrum with the harmonic attractor model, in which the scalar field has its harmonic effective potential with curvature $β$ in the Einstein conformal frame and the theory relaxes toward Einstein gravity with time. We found that the present value of the scalar coupling, i.e. the present level of deviation from Einstein gravity $(α_0^2)$, is bounded to be smaller than $5\times 10^{-4-7β}$ ($2σ$), and $10^{-2-7β}$ ($4σ$) for $0< β<0.45$. This constraint is much stronger than the bound from the solar system experiments for large $β$ models, i.e., $β> 0.2$ and 0.3 in $2σ$ and $4σ$ limits, respectively. Furthermore, within the framework of this model, the variation of the gravitational constant at the recombination epoch is constrained as $|G(z=z_{rec})-G_0|/G_0 < 0.05(2σ)$, and $0.23(4σ)$.

astro-ph↗
arXiv2003

1/R gravity and Scalar-Tensor Gravity

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 corresponding gravity theory is $ω=0$ Brans-Dicke gravity with a potential for the Brans-Dicke scalar field, which is not compatible with solar system experiments if the field is very light: the case when such modifications are important recently.

astro-ph↗
arXiv2003

Supernova Cosmology and the Fine Structure Constant

We study the dependence of the peak luminosity of Type Ia supernovae on the fine structure constant $α$. We find that decreasing (increasing) $α$ enhances (reduces) the luminosity. Future experiments like SNAP could determine the variation of $α$ to a precision of $10^{-2}$.

astro-ph↗
arXiv2002

Lens Model Degeneracy and Cosmological Tests by Strong Gravitational Lensing

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 observables are primarily dependent on the lens model parameters, while the dependence on cosmological parameters is minor (especially so for the dark energy parameters). We demonstrate the lens model degeneracy by deriving both the projected mass density profile and the circular velocity profile. We also identify a possible source of the problem of fitting the averaged mass profile of CL 0024+1654 with the Navarro-Frenk-White profile.

astro-ph↗
arXiv2002

Observational Consequences of Evolution of Primordial Fluctuations in Scalar-Tensor Cosmology

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 effective potential in the Einstein conformal frame and the theory relaxes toward Einstein gravity with time, is considered. The evolution of adiabatic initial perturbations in flat SCDM models is examined from the radiation-dominated epoch up to the present. We discuss how the scalar-tensor gravity affects the evolution of metric and matter perturbations, mainly focusing on the observational consequences, i.e., the matter power spectrum and the power spectrum of cosmic microwave background temperature. We find that the early time deviation is characterized only by the large static gravitational constant while the late time behavior is qualitatively different from that in Einstein gravity because the time variation of the gravitational constant and its fluctuation have non-negligible effects. The attracting scalar-tensor gravity affects only small scale modes due to its attracting nature, the degree of which is far beyond the post-Newtonian deviation at the present epoch.

astro-ph↗
arXiv2002

Tracking K-essence

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 X=dot{phi}^2/2, we find tracker solutions with w_{phi} < w_B exist if Gamma equivalent V''V/(V')^2 > 3/2. Here w_{phi}(w_B) is the equation-of-state of the scalar field (background radiation/matter). Our condition may be useful for examining the existence of the attractor-like behavior in cosmology with k-essence.

astro-ph↗
arXiv2002

Quintessence Cosmology and Varying $α$

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 of the quintessence potential. If these results prove valid, future satellite experiment (STEP) should measure the induced violation of the weak equivalence principle. Future cosmological observations of nearby $(z \siml 0.5)$ absorption systems would make it clear whether the variation is significant or not.

hep-ph↗
arXiv2001

Gravitational Lens Statistics and The Density Profile of Dark Halos

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 halo profiles. We take the dark halos as the lens objects and consider the following three models for the density profile of dark halos; SIS (singular isothermal sphere), the NFW (Navarro Frenk White) profile, and the generalized NFW profile which has a different slope at smaller radii. The mass function of dark halos is assumed to be given by the Press-Schechter function. We find that magnification bias for the NFW profile is order of magnitude larger than that for SIS. We estimate the sensitivity of the lensing probability of distant sources to the inner profile of lenses and to the cosmological parameters. It turns out that the lensing probability is strongly dependent on the inner density profile as well as on the cosmological constant. We compare the predictions with the largest observational sample, the Cosmic Lens All-Sky Survey. The absence or presence of large splitting events in larger surveys currently underway such as the 2dF and SDSS could set constraints on the inner density profile of dark halos.

astro-ph↗
arXiv2001

Extended Quintessence and its Late-time Domination

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 a scenario based on extended quintessence models to explain the late-time domination of dark energy without severe fine-tuning of initial conditions and model parameters.

astro-ph↗
arXiv2000

Feasibility of Reconstructing the Quintessential Potential Using SNIa Data

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 fit the luminosity distance with good accuracy (the relative error is less than 0.1%). We assess the ambiguity in reconstructing the equation of state and the effective potential which originates from the uncertainty in $Ω_M$. We find that the equation of state is sensitive to the assumed $Ω_M$, while the shape of the effective potential is not. We also demonstrate the actual reconstruction procedure using the data created by Monte-Carlo simulation. Future high precision measurements of distances to thousands of SNIa could reveal the shape of the quintessential potential.

astro-ph↗
arXiv2000

Determining the Equation of State of the Expanding Universe Using a New Independent Variable

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 luminosity distance, respectively. For the flat universe suggested from the observation of the anisotropy of cosmic microwave background, the density and the pressure are expressed as $ρ/ρ_0=4(df/dR)^2/f^6$ and $p/ρ_0=-4/3(d^2f/dR^2)/f^5$ where $ρ_0$ is the present density and $f(R)=1/\sqrt{1+z(R)}$. In $(R, f)$ plane the sign as well as the strength of the pressure is in proportion to the curvature of the curve $f(R)$. We propose to adopt a Pade-like expression of $f(R)=1/\sqrt{u}$ with $u\equiv 1+\sum\limits_{n=1}^{N}u_nR^n$. For flat $Λ$ model the expansion up to N=7 has at most an error $< 0.2%$ for $z < 1.7$ and any value of $Λ$. We also propose a general method to determine the equation of state of the universe which has $N-1$ free parameters. If the number of parameters are smaller than $N-1$, there is a consistency check of the equation of state so that we may confirm or refute each model.

astro-ph↗
arXiv2000

Scalar-Tensor Gravity in Two 3-brane System

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 reduction with the Ansatz for the radion field by Charmousis et al., which solves five-dimensional linearized field equations, results in a class of scalar-tensor gravity theories. In the limit of vanishing radion fluctuations, the effective action reduces to the Brans-Dicke gravity in accord with the results of Garriga and Tanaka: Brans-Dicke gravity with the corresponding Brans-Dicke parameter $0< ω< \infty$ (for positive tension brane) and $-3/2< ω<0$ (for negative tension brane). In general the gravity induced a brane belongs to a class of scalar-tensor gravity with the Brans-Dicke parameter which is a function of the interval and the radion. In particular, gravity on a positive tension brane contains an attractor mechanism toward the Einstein gravity.

gr-qc↗
arXiv2000

Kinetically Driven Quintessence

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 extension, we show that a scalar field with non-canonical kinetic terms alone behaves like an energy component which is time-varying and has negative pressure presently, i.e. quintessence. We present a model which has a constant equation of state, that is, a ``kinetic'' counterpart of the Ratra-Peebles model of a quintessence field with a potential term. We make clear the structure of the phase plane and show that the quintessential solution is a late-time attractor. We also give a model for the ``phantom'' component which has an equation of state with $w=p/ρ<-1$.

astro-ph↗
arXiv1999

Extended Open Inflation

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 inflation after the bubble nucleation is realized.

hep-ph↗
arXiv1999

Quintessence, the Gravitational Constant, and Gravity

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 potential difficulty with the idea of quintessence is that couplings to ordinary matter should be strongly suppressed so as not to lead to observable time variations of the constants of nature. We further explore the possibility of an explicit coupling between the quintessence field and the curvature. Since such a scalar field gives rise to another gravity force of long range ($\simg H^{-1}_0$), the solar system experiments put a constraint on the non-minimal coupling: $|ξ| \siml 10^{-2}$.

gr-qc↗