arXiv subjects

S. Nojiri

Publications and source records attributed to S. Nojiri.

At least 19 recordsLinked to original sources

arXiv2022

Black Holes with Electric and Magnetic Charges in $F(R)$ Gravity

We construct spherically symmetric and static solutions in $F(R)$ gravity coupled with electromagnetic fields. The solutions include new types of black holes with electric and magnetic charges. We show that the higher-derivative terms make the curvature singularity much softer than that in the charged black holes in Einstein's general relativity. We calculate some thermodynamical quantities of the obtained black holes like entropy, Hawking radiation, and quasi-local energy and we confirm that the black hole solutions satisfy the first law of thermodynamics. Finally, we study the stability analysis using the odd-type mode and show that there are stable black hole solutions and the radial speed of the parity-odd mode is unit, that is, the speed of light.

gr-qc↗

arXiv2022

Formalizing Anisotropic Inflation in Modified Gravity

Motivated by the fact that the pre-inflationary era may evolve in an exotic way, in this work we formalize anisotropic evolution in the context of modified gravity, focusing on pre-inflationary and near the vicinity of the inflationary epochs. We specialize on specific metrics like Bianchi and Taub and we formalize the inflationary theory in vacuum $F(R)$ gravity, in $F(R)$ gravity with an extra scalar field and in Gauss-Bonnet gravity. We discuss the qualitative effects of the anisotropies on the evolution of the Universe and also we consider several specific solutions, like the de Sitter solution, in both the isotropic and anisotropic contexts. Furthermore, several exotic modified gravity cosmological solutions, like the ones which contain finite time singularities, are also discussed in brief.

gr-qc↗

arXiv2022

2D $F(R)$ gravity and AdS$_2$/CFT$_1$ correspondence

We studied the canonical structure of 2D $F(R)$ gravity. Its equivalence with Jackiw-Teitelboim gravity is demonstrated when no matter presents. Then, due to AdS$_2$/CFT$_1$ correspondence, such $F(R)$ gravity is equivalent to the Sachdev-Ye--Kitaev models. The singular $D\to 2$ limit of $F(R)$ gravity is also studied. It is shown that in such a limit AdS$_2$/CFT$_1$ correspondence is not realized.

hep-th↗

arXiv2022

Integral $F(R)$ Gravity and Saddle Point Condition as a Remedy for the $H_0$-tension

In this work, we shall provide an $F(R)$ gravity theoretical framework for solving the $H_0$-tension. Specifically, by exploiting the $F(R)$ gravity correspondence with a scalar-tensor theory, we shall provide a condition in which when it is satisfied, the $H_0$-tension is alleviated. The condition that remedies the $H_0$-tension restricts the corresponding $F(R)$ gravity, and we present in brief the theoretical features of the constrained $F(R)$ gravity theory in both the Jordan and Einstein frames. The condition that may remedy the $H_0$-tension is based on the existence of a metastable de Sitter point that occurs for redshifts near the recombination. This metastable de Sitter vacuum restricts the functional form of the $F(R)$ gravity in the Jordan frame. We also show that by appropriately choosing the $F(R)$ gravity, along with the theoretical solution offered for the $H_0$-tension problem, one may also provide a unified description of the inflationary era with the late-time accelerating era, in terms of two extra de Sitter vacua. We propose a new approach to $F(R)$ gravity by introducing a new class of integral $F(R)$ gravity functions, which may be wider than the usual class expressed in terms of elementary $F(R)$ gravity functions. Finally, the Einstein frame inflationary dynamics formalism is briefly discussed.

gr-qc↗

arXiv2021

Ghost-free $F\left( R,\mathcal{G} \right)$ Gravity

In this work we shall address the ghost issue of $F\left( R,\mathcal{G} \right)$ gravity, which is known to be plagued with ghost degrees of freedom. These ghosts occur due to the presence of higher than two derivatives in the field equations, and can arise even when considering cosmological perturbations, where superluminal modes may arise in the theory. If we consider the quantum theory, the ghosts generate the negative norm states, which give the negative probabilities, and therefore the ghosts are physically inconsistent. Motivated by the importance of $F\left( R,\mathcal{G} \right)$ gravity for providing viable inflationary and dark energy phenomenologies, in this work we shall provide a technique that can render $F\left( R,\mathcal{G} \right)$ gravity theories free from ghost degrees of freedom. This will be done by introducing two auxiliary scalar fields, and by employing the Lagrange multiplier technique, the theory is ghost free in the Einstein frame. Also the framework can be viewed as a reconstruction technique and can be used as a method in order to realize several cosmological evolutions of interest. We demonstrate how we can realize several cosmologically interesting phenomenologies by using the reconstruction technique.

gr-qc↗

arXiv2021

Modified Gravity Theories on a Nutshell: Inflation, Bounce and Late-time Evolution

We systematically review some standard issues and also the latest developments of modified gravity in cosmology, emphasizing on inflation, bouncing cosmology and late-time acceleration era. Particularly, we present the formalism of standard modified gravity theory representatives, like $F(R)$, $F(\mathcal{G})$ and $F(T)$ gravity theories, but also several alternative theoretical proposals which appeared in the literature during the last decade. We emphasize on the formalism developed for these theories and we explain how these theories can be considered as viable descriptions for our Universe. Using these theories, we present how a viable inflationary era can be produced in the context of these theories, with the viability being justified if compatibility with the latest observational data is achieved. Also we demonstrate how bouncing cosmologies can actually be described by these theories. Moreover, we systematically discuss several qualitative features of the dark energy era by using the modified gravity formalism, and also we critically discuss how a unified description of inflation with dark energy era can be described by solely using the modified gravity framework. Finally, we also discuss some astrophysical solutions in the context of modified gravity, and several qualitative features of these solutions. The aim of this review is to gather the different modified gravity techniques and form a virtual modified gravity "toolbox", which will contain all the necessary information on inflation, dark energy and bouncing cosmologies in the context of the various forms of modified gravity.

gr-qc↗

arXiv2020

Non-trivial black hole solutions in $\mathit{f(R)}$ gravitational theory

Recent observation shows that general relativity (GR) is not valid in the strong regime. $\mathit{f(R)}$ gravity where $\mathit{R}$ is the Ricci scalar, is regarded to be one of good candidates able to cure the anomalies appeared in the conventional general relativity. In this realm, we apply the equation of motions of $\mathit{f(R)}$ gravity to a spherically symmetric spacetime with two unknown functions and derive original black hole (BH) solutions without any constrains on the Ricci scalar as well as on the form of $\mathit{f(R)}$ gravity. Those solutions depend on a convolution function and are deviating from the Schwarzschild solution of the Einstein GR. These solutions are characterized by the gravitational mass of the system and the convolution function that in the asymptotic form gives extra terms that are responsible to make such BHs different from GR. Also, we show that these extra terms make the singularities of the invariants much weaker than those of the GR BH. We analyze such BHs using the trend of thermodynamics and show their consistency with the well known quantities in thermodynamics like the Hawking radiation, entropy and quasi-local energy. We also show that our BH solutions satisfy the first law of thermodynamics. Moreover, we study the stability analysis using the odd-type mode and shows that all the derived BHs are stable and have radial speed equal to one. Finally, using the geodesic deviations we derive the stability conditions of these BHs.

gr-qc↗

arXiv2020

Spherically symmetric black holes with electric and magnetic charge in extended gravity: Physical properties, causal structure, and stability analysis in Einstein's and Jordan's frames

Novel static black hole solutions with electric and magnetic charges are derived for the class of modified gravities: $f({\cal R})={\cal R}+2β\sqrt{\cal R}$, with or without a cosmological constant. The new black holes behave asymptotically as flat or (A)dS space-times with a dynamical value of the Ricci scalar given by $R=\frac{1}{r^2}$ and $R=\frac{8r^2Λ+1}{r^2}$, respectively. They are characterized by three parameters, namely their mass and electric and magnetic charges, and constitute black hole solutions different from those in Einstein's general relativity. Their singularities are studied by obtaining the Kretschmann scalar and Ricci tensor, which shows a dependence on the parameter $β$ that is not permitted to be zero. A conformal transformation is used to display the black holes in Einstein's frame and check if its physical behavior is changed w.r.t. the Jordan one. The thermal stability of the solutions is discussed by using thermodynamical quantities, in particular the entropy, the Hawking temperature, the quasi-local energy, and the Gibbs free energy. Also, the casual structure of the new black holes is studied, and a stability analysis is performed in both frames using the odd perturbations technique and the study of the geodesic deviation. It is concluded that, generically, there is coincidence of the physical properties of the novel black holes in both frames, although this turns not to be the case for the Hawking temperature.

gr-qc↗

arXiv2019

Ghost-free Gauss-Bonnet Theories of Gravity

In this work we develop a theoretical framework for Gauss-Bonnet modified gravity theories, in which ghost modes can be eliminated at the equations of motion level. Particularly, after we present how the ghosts can occur at the level of equations of motion, we employ the Lagrange multipliers technique, and by means of constraints we are able to eliminate the ghost modes from Gauss-Bonnet theories of the form $f(\mathcal{G})$ and $F(R,\mathcal{G})$ types. Some cosmological realizations in the context of the ghost free $f(\mathcal{G})$ gravity are presented, by using the reconstruction technique we developed. Finally, we explore the modifications to the Newton law of gravity generated by the ghost-free $f(\mathcal{G})$ theory.

gr-qc↗

arXiv2019

$k$-essence $f(R)$ Gravity Inflation

In this work we study a modified version of $f(R)$ gravity in which higher order kinetic terms of a scalar field are added in the action of vacuum $f(R)$ gravity. This type of theory is a type of $k$-essence $f(R)$ gravity, and it belongs to the general class of $f(R,ϕ,X)$ theories of gravity, where $ϕ$ is a scalar field and $X=\frac{1}{2}\partial^μϕ\partial_μϕ$. We focus on the inflationary phenomenology of the model, in the slow-roll approximation, and we investigate whether viable inflationary evolutions can be realized in the context of this theory. We use two approaches, firstly by imposing the slow-roll conditions and by using a non-viable vacuum $f(R)$ gravity. As we demonstrate, the spectral index of the primordial scalar perturbations and the tensor-to-scalar ratio of the resulting theory can be compatible with the latest observational data. In the second approach, we fix the functional form of the Hubble rate as a function of the $e$-foldings number, and we modify well-known vacuum $f(R)$ gravity reconstruction techniques, in order to find the $k$-essence $f(R)$ gravity which can realize the given Hubble rate. Accordingly, we calculate the slow-roll indices and the corresponding observational indices, and we also provide general formulas of these quantities in the slow-roll approximation. As we demonstrate, viability can be obtained in this case too, however the result is strongly model dependent. In addition, we discuss when ghosts can occur in the theory, and we investigate under which conditions ghosts can be avoided by using a particular class of models. Finally, we qualitatively discuss the existence of inflationary attractors for the non-slow-roll theory, and we provide hints towards finding general de Sitter attractors for the theory at hand.

gr-qc↗

arXiv2017

Ghost-Free $F(R)$ Gravity with Lagrange Multiplier Constraint

We propose two new versions of ghost-free generalized $F(R)$ gravity with Lagrange multiplier constraint. The first version of such theory for a particular degenerate choice of the Lagrange multiplier, corresponds to mimetic $F(R)$ gravity. The second version of such theory is just the Jordan frame description of mimetic gravity with potential. As we demonstrate, it is possible to realize several cosmological scenarios in such theory. In particulary, de Sitter solutions may also be found.

gr-qc↗

arXiv2017

Constant-roll Inflation in $F(R)$ Gravity

We propose the study of constant-roll inflation in $F(R)$ gravity. We use two different approaches, one that relates an $F(R)$ gravity to well known scalar models of constant-roll and a second that examines directly the constant-roll condition in $F(R)$ gravity. With regards to the first approach, by using well known techniques, we find the $F(R)$ gravity which realizes a given constant-roll evolution in the scalar-tensor theory. We also perform a conformal transformation in the resulting $F(R)$ gravity and we find the Einstein frame counterpart theory. As we demonstrate, the resulting scalar potential is different in comparison to the original scalar constant-roll case, and the same applies for the corresponding observational indices. Moreover, we discuss how cosmological evolutions that can realize constant-roll to constant-roll eras transitions in the scalar-tensor description, can be realized by vacuum $F(R)$ gravity. With regards to the second approach, we examine directly the effects of the constant-roll condition on the inflationary dynamics of vacuum $F(R)$ gravity. We present in detail the formalism of constant-roll $F(R)$ gravity inflationary dynamics and we discuss how the inflationary indices become in this case. We use two well known $F(R)$ gravities in order to illustrate our findings, the $R^2$ model and a power-law $F(R)$ gravity in vacuum. As we demonstrate, in both cases the parameter space is enlarged in comparison to the slow-roll counterparts of the models, and in effect, the models can also be compatible with the observational data. Finally, we briefly address the graceful exit issue.

gr-qc↗

arXiv2016

Viable Mimetic Completion of Unified Inflation-Dark Energy Evolution in Modified Gravity

In this paper, we demonstrate that a unified description of early and late-time acceleration is possible in the context of mimetic $F(R)$ gravity. We study the inflationary era in detail and demonstrate that it can be realized even in mimetic $F(R)$ gravity where traditional $F(R)$ gravity fails to describe the inflation. By using standard methods we calculated the spectral index of primordial curvature perturbations and the scalar-to-tensor ratio. We use two $F(R)$ gravity models and as it turns out, for both the models under study the observational indices are compatible with both the latest Planck and the BICEP2/Keck array data. Finally, the graceful exit from inflation is guaranteed by the existence of growing curvature perturbations when the slow-roll era ends.

gr-qc↗

arXiv2016

Unimodular-Mimetic Cosmology

We combine the unimodular gravity and mimetic gravity theories into a unified theoretical framework, which is proposed to provide a suggestive proposal for a framework that may assist in the discussion and solution search of the cosmological constant problem and of the dark matter issue. After providing the formulation of the unimodular mimetic gravity and investigating all the new features that the vacuum unimodular gravity implies, by using the underlying reconstruction method, we realize some well known cosmological evolutions, with some of these being exotic for the ordinary Einstein-Hilbert gravity. Specifically we provide the vacuum unimodular mimetic gravity description of the de Sitter cosmology and of the perfect fluid with constant equation of state cosmology. As we demonstrate, these cosmologies can be realized by vacuum mimetic unimodular gravity, without the existence of any matter fluid source. Moreover, we investigate how cosmologically viable cosmologies, which are compatible with the recent observational data, can be realized by the vacuum unimodular mimetic gravity. Since in some cases, the graceful exit from inflation problem might exist, we provide a qualitative description of the mechanism that can potentially generate the graceful exit from inflation in these theories, by searching for the unstable de Sitter solutions in the context of unimodular mimetic theories of gravity.

gr-qc↗

arXiv2016

Newton law in covariant unimodular $F(R)$ gravity

We propose a covariant ghost-free unimodular $F(R)$ gravity theory, which contains a three-form field and study its structure using the analogy of the proposed theory with a quantum system which describes a charged particle in uniform magnetic field. Newton's law in non-covariant unimodular $F(R)$ gravity as well as in unimodular Einstein gravity is derived and it is shown to be just the same as in General Relativity. The derivation of Newton's law in covariant unimodular $F(R)$ gravity shows that it is modified precisely in the same way as in the ordinary $F(R)$ theory. We also demonstrate that the cosmology of a Friedmann-Robertson-Walker background, is equivalent in the non-covariant and covariant formulations of unimodular $F(R)$ theory.

gr-qc↗

arXiv2016

The bounce universe history from unimodular $F(R)$ gravity

In this paper we investigate how to realize various quite well known cosmological bouncing models in the context of the recently developed unimodular $F(R)$ gravity. Particularly, we shall study the matter bounce scenario, the singular bounce, the superbounce and a symmetric bounce scenario. We present the behavior of the Hubble radius for each of the bouncing models we shall take into account and we investigate which era of the bouncing model is responsible for the cosmological perturbations. As we shall demonstrate, the various bouncing models do not behave in the same way, so the cosmological perturbations for each model may correspond to a different era, in comparison to other models. Also we present which unimodular $F(R)$ gravity realizes each model. We also show that Newton's law is not modified in the unimodular $F(R)$ gravity, which also is proven to be a ghost-free theory, and in addition we discuss the matter stability issue. Finally, we demonstrate how it is possible to solve a cosmological constant problem in the context of unimodular $F(R)$ gravity.

gr-qc↗

arXiv2016

Unimodular $F(R)$ Gravity

We extend the formalism of the Einstein-Hilbert unimodular gravity in the context of modified $F(R)$ gravity. After appropriately modifying the Friedmann-Robertson-Walker metric in a way that it becomes compatible to the unimodular condition of having a constant metric determinant, we derive the equations of motion of the unimodular $F(R)$ gravity by using the metric formalism of modified gravity with Lagrange multiplier constraint. The resulting equations are studied in frames of reconstruction method, which enables us to realize various cosmological scenarios, which was impossible to realize in the standard Einstein-Hilbert unimodular gravity. Several unimodular $F(R)$ inflationary scenarios are presented, and in some cases, concordance with Planck and BICEP2 observational data can be achieved.

gr-qc↗

arXiv2015

Singular Accelerated Evolution in Massive $F(R)$ Bigravity

The possibility to have singular accelerated evolution in the context of $F(R)$ bimetric gravity is investigated. Particularly, we study two singular models of cosmological evolution, one of which is a singular modified version of the Starobinsky $R^2$ inflation model. As we demonstrate, for both models in some cases, the slow-roll parameters become singular at the Type IV singularity, a fact that we interpret as a dynamical instability of the theory under study. This dynamically instability may be an indicator of graceful exit from inflation and we thoroughly discuss this scenario and the interpretation of the singular slow-roll parameters. Furthermore, it is demonstrated that for some versions of $F(R)$ bigravity, singular inflation is realized in consistent way so that inflationary indices are compatible with Planck data. Moreover, we study the late-time behavior of the two singular models and we show that the unified description of early and late-time acceleration can be achieved in the context of bimetric $F(R)$ gravity.

gr-qc↗