arXiv subjects

Shin'ichi Nojiri

Publications and source records attributed to Shin'ichi Nojiri.

At least 127 records · Page 7Linked to original sources

arXiv2016

Relativistic Stars in dRGT Massive Gravity

We study relativistic stars in the simplest model of the de Rham-Gabadadze-Tolley massive gravity which describes the massive graviton without ghost propagating mode. We consider the hydrostatic equilibrium, and obtain the modified Tolman-Oppenheimer-Volkoff equation and the constraint equation coming from the potential terms in the gravitational action. We give analytical and numerical results for quark and neutron stars and discuss the deviations compared with General Relativity and $F(R)$ gravity. It is shown that theory under investigation leads to small deviation from the General Relativity in terms of density profiles and mass-radius relation. Nevertheless, such deviation may be observable in future astrophysical probes.

gr-qc↗

arXiv2016

Palatini-Born-Infeld Gravity, Bouncing Universe, and Black Hole Formation

We consider the Palatini formalism of the Born-Infeld gravity. In the Palatini formalism, the propagating mode is only graviton, whose situation is different from that in the metric formalism. We discuss about the FRW cosmology by using an effective potential. Especially we consider the condition that the bouncing could occur. We also give some speculations about the black hole formation

hep-th↗

arXiv2016

Dynamical Domain Wall and Localization

Based on the previous works (arXiv:1202.5375 and 1402.1346), we investigate the localization of the fields on the dynamical domain wall, where the four dimensional FRW universe is realized on the domain wall in the five dimensional space-time. Especially we show that the chiral spinor can localize on the domain wall, which has not been succeeded in the past works as the seminal work in arXiv:0810.3746.

hep-th↗

arXiv2015

Singular cosmological evolution using canonical and phantom scalar fields

We demonstrate that finite time singularities of Type IV can be consistently incorporated in the Universe's cosmological evolution, either appearing in the inflationary era, or in the late-time regime. While using only one scalar field instabilities can in principle occur at the time of the phantom-divide crossing, when two fields are involved we are able to avoid such instabilities. Additionally, the two-field scalar-tensor theories prove to be able to offer a plethora of possible viable cosmological scenarios, at which various types of cosmological singularities can be realized. Amongst others, it is possible to describe inflation with the appearance of a Type IV singularity, and phantom late-time acceleration which ends in a Big Rip. Finally, for completeness, we also present the Type IV realization in the context of suitably reconstructed $F(R)$ gravity.

gr-qc↗

arXiv2015

Stability analysis for new theories of massive spin-two particle and black hole entropy of new bigravity

In Phys.\ Rev.\ D 90 043006 (2014), we proposed a new ghost-free massive spin-two model in flat spacetime. Furthermore, as some extension, we couple the new model with a non-dynamical curved background in Phys.\ Rev.\ D 90 123013 (2014) and constructed new interaction terms without appearance of an extra mode. The characteristic property of the new model is the existence of nonlinear potential terms which give the nontrivial vacua. The presence of the nontrivial vacua, however, does not mean that the particle can be defined around all vacua. Therefore, in this paper, we discuss the condition for the new model to have stable vacua in flat spacetime and curved spacetime. Then, we couple this spin-two theory with a dynamical background and obtain the solutions. Moreover, we investigate the effect of this new spin-two model to the Einstein gravity by calculating the black hole entropy since the gravity coupled massive spin-two theory admits a black hole solution in addition to (anti-) de Sitter space solution.

hep-th↗

arXiv2015

Inflation without self-reproduction in $F(R)$ gravity

We investigate inflation in frames of two classes of $F(R)$ gravity and check its consistency with Planck data. It is shown that $F(R)$ inflation without self-reproduction may be constructed in close analogy with the corresponding scalar example proposed by Mukhanov for the resolution the problems of multiverse, predictability and initial conditions.

gr-qc↗

arXiv2014

Mimetic $F(R)$ gravity: inflation, dark energy and bounce

We propose the mimetic $F(R)$ theory and investigate the early-time and late-time acceleration in such theory. It is demonstrated that inflation consistent with observable data may be realized in such theory. The reconstruction of realistic $Λ$CDM era is also possible as well as the unification of early-time inflation with late-time acceleration or bounce universe. It is stressed that specific universe evolution is governed by mimetic $F(R)$ theory which is different from convenient $F(R)$ gravity. The corresponding examples are presented. Mimetic $F(R)$ gravity is generalized by the addition of the scalar potential in the formulation of convenient $F(R)$ gravity with specific Lagrange multiplier constraint. It is demonstrated that such theory may admit the arbitrary universe evolution via the corresponding choice of the scalar potential and/or function $F(R)$.

hep-th↗

arXiv2014

New massive spin two model on curved space-time

We have proposed a new ghost-free model with interactions of massive spin two particles in Phys.\ Rev.\ D {\bf 90} (2014) 043006 [arXiv:1402.5737 [hep-th]]. Although the model is ghost-free on the Minkowski space-time, it is not obvious whether or not this desirable property is preserved on curved space-time. In fact, Buchbinder et al. already pointed out that the Fierz-Pauli theory is not ghost-free on curved space-time without non-minimal coupling terms. In this paper, we construct a new theory of massive spin two particles with non-minimal coupling on curved space-time and show that the model can be ghost-free. Furthermore, we propose new non-minimal coupling terms.

hep-th↗

arXiv2014

New model of massive spin two field

We propose a new ghost-free model describing massive spin two field. This model consists of a kinetic term and interaction terms without derivative. We report on the properties of this model, especially we consider what could happen when this model couples with gravity. Although the model does not generate any ghost on the Minkowski space-time, it is not so clear whether or not this property is preserved even on curved space-time. In fact, Buchbinder et al. have found that the ghost appears even in the Fierz-Pauli theory on curved space-time if we do not include non-minimal coupling terms. We report on the model with interactions on curved space-time and show that we can construct a model without ghost by including non-minimal coupling terms.

hep-th↗

arXiv2014

Inflationary universe from perfect fluid and $F(R)$ gravity and its comparison with observational data

We investigate the descriptions for the observables of inflationary models, in particular, the spectral index of curvature perturbations, the tensor-to-scalar ratio, and the running of the spectral index, in the framework of perfect fluid models and $F(R)$ gravity theories through the reconstruction methods. Furthermore, the perfect fluid and $F(R)$ gravity descriptions of inflation are compared with the recent cosmological observations such as the Planck satellite and BICEP2 experiment. It is demonstrated with explicit examples that perfect fluid may lead to the inflationary universe consistent with the Planck data. It is also shown that several $F(R)$ gravity models, especially, a power-law model gives the best fit values compatible with the spectral index and tensor-to-scalar ratio within the allowed ranges suggested by the Planck and BICEP2 results.

hep-th↗

arXiv2014

Stability and Anti-evaporation of the Schwarzschild-de Sitter Black Holes in Bigravity

We study the stability under the perturbation and the related anti-evaporation of the Nariai space-time in bigravity. If we impose specific condition for the solutions and parameters, we obtain asymptotically de Sitter space-time, and show the existence of the Nariai space-time as a background solution. Considering the perturbation around the Nariai space-time up to first order, we investigate the behavior of black hole horizon. We show that the anti-evaporation does not occur on the classical level in the bigravity.

hep-th↗

arXiv2014

Instabilities and Anti-Evaporation of Reissner-Nordström Black Holes in modified $F(R)$ gravity

We study the instabilities and related anti-evaporation of the extremal Reissner-Nordström (RN) black hole in $F(R)$ gravity. It is remarkable that the effective electric charge can be generated for some solutions of $F(R)$ gravity without electromagnetic field. The anti-evaporation effect occurs but it emerges only in the strong coupling limit of the effective gravitational coupling. The instabilities of RN black hole are also investigated when the electromagnetic sector is added to the action of $F(R)$ gravity. We show the anti-evaporation occurs in the Maxwell-$F(R)$ gravity with the arbitrary gravitational coupling constant although it does not occur in the Maxwell-Einstein gravity. Furthermore, general spherically-symmetric solution of $F(R)$ gravity in the Einstein frame is obtained.

gr-qc↗

arXiv2014

Reconstruction of scalar field theories realizing inflation consistent with the Planck and BICEP2 results

We reconstruct scalar field theories to realize inflation compatible with the BICEP2 result as well as the Planck. In particular, we examine the chaotic inflation model, natural (or axion) inflation model, and an inflationary model with a hyperbolic inflaton potential. We perform an explicit approach to find out a scalar field model of inflation in which any observations can be explained in principle.

hep-th↗

arXiv2014

Renormalizable toy model of massive spin two field and new bigravity

In this paper, we propose a toy model of the renormalizable theory describing massive spin two field. Although the model is renormalizable, we show that the model contains ghost. The coupling of the theory with gravity can be regarded as a new kind of bimetric gravity or bigravity. We show that the massive spin two field plays the role of the cosmological constant.

hep-th↗

arXiv2014

Cosmology and Stability in Scalar-Tensor bigravity

The bigravity models coupled with two scalar fields are constructed. We show that a wide class of the expansion history of the universe, especially corresponding to dark energy and/or inflation, can be described by a solution of the bigravity model. We discuss the stability of the solution and give the conditions for the stability. We also explicitly construct a model which gives a stable solution. By using the stable model, for an arbitrary evolution of the universe expansion, we construct the Brans-Dicke like model which reproduces the evolution.

hep-th↗

arXiv2014

Trace-anomaly driven inflation in $f(T)$ gravity and in minimal massive bigravity

We explore trace-anomaly driven inflation in modified gravity. It is explicitly shown that in $T^2$ teleparallel gravity, the de Sitter inflation can occur, although quasi de Sitter inflation happens in $R^2$ gravity. Furthermore, we investigate the influence of the trace anomaly on inflation. It is found that in $f(T)$ gravity, the de Sitter inflation can end because it becomes unstable due to the trace anomaly, whereas also in higher derivative gravity, the de Sitter inflation can be realized and it will be over thanks to the trace anomaly for smaller parameter regions in comparison with those in teleparallelism. The instability of the de Sitter inflation in $T^2$ gravity and $R^2$ gravity (both with taking account of the trace anomaly) is examined. In addition, we study trace-anomaly driven inflation in minimal massive bigravity, where the contribution from the massive graviton acts as negative cosmological constant. It is demonstrated that the de Sitter inflation can occur and continue for long enough duration.

gr-qc↗

arXiv2014

Reconstruction of Domain Wall Universe and Localization of Gravity

We construct a four-dimensional domain wall universe by using the Brans-Dicke type gravity with two scalar field. We give a formulation where for arbitrarily given warp factor and scale factor, we construct an action which reproduces both of the warp and scale factors as a solution of the Einstein equation and the field equations given by the action. This formulation could be called as reconstruction. We show that the model does not contain ghost with negative kinetic term and there occur the localization of gravity as in the Randall-Sundrum model. It should be noted that in the equation of the graviton, there appear extra terms related with the extra dimension, which might affect the tensor mode in the fluctuations

hep-th↗

arXiv2014

Possible antigravity regions in $F(R)$ theory?

We construct an $F(R)$ gravity theory corresponding to the Weyl invariant two scalar field theory. We investigate whether such $F(R)$ gravity can have the antigravity regions where the Weyl curvature invariant does not diverge at the Big Bang and Big Crunch singularities. It is revealed that the divergence cannot be evaded completely but can be much milder than that in the original Weyl invariant two scalar field theory.

hep-th↗