arXiv subjects

Shin'ichi Nojiri

Publications and source records attributed to Shin'ichi Nojiri.

At least 199 records · Page 11Linked to original sources

arXiv2009

Cosmological reconstruction of realistic modified F(R) gravities

The cosmological reconstruction scheme for modified $F(R)$ gravity is developed in terms of e-folding (or, redshift). It is demonstrated how any FRW cosmology may emerge from specific $F(R)$ theory. The specific examples of well-known cosmological evolution are reconstructed, including $Λ$CDM cosmology, deceleration with transition to phantom superacceleration era which may develop singularity or be transient. The application of this scheme to viable $F(R)$ gravities unifying inflation with dark energy era is proposed. The additional reconstruction of such models leads to non-leading gravitational correction mainly relevant at the early/late universe and helping to pass the cosmological bounds (if necessary). It is also shown how cosmological reconstruction scheme may be generalized in the presence of scalar field.

hep-th↗

arXiv2009

Crossing of the phantom divide in modified gravity

We reconstruct an explicit model of modified gravity in which a crossing of the phantom divide can be realized. It is shown that the (finite-time) Big Rip singularity appears in the model of modified gravity (i.e., in the so-called Jordan frame), whereas that in the corresponding scalar field theory obtained through the conformal transformation (i.e., in the so-called Einstein frame) the singularity becomes the infinite-time one. Furthermore, we investigate the relations between the scalar field theories with realizing a crossing of the phantom divide and the corresponding modified gravitational theories by using the inverse conformal transformation. It is demonstrated that the scalar field theories describing the non-phantom phase (phantom one with the Big Rip) can be represented as the theories of real (complex) $F(R)$ gravity through the inverse (complex) conformal transformation. We also study a viable model of modified gravity in which the transition from the de Sitter universe to the phantom phase can occur. In addition, we explore the stability for the obtained solutions of the crossing of the phantom divide under a quantum correction coming from conformal anomaly.

hep-th↗

arXiv2009

Gauss-Bonnet Chameleon Mechanism of Dark Energy

As a model of the current accelerated expansion of the universe, we consider a model of the scalar-Einstein-Gauss-Bonnet gravity. This model includes the propagating scalar modes, which might give a large correction to the Newton law. In order to avoid this problem, we propose an extension of the Chameleon mechanism where the scalar mode becomes massive due to the coupling with the Gauss-Bonnet term. Since the Gauss-Bonnet invariant does not vanish near the earth or in the Solar System, even in the vacuum, the scalar mode is massive even in the vacuum and the correction to the Newton law could be small. We also discuss about the possibility that the model could describe simultaneously the inflation in the early universe, in addition to the current accelerated expansion.

hep-th↗

arXiv2009

Singularity of spherically-symmetric spacetime in quintessence/phantom dark energy universe

We consider ideal fluid and equivalent scalar field dark energy universes where all four known types of finite-time, future singularities occur at some parameter values. It is demonstrated that pressure/energy density of such quintessence/phantom dark energy diverges in spherically-symmetric spacetime at finite radius or at the center. This may cause the instability of the relativistic star or black hole in such universe. The resolution of the problem via the extra modification of the equation of state is briefly discussed.

hep-th↗

arXiv2008

Accelerating cosmologies from non-local higher-derivative gravity

We study accelerating cosmological solutions of a general class of non-linear gravities which depend on Gauss-Bonnet and other higher derivative invariants. To achieve this goal a local formulation with auxiliary scalars for arbitrary higher-derivative non-local gravity is developed. It is demonstrated that non-local Gauss-Bonnet gravity can be reduced, in the local formulation, to a model of string-inspired scalar-Gauss-Bonnet gravity. A natural unification, in the theory here developed, of the early-time inflation epoch with a late-time acceleration stage can also be realized.

hep-th↗

arXiv2008

Cosmology in non-minimal Yang-Mills/Maxwell theory

We review cosmology in non-minimal Yang-Mills/Maxwell theory, in which the Yang-Mills/electromagnetic field couples to a function of the scalar curvature. We show that power-law inflation can be realized due to the non-minimal gravitational coupling of the Yang-Mills field which may be caused by quantum corrections. Moreover, we study non-minimal vector model in the framework of modified gravity and demonstrate that both inflation and the late-time accelerated expansion of the universe can be realized. We also discuss the cosmological reconstruction of the Yang-Mills theory. Furthermore, we investigate late-time cosmology in non-minimal Maxwell-Einstein theory. We explore the forms of the non-minimal gravitational coupling which generate the finite-time future singularities and the general conditions for this coupling in order that the finite-time future singularities cannot appear.

hep-th↗

arXiv2008

Future of the universe in modified gravitational theories: Approaching to the finite-time future singularity

We investigate the future evolution of the dark energy universe in modified gravities including $F(R)$ gravity, string-inspired scalar-Gauss-Bonnet and modified Gauss-Bonnet ones, and ideal fluid with the inhomogeneous equation of state (EoS). Modified Friedmann-Robertson-Walker (FRW) dynamics for all these theories may be presented in universal form by using the effective ideal fluid with an inhomogeneous EoS without specifying its explicit form. We construct several examples of the modified gravity which produces accelerating cosmologies ending at the finite-time future singularity of all four known types by applying the reconstruction program. Some scenarios to resolve the finite-time future singularity are presented. Among these scenarios, the most natural one is related with additional modification of the gravitational action in the early universe. In addition, late-time cosmology in the non-minimal Maxwell-Einstein theory is considered. We investigate the forms of the non-minimal gravitational coupling which generates the finite-time future singularities and the general conditions for this coupling in order that the finite-time future singularities cannot emerge. Furthermore, it is shown that the non-minimal gravitational coupling can remove the finite-time future singularities or make the singularity stronger (or weaker) in modified gravity.

hep-th↗

arXiv2008

Modified gravity as realistic candidate for dark energy, inflation and dark matter

We review the unification of early-time inflation with late-time acceleration in several local modified gravity models which pass Solar System and cosmological tests. It is also demonstrated that account of non-local gravitational corrections to the action does not destroy the possibility of such unification. Dark matter effect is caused by composite graviton degree of freedom in such models.

hep-th↗

arXiv2008

Dark energy, inflation and dark matter from modified F(R) gravity

We review modified $F(R)$ gravity as realistic candidate to describe the observable universe expansion history. We show that recent cosmic acceleration, radiation/matter-dominated epoch and inflation could be realized in the framework of $F(R)$-gravity in the unified way. For some viable classes of $F(R)$-gravity, the Newton law is respected and there is no so-called matter instability (the very heavy positive mass for additional scalar degree of freedom is generated). The reconstruction program in modified gravity is also reviewed and it is demonstrated that {\it any} time-evolution of the universe expansion could be realized in $F(R)$-gravity. These models remain to be realistic also in the presence of non-minimal gravitational coupling with usual matter. It is shown that same model which passes local tests and predicts the unification of inflation with cosmic acceleration also describes dark matter thanks to presence of additional scalar degree of freedom and chameleon mechanism.

hep-th↗

arXiv2008

The future evolution and finite-time singularities in $F(R)$-gravity unifying the inflation and cosmic acceleration

We study the future evolution of quintessence/phantom dominated epoch in modified $F(R)$-gravity which unifies the early-time inflation with late-time acceleration and which is consistent with observational tests. Using the reconstruction technique it is demonstrated that there are models where any known (Big Rip, II, III or IV Type) singularity may classically occur. From another side, in Einstein frame (scalar-tensor description) only IV Type singularity occurs. Near the singularity the classical description breaks up, it is demonstrated that quantum effects act against the singularity and may prevent its appearance. The realistic $F(R)$-gravity which is future singularity free is proposed. We point out that additional modification of any $F(R)$-gravity by the terms relevant at the early universe is possible, in such a way that future singularity does not occur even classically.

hep-th↗

arXiv2008

Inflationary cosmology and the late-time accelerated expansion of the universe in non-minimal Yang-Mills-$F(R)$ gravity and non-minimal vector-$F(R)$ gravity

We study inflationary cosmology and the late-time accelerated expansion of the universe in non-minimal Yang-Mills (YM) theory, in which the YM field couples to a function of the scalar curvature. It is shown that power-law inflation can be realized due to the non-minimal YM field-gravitational coupling which maybe caused by quantum corrections. Moreover, it is demonstrated that both inflation and the late-time accelerated expansion of the universe can be realized in a modified YM-$F(R)$ gravity which is consistent with solar system tests. Furthermore, it is shown that this result can be realized also in a non-minimal vector-$F(R)$ gravity. In addition, we consider the duality of the non-minimal electromagnetic theory and that of the non-minimal YM theory, and also discuss the cosmological reconstruction of the YM theory.

hep-th↗

arXiv2008

Reconstructing the universe history, from inflation to acceleration, with phantom and canonical scalar fields

We consider the reconstruction technique in theories with a single or multiple (phantom and/or canonical) scalar fields. With the help of several examples, it is demonstrated explicitly that the universe expansion history, unifying early-time inflation and late-time acceleration, can be realized in scalar-tensor gravity. This is generalized to the theory of a scalar field coupled non-minimally to the curvature and to a Brans-Dicke-like theory. Different examples of unification of inflation with cosmic acceleration, in which de Sitter, phantom, and quintessence type fields play the fundamental role--in different combinations--are worked out. Specifically, the frame dependence and stability properties of de Sitter space scalar field theory are studied. Finally, for two-scalar theories, the late-time acceleration and early-time inflation epochs are successfully reconstructed, in realistic situations in which the more and more stringent observational bounds are satisfied, using the freedom of choice of the scalar field potential, and of the kinetic factor.

hep-th↗

arXiv2008

Can F(R)-gravity be a viable model: the universal unification scenario for inflation, dark energy and dark matter

We review on the viability of $F(R)$-gravity. We show that recent cosmic acceleration, radiation/matter-dominated epoch and inflation could be realized in the framework of $F(R)$-gravity in the unified way. For some classes of $F(R)$-gravity, the correction to the Newton law is extremely small and there is no so-called matter instability (the very heavy positive mass for additional scalar degree of freedom is generated). The reconstruction program in modified gravity is also reviewed and it is demonstrated that {\it any} time-evolution of the universe expansion could be realized in $F(R)$-gravity. Special attention is paid to modified gravity which unifies inflation with cosmic acceleration and passes local tests. It turns out that such a theory may describe also dark matter.

astro-ph↗

arXiv2007

Modified f(R) gravity unifying R^m inflation with \LambdaCDM epoch

We consider modified $f(R)$ gravity which may unify $R^m$ early-time inflation with late-time $Λ$CDM epoch. It is shown that such model passes the local tests (Newton law, stability of Earth-like gravitational solution, very heavy mass for additional scalar degree of freedom) and suggests the realistic alternative for General Relativity. Various scenarios for future evolution of $f(R)$ $Λ$CDM era are discussed.

hep-th↗

arXiv2007

Modified non-local-F(R) gravity as the key for the inflation and dark energy

We consider FRW cosmology in non-local modified gravity. Its local scalar-tensor formulation is developed. It is explicitly demonstrated that such theory may lead to the unification of early-time inflation with late-time cosmic acceleration. The quintessence or phantom era may emerge for specific form of the action. The coupled non-local-F(R) gravity is also investigated. It is shown that such theory being consistent with Solar System tests may lead to the known universe history sequence: inflation, radiation/matter dominance and dark epoch.

hep-th↗

arXiv2007

From inflation to dark energy in the non-minimal modified gravity

We consider the modified gravity non-minimally coupled with matter Lagrangian for the description of early-time and late-time universe. Such $F(R)$ ($F(G)$) gravity in the absence of non-minimal coupling is viable theory which passes the local tests and reproduces the $Λ$CDM era. For qualitatively similar choice of non-minimal gravitational coupling function it is shown that the unified description of early-time inflation and late-time cosmic acceleration is possible. It is interesting that matter (scalar) which supports the inflationary era is gravitationally screened at late times. Hence, it may be effectively invisible at current universe.

hep-th↗

arXiv2007

Unifying inflation with LambdaCDM epoch in modified f(R) gravity consistent with Solar System tests

We suggest two realistic f(R) and one F(G) modified gravities which are consistent with local tests and cosmological bounds. The typical property of such theories is the presence of the effective cosmological constant epochs in such a way that early-time inflation and late-time cosmic acceleration are naturally unified within single model. It is shown that classical instability does not appear here and Newton law is respected. Some discussion of possible anti-gravity regime appearence and related modification of the theory is done.

hep-th↗

arXiv2007

Newton law corrections and instabilities in $f(R)$ gravity with the effective cosmological constant epoch

We consider class of modified $f(R)$ gravities with the effective cosmological constant epoch at the early and late universe. Such models pass most of solar system tests as well they satisfy to cosmological bounds. Despite their very attractive properties, it is shown that one realistic class of such models may lead to significant Newton law corrections which become small at the future universe only. The model with acceptable Newton law regime shows the matter instability. This suggests that more complicated version of such theory (or extended parameters space) should be investigated.

hep-th↗