arXiv subjects

Takeshi Chiba

Publications and source records attributed to Takeshi Chiba.

At least 37 records · Page 2Linked to original sources

arXiv2015

Consistency Relations for Large Field Inflation: Non-minimal Coupling

We derive the consistency relations for a chaotic inflation model with a non-minimal coupling to gravity. For a quadratic potential in the limit of a small non-minimal coupling parameter $ξ$ and for a quartic potential without assuming small $ξ$, we give the consistency relations among the spectral index $n_s$, the tensor-to-scalar ratio $r$ and the running of the spectral index $α$. We find that unlike $r$, $α$ is less sensitive to $ξ$. If $r<0.1$, then $ξ$ is constrained to $ξ<0$ and $α$ is predicted to be $α\simeq -8\times 10^{-4}$ for a quartic potential. For a general monomial potential, $α$ is constrained in the range $-2.7\times 10^{-3}<α< -6\times 10^{-4}$ as long as $|ξ|\leq 10^{-3}$ if $r<0.1$.

astro-ph.CO↗

arXiv2014

Motion of Charged Particles around a Weakly Magnetized Rotating Black Hole

We study the motion of a charged particle around a weakly magnetized rotating black hole. We classify the fate of a charged particle kicked out from the innermost stable circular orbit. We find that the final fate of the charged particle depends mostly on the energy of the particle and the radius of the orbit. The energy and the radius in turn depend on the initial velocity, the black hole spin, and the magnitude of the magnetic field. We also find possible evidence for the existence of bound motion in the vicinity of the equatorial plane.

gr-qc↗

arXiv2014

Implications of the B-mode Polarization Measurement for Direct Detection of Inflationary Gravitational Waves

The prospects for direct measurements of inflationary gravitational waves by next generation interferometric detectors inferred from the possible detection of B-mode polarization of the cosmic microwave background are studied. We compute the spectra of the gravitational wave background and the signal-to-noise ratios by two interferometric detectors (DECIGO and BBO) for large-field inflationary models in which the tensor-to-scalar ratio is greater than the order of 0.01. If the reheating temperature $T_{\rm RH}$ of chaotic inflation with the quadratic potential is high ($T_{\rm RH}>7.9\times10^6$ GeV for upgraded DECIGO and $T_{\rm RH}> 1.8\times 10^{6}$ GeV for BBO), it will be possible to reach the sensitivity of the gravitational background in future experiments at $3σ$ confidence level. The direct detection is also possible for natural inflation with the potential $V(ϕ)=Λ^4 [1-\cos(ϕ/f)]$, provided that $f>4.2 M_{\rm pl}$ (upgraded DECIGO) and $f>3.6 M_{\rm pl}$ (BBO) with $T_{\rm RH}$ higher than $10^8$ GeV. The quartic potential $V(ϕ)=λϕ^4/4$ with a non-minimal coupling $ξ$ between the inflaton field $ϕ$ and the Ricci scalar $R$ gives rise to a detectable level of gravitational waves for $|ξ|$ smaller than the order of 0.01, irrespective of the reheating temperature.

astro-ph.CO↗

arXiv2014

Consistency Relations for Large Field Inflation

Consistency relations for chaotic inflation with a monomial potential and natural inflation and hilltop inflation are given which involve the scalar spectral index $n_s$, the tensor-to-scalar ratio $r$ and the running of the spectral index $α$. The measurement of $α$ with $O(10^{-3})$ and the improvement in the measurement of $n_s$ could discriminate monomial model from natural/hilltop inflation models. A consistency region for general large field models is also presented.

astro-ph.CO↗

arXiv2014

Cosmological Scaling Solutions for Multiple Scalar Fields

The general k-essence Lagrangian for the existence of cosmological scaling solutions is derived in the presence of multiple scalar fields coupled to a barotropic perfect fluid. In addition to the scaling fixed point associated with the dynamics during the radiation and matter eras, we also obtain a scalar-field dominated solution relevant to dark energy and discuss the stability of them in the two-field set-up. We apply our general results to a model of two canonical fields with coupled exponential potentials arising in string theory. Depending on model parameters and initial conditions, we show that the scaling matter-dominated epochs followed by an attractor with cosmic acceleration can be realized with/without the couplings to scalar fields. The different types of scaling solutions can be distinguished from each other by the evolution of the dark energy equation of state from high-redshifts to today.

gr-qc↗

arXiv2013

Conformal-Frame (In)dependence of Cosmological Observations in Scalar-Tensor Theory

We provide the correspondence between the variables in the Jordan frame and those in the Einstein frame in scalar-tensor gravity and consider the frame-(in)dependence of the cosmological observables. In particular, we show that the cosmological observables/relations (redshift, luminosity distance, temperature anisotropies) are frame-independent. We also study the frame-dependence of curvature perturbations and find that the curvature perturbations are conformal invariant if the perturbation is adiabatic and the entropy perturbation between matter and the Brans-Dicke scalar is vanishing. The relation among various definitions of curvature perturbations in the both frames is also discussed, and the condition for the equivalence is clarified.

gr-qc↗

arXiv2013

Observational Constraints on Quintessence: Thawing, Tracker, and Scaling models

For two types of quintessence models having thawing and tracking properties, there exist analytic solutions for the dark energy equation of state w expressed in terms of several free parameters. We put observational bounds on the parameters in such scenarios by using the recent data of Supernovae type Ia, Cosmic Microwave Background, and Baryon Acoustic Oscillations. The observational constraints are quite different depending on whether or not the recent BAO data from BOSS are taken into account. With the BOSS data the upper bounds of today's values of w (=w_0) in thawing models is very close to -1, whereas without this data the values of w_0 away from -1 can be still allowed. The tracker equation of state w_{(0)} during the matter era is constrained to be w_{(0)}<-0.949 at 95 % confidence level even without the BOSS data, so that the tracker models with w away from -1 are severely disfavored. We also study observational constraints on scaling models in which w starts to evolve from 0 in the deep matter era and show that the transition to the equation of state close to w=-1 needs to occur at an early cosmological epoch. In the three classes of quintessence models studied in this paper, the past evolution of the Hubble parameters in the best-fit models shows only less than the 2.5 % difference compared to the LCDM.

astro-ph.CO↗

arXiv2012

Shadows of Multi-Black Holes: Analytic Exploration

Shadows of multi-black holes have structures distinct from the mere superposition of the shadow of a single black hole: the eyebrow-like structures outside the main shadows and the deformation of the shadows. We present analytic estimates of these structures using the static multi-black hole solution (Majumdar-Papapetrou solution). We show that the width of the eyebrow is related with the distance between the black holes and that the shadows are deformed into ellipses due to the presence of the second black holes. These results are helpful to understand qualitatively the features of the shadows of colliding black holes. We also present the shadows of colliding/coalescing black holes in the Kastor-Traschen solution.

gr-qc↗

arXiv2011

The Constancy of the Constants of Nature: Updates

The current observational and experimental bounds on the time variation of the constants of nature (the fine structure constant $α$, the gravitational constant $G$ and the proton-electron mass ratio $μ=m_p/m_e$) are reviewed.

gr-qc↗

arXiv2011

Shadows of Colliding Black Holes

We compute the shadows of colliding black holes using the Kastor-Traschen cosmological multiblack hole solution that is an exact solution describing the collision of maximally charged black holes with a positive cosmological constant. We find that in addition to the shadow of each black hole, an eyebrowlike structure appears as the black holes come close to each other. These features can be used as probes to find the multiblack hole system at the final stage of its merger process.

gr-qc↗

arXiv2011

Runaway Domain Wall and Space-time Varying alpha

Recently spatial as well as temporal variations of the fine structure constant alpha have been reported. We show that a "runaway domain wall", which arises for the scalar field potential without minima, can account for such variations simultaneously. The time variation is induced by a runaway potential and the spatial variation is induced by the formation of a domain wall. The model is consistent with the current cosmological data and can be tested by the future experiments to test the equivalence principle.

astro-ph.CO↗

arXiv2011

Prospects for Direct Detection of Inflationary Gravitational Waves by Next Generation Interferometric Detectors

We study the potential impact of detecting the inflationary gravitational wave background by the future space-based gravitational wave detectors, such as DECIGO and BBO. The signal-to-noise ratio of each experiment is calculated for chaotic/natural/hybrid inflation models by using the precise predictions of the gravitational wave spectrum based on numerical calculations. We investigate the dependence of each inflation model on the reheating temperature which influences the amplitude and shape of the spectrum, and find that the gravitational waves could be detected for chaotic/natural inflation models with high reheating temperature. From the detection of the gravitational waves, a lower bound on the reheating temperature could be obtained. The implications of this lower bound on the reheating temperature for particle physics are also discussed.

astro-ph.CO↗

arXiv2010

Gravitational Waves from Q-ball Formation

We study the detectability of the gravitational waves (GWs) from the Q-ball formation associated with the Affleck-Dine (AD) mechanism, taking into account both of the dilution effect due to Q-ball domination and of finite temperature effects. The AD mechanism predicts the formation of non-topological solitons, Q-balls, from which GWs are generated. Q-balls with large conserved charge $Q$ can produce a large amount of GWs. On the other hand, the decay rate of such Q-balls is so small that they may dominate the energy density of the universe, which implies that GWs are significantly diluted and that their frequencies are redshifted during Q-ball dominated era. Thus, the detectability of the GWs associated with the formation of Q-balls is determined by these two competing effects. We find that there is a finite but small parameter region where such GWs may be detected by future detectors such as DECIGO or BBO, only in the case when the thermal logarithmic potential dominates the potential of the AD field. Otherwise GWs from Q-balls would not be detectable even by these futuristic detectors: $Ω_{\rm GW}^0<10^{-21}$. Unfortunately, for such parameter region the present baryon asymmetry of the universe can hardly be explained unless one fine-tunes A-terms in the potential. However the detection of such a GW background may give us an information about the early universe, for example, it may suggest that the flat directions with $B-L=0$ are favored.

astro-ph.CO↗

arXiv2010

Fate of thermal log type Q balls

We study time evolution of the $Q$ ball in thermal logarithmic potential using lattice simulations. As the temperature decreases due to the cosmic expansion, the thermal logarithmic term in the potential is eventually overcome by a mass term, and we confirm that the $Q$ ball transforms from the thick-wall type to the thin-wall type for a positive coefficient of radiative corrections to the mass term, as recently suggested. Moreover, we find that the $Q$ ball finally "melts down" when the $Q$-ball solution disappears. We also discuss the effects of this phenomenon on the detectability of gravitational waves from the $Q$-ball formation.

hep-ph↗

arXiv2010

Slow-roll Extended Quintessence

We derive the slow-roll conditions for a non-minimally coupled scalar field (extended quintessence) during the radiation/matter dominated era extending our previous results for thawing quintessence. We find that the ratio $\ddotϕ/3H\dotϕ$ becomes constant but negative, in sharp contrast to the ratio for the minimally coupled scalar field. We also find that the functional form of the equation of state of the scalar field asymptotically approaches that of the minimally coupled thawing quintessence.

astro-ph.CO↗

arXiv2010

The Equation of State of Tracker Fields

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 late-time departure from the tracker solution due to the nonzero density parameter of dark energy $\Omp$. We calculate the equation of state as a function of $\Omp$ for constant $Γ=VV"/(V')^2$ (during matter era) models. The derived equation of state contains a single parameter, $w_{(0)}$, which parametrizes the equation of state during the matter-dominated epoch. We derive observational constraints on $w_{(0)}$ and find that observational data are consistent with the cosmological constant: $-1.11< \wzero< -0.96 (1 σ)$.

astro-ph.CO↗

arXiv2009

w and w' of Scalar Field Models of Dark Energy

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 models of dark energy (quintessence) occupy rather narrow regions in the $w-w'$ plane. We extend the $w-w'$ plane to $w<-1$ and derive bounds on $w'$ as a function of $w$ for tracker phantom dark energy. We also derive bounds on tracker k-essence.

astro-ph↗

arXiv2009

Slow-Roll Thawing Quintessence

We derive slow-roll conditions for thawing quintessence. We solve the equation of motion of $ϕ$ for a Taylor expanded potential (up to the quadratic order) in the limit where the equation of state $w$ is close to -1 to derive the equation of state as a function of the scale factor. We find that the evolution of $ϕ$ and hence $w$ are described by only two parameters. The expression for $w(a)$, which can be applied to general thawing models, coincides precisely with that derived recently by Dutta and Scherrer for hilltop quintessence. The consistency conditions of $|w+1|\ll 1$ are derived. The slow-roll conditions for freezing quintessence are also derived.

astro-ph.CO↗