arXiv subjects

Kei-ichi Maeda

Publications and source records attributed to Kei-ichi Maeda.

At least 37 records · Page 2Linked to original sources

arXiv2020

Stability of Hybrid Higgs Inflation

Hybrid Higgs inflation model is a hybrid type of the so-called Higgs inflation model with the conventional non-minimal $ξ$ coupling to a scalar curvature $R$ and new Higgs inflation model with the derivative coupling to the Einstein curvature tensor $G_{μν}$. This model can explain all possible value of the tensor-to-scalar ratio $r$ allowed by the present CMB constraints with an appropriate choice of coupling parameters. However the derivative coupling may causes a gradient instability during oscillation phase after inflation just as in new Higgs inflation model. Analyzing the behaviours of perturbations during oscillation phase in the hybrid Higgs inflation model, we show that the unstable scalar modes found in new Higgs inflation model are stabilized by the non-minimal $ξ$ coupling for some range of the coupling parameters.

gr-qc↗

arXiv2019

Cosmological Dynamics and Double Screening of DBI Galileon Gravity

We investigate cosmological dynamics and screening mechanism of the Dirac-Born-Infeld (DBI) Galileon model. The model has been divided into two regimes, one has positive signs in front of scalar field kinetic terms so-called the DBI galileon, another one has negative signs and it is dubbed as the DBIonic galileon. We find de Sitter solution and evolution of the Universe starting from radiation dominated era to late-time accelerated expansion in the DBI galileon model without the presence of potential term. In one of the attractors, the ghost and Laplacian instabilities vanishes for the whole evolution. We find mixing of screening mechanisms between the Vainshtein mechanism and the DBIonic screening mechanism in the DBIonic galileon model, in which a scale changing between these two mechanisms depends on a mass of a source.

gr-qc↗

arXiv2019

Cumulative Shift of Periastron Time of Binary Pulsar with Kozai-Lidov Oscillation

We study a hierarchical triple system with the Kozai-Lidov mechanism, and analyse the cumulative shift of periastron time of a binary pulsar by the emission of gravitational waves. Time evolution of the osculating orbital elements of the triple system is calculated by directly integrating the first-order post-Newtonian equations of motion. The Kozai-Lidov mechanism will bend the evolution curve of the cumulative shift when the eccentricity becomes large. We also investigate the parameter range of mass and semi-major axis of the third companion with which the bending of the cumulative-shift curve could occur within 100 years.

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arXiv2019

$α$-attractor-type Double Inflation

We study the background dynamics and primordial perturbation in $α$-attractor-type double inflation. The model is composed of two minimally coupled scalar fields, where each of the fields has a potential of the $α$-attractor type. We find that the background trajectory in this double inflation model is given by two almost straight lines connected with a turn in the field space. With this simple background, we can classify the property of the perturbations generated in this double inflation model just depending on whether the turn occurs before the horizon exit for the mode corresponding to the scale of interest or not. In the former case, the resultant primordial curvature perturbation coincides with the one obtained by the single-field $α$-attractor. On the other hand, in the latter case, it is affected by the multifield effects, like the mixing with the isocurvature perturbation and the change of the Hubble expansion rate at the horizon exit. We obtain the approximated analytic solutions for the background, with which we can calculate the primordial curvature perturbation by the $δN$ formalism analytically, even when the multifield effects are significant. We also impose observational constraints on the model parameters and initial values of the fields in this double inflation model based on the Planck result.

hep-th↗

arXiv2018

Metric-affine Gravity and Inflation

We classify the metric-affine theories of gravitation, in which the metric and the connections are treated as independent variables, by use of several constraints on the connections. Assuming the Einstein-Hilbert action, we find that the equations for the distortion tensor (torsion and non-metricity) become algebraic, which means that those variables are not dynamical. As a result, we can rewrite the basic equations in the form of Riemannian geometry. Although all classified models recover the Einstein gravity in the Palatini formalism (in which we assume there is no coupling between matter and the connections), but when matter field couples to the connections, the effective Einstein equations include an additional hyper energy-momentum tensor obtained from the distortion tensor. Assuming a simple extension of a minimally coupled scalar field in metric-affine gravity, we analyze an inflationary scenario. Even if we adopt a chaotic inflation potential, certain parameters could satisfy observational constraints. Furthermore, we find that a simple form of Galileon scalar field in metric-affine could cause G-inflation.

gr-qc↗

arXiv2018

Maximal Efficiency of Collisional Penrose Process with Spinning Particles

We analyze collisional Penrose process of spinning test particles in an extreme Kerr black hole. We consider that two particles plunge into the black hole from infinity and collide near the black hole. For the collision of two massive particles, if the spins of particles are $s_1 \approx 0.01379μM$ and $s_2 \approx -0.2709μM$, we obtain the maximal efficiency is about $η_{max} = (\text{extracted energy})=(\text{input energy}) \approx 15.01$, which is more than twice as large as the case of the collision of non-spinning particles ($η_{max} \approx 6.32$). We also evaluate the collision of a massless particle without spin and a massive particle with spin (Compton scattering), in which we find the maximal efficiency is $η_{max} \approx 26.85$ when $s_2 \approx -0.2709μM$, which should be compared with $η_{max} \approx 13.93$ for the nonspinning case.

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arXiv2018

Condensate of Massive Graviton and Dark Matter

We study coherently oscillating massive gravitons in the ghost-free bigravity theory. This coherent field can be interpreted as a condensate of the massive gravitons. We first define the effective energy-momentum tensor of the coherent massive gravitons in a curved spacetime. We then study the background dynamics of the universe and the cosmic structure formation including the effects of the coherent massive gravitons. We find that the condensate of the massive graviton behaves as a dark matter component of the universe. From the geometrical point of view the condensate is regarded as a spacetime anisotropy. Hence, in our scenario, dark matter is originated from the tiny deformation of the spacetime. We also discuss a production of the spacetime anisotropy and find that the extragalactic magnetic field of a primordial origin can yield a sufficient amount for dark matter.

hep-th↗

arXiv2018

Massive Graviton Geons

We find vacuum solutions such that massive gravitons are confined in a local spacetime region by their gravitational energy in asymptotically flat spacetimes in the context of the bigravity theory. We call such self-gravitating objects massive graviton geons. The basic equations can be reduced to the Schrödinger-Poisson equations with the tensor "wavefunction" in the Newtonian limit. We obtain a non-spherically symmetric solution with $j=2,\ell=0$ as well as a spherically symmetric solution with $j=0,\ell=2$ in this system where $j$ is the total angular momentum quantum number and $\ell$ is the orbital angular momentum quantum number, respectively. The energy eigenvalue of the Schrödinger equation in the non-spherical solution is smaller than that in the spherical solution. We then study the perturbative stability of the spherical solution and find that there is an unstable mode in the quadrupole mode perturbations which may be interpreted as the transition mode to the non-spherical solution. The results suggest that the non-spherically symmetric solution is the ground state of the massive graviton geon. The massive graviton geons may decay in time due to emissions of gravitational waves but this timescale can be quite long when the massive gravitons are non-relativistic and then the geons can be long-lived. We also argue possible prospects of the massive graviton geons: applications to the ultralight dark matter scenario, nonlinear (in)stability of the Minkowski spacetime, and a quantum transition of the spacetime.

gr-qc↗

arXiv2017

Hybrid Higgs Inflation: The Use of Disformal Transformation

We propose a hybrid type of the conventional Higgs inflation and new Higgs inflation models. We perform a disformal transformation into the Einstein frame and analyze the background dynamics and the cosmological perturbations in the truncated model, in which we ignore the higher-derivative terms of the Higgs field. From the observed power spectrum of the density perturbations, we obtain the constraint on the non-minimal coupling constant $ξ$ and the mass parameter $M$ in the derivative coupling. Although the primordial tilt $n_s$ in the hybrid model barely changes, the tensor-to-scalar ratio $r$ moves from the value in new Higgs inflationary model to that in the conventional Higgs inflationary model as $|ξ|$ increases. We confirm our results by numerical analysis by ADM formalism of the full theory in the Jordan frame.

gr-qc↗

arXiv2017

Non-linear collisional Penrose process: How large energy can a black hole release?

Energy extraction from a rotating or charged black hole is one of fascinating issues in general relativity. The collisional Penrose process is one of such extraction mechanisms and has been reconsidered intensively since Banados, Silk and West pointed out the physical importance of very high energy collisions around a maximally rotating black hole. In order to get results analytically, the test particle approximation has been adopted so far. Successive works based on this approximation scheme have not yet revealed the upper bound on the efficiency of the energy extraction because of lack of the back reaction. In the Reissner-Nordstrom spacetime, by fully taking into account the self-gravity of the shells, we find that there is an upper bound on the extracted energy, which is consistent with the area law of a black hole. We also show one particular scenario in which the almost maximum energy extraction is achieved even without the Banados-Silk-West collision.

gr-qc↗

arXiv2017

Cosmological Dynamics of D-BIonic and DBI Scalar Field and Coincidence Problem of Dark Energy

We study the cosmological dynamics of D-BIonic and DBI scalar field, which is coupled to matter fluid. For the exponential potential and the exponential couplings, we find a new analytic scaling solution yielding the accelerated expansion of the Universe. Since it is shown to be an attractor for some range of the coupling parameters, the density parameter of matter fluid can be the observed value, as in the coupled quintessence with a canonical scalar field. Contrary to the usual coupled quintessence, where the value of matter couple giving observed density parameter is too large to satisfy observational constraint from CMB, we show that the D-BIonic theory can give similar solution with much smaller value of matter coupling. As a result, together with the fact that the D-BIonic theory has a screening mechanism, the D-BIonic theory can solve the so-called coincidence problem as well as the dark energy problem.

hep-th↗

arXiv2016

AdS Monopole Black Hole and Phase Transition

We study the Einstein-SO(3)Yang-Mills-Higgs system with a negative cosmological constant, and find the monopole black hole solutions as well as the trivial Reissner-Nordström black hole. We discuss thermodynamical stability of the monopole black hole in an isolated system. We expect a phase transition between those two black holes when the mass of a black hole increases or decreases. The type of phase transition depends on the cosmological constant $Λ$ as well as the vacuum expectation value $v$ and the coupling constant $λ$ of the Higgs field. Fixing $λ$ small, we find there are two critical values of the cosmological constant $Λ_{\rm cr (1)}(v)$ and $Λ_{\rm cr(2)}(v)$, which depend on $v$. If $Λ_{\rm cr(1)}(v)<Λ(<0)$, we find the first order transition, while if $Λ_{\rm cr(2)}(v)<Λ<Λ_{\rm cr(1)}(v)$, the transition becomes second order. For the case of $Λ_{b}(v)<Λ<Λ_{\rm (2)}(v)$, we again find the first order irreversible transition from the monopole black hole to the extreme Reissner-Nordström black hole. Beyond $Λ_{b}(v)$, no monopole black hole exists. We also discuss thermodynamical properties of the monopole black hole in a thermal bath system.

gr-qc↗

arXiv2016

Gravitational Baryogenesis after Anisotropic Inflation

The gravitational baryogensis may not generate a sufficient baryon asymmetry in the standard thermal history of the Universe when we take into account the gravitino problem. Hence it has been suggested that anisotropy of the Universe can enhance the generation of the baryon asymmetry through the increase of the time change of the Ricci scalar curvature. We study the gravitational baryogenesis in the presence of anisotropy, which is produced at the end of an anisotropic inflation. Although we confirm that the generated baryon asymmetry is enhanced compared with the original isotropic cosmological model, taking into account the constraint on the anisotropy by the recent CMB observations, we find that it is still difficult to obtain the observed baryon asymmetry only through the gravitational baryogenesis without suffering from the gravitino problem.

hep-ph↗

arXiv2016

Dynamical angled brane

We discuss the dynamical D$p$-brane solutions describing any number of D$p$-branes whose relative orientations are given by certain SU(2) rotations. These are the generalization of the static angled D$p$-brane solutions. We study the collision of dynamical D3-brane with angles in type II string theory, and show that the particular orientation of the smeared D3-brane configuration can provide an example of colliding branes if they have the same charges. Otherwise a singularity appears before D3-branes collide.

hep-th↗

arXiv2016

Relativistic stars in bigravity theory

Assuming static and spherically symmetric spacetimes in the ghost-free bigravity theory, we find a relativistic star solution, which is very close to that in general relativity. The coupling constants are classified into two classes: Class [I] and Class [II]. Although the Vainshtein screening mechanism is found in the weak gravitational field for both classes, we find that there is no regular solution beyond the critical value of the compactness in Class [I]. This implies that the maximum mass of a neutron star in Class [I] becomes much smaller than that in GR. On the other hand, for the solution in Class [II], the Vainshtein screening mechanism works well even in a relativistic star and the result in GR is recovered.

gr-qc↗

arXiv2015

Black Holes and Thunderbolt Singularities with Lifshitz Scaling Terms

We study a static, spherically symmetric and asymptotic flat spacetime, assuming the hypersurface orthogonal Einstein-aether theory with an ultraviolet modification motivated by the Horava-Lifshitz theory, which is composed of the $z=2$ Lifshitz scaling terms such as scalar combinations of a three-Ricci curvature and the acceleration of the aether field. For the case with the quartic term of the acceleration of the aether field, we obtain a two-parameter family of black hole solutions, which possess a regular universal horizon. While, if three-Ricci curvature squared term is joined in ultraviolet modification, we find a solution with a thunderbolt singularity such that the universal horizon turns to be a spacelike singularity.

gr-qc↗

arXiv2015

Stability of the Early Universe in Bigravity Theory

We study the stability of a spherically symmetric perturbation around the flat Friedmann-Lema$\hatı$tre-Robertson-Walker spacetime in the ghost-free bigravity theory, retaining nonlinearities of the helicity-$0$ mode of the massive graviton. It has been known that, when the graviton mass is smaller than the Hubble parameter, homogeneous and isotropic spacetimes suffer from the Higuchi-type ghost or the gradient instability against the linear perturbation in the bigravity. Hence, the bigravity theory has no healthy massless limit for cosmological solutions at linear level. In this paper we show that the instabilities can be resolved by taking into account nonlinear effects of the scalar graviton mode for an appropriate parameter space of coupling constants. The growth history in the bigravity can be restored to the result in general relativity in the early stage of the Universe, in which the Stückelberg fields are nonlinear and there is neither ghost nor gradient instability. Therefore, the bigravity theory has the healthy massless limit, and cosmology based on it is viable even when the graviton mass is smaller than the Hubble parameter.

hep-th↗

arXiv2015

Bubble Universes With Different Gravitational Constants

We argue a scenario motivated by the context of string landscape, where our universe is produced by a new vacuum bubble embedded in an old bubble and these bubble universes have not only different cosmological constants, but also their own different gravitational constants. We study these effects on the primordial curvature perturbations. In order to construct a model of varying gravitational constants, we use the Jordan-Brans-Dicke (JBD) theory where different expectation values of scalar fields produce difference of constants. In this system, we investigate the nucleation of bubble universe and dynamics of the wall separating two spacetimes. In particular, the primordial curvature perturbation on superhorizon scales can be affected by the wall trajectory as the boundary effect. We show the effect of gravitational constant in the exterior bubble universe can provide a peak like a bump feature at a large scale in a modulation of power spectrum.

gr-qc↗