arXiv subjects

Shin'ichi Nojiri

Publications and source records attributed to Shin'ichi Nojiri.

At least 91 records · Page 5Linked to original sources

arXiv2021

Different faces of generalized holographic dark energy

In the formalism of generalized holographic dark energy (HDE), the holographic cut-off is generalized to depend upon $L_\mathrm{IR} = L_\mathrm{IR} \left( L_\mathrm{p}, \dot L_\mathrm{p}, \ddot L_\mathrm{p}, \cdots, L_\mathrm{f}, \dot L_\mathrm{f}, \cdots, a\right)$ with $L_\mathrm{p}$ and $L_\mathrm{f}$ are the particle horizon and the future horizon, respectively (moreover $a$ is the scale factor of the universe). Based on such formalism, in the present paper, we show that a wide class of dark energy (DE) models can be regarded as different candidates of the generalized HDE family, with respective cut-offs. This can be thought as a symmetry between the generalized HDE and different DE models. In this regard, we consider several entropic dark energy models - like Tsallis entropic DE, the Rényi entropic DE, and the Sharma-Mittal entropic DE - and showed that they are indeed equivalent with the generalized HDE. Such equivalence between the entropic DE and the generalized HDE is extended to the scenario where the respective exponents of the entropy functions are allowed to vary with the expansion of the universe. Besides the entropic DE models, the correspondence with the generalized HDE is also established for the Quintessence and for the Ricci DE models. In all the above cases, the effective equation of state (EoS) parameter corresponds to the holographic energy density are determined, by which the equivalence of various DE models with the respective generalized HDE models are further confirmed. The equivalent holographic cut-offs are determined by two ways: (1) in terms of the particle horizon and its derivatives, (2) in terms of the future horizon horizon and its derivatives.

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arXiv2021

Modelling and testing the equation of state for (Early) dark energy

A general equation of state is considered for analysing the possible behaviors for (Early) dark energy that alleviates the Hubble parameter tension problem. By departing from the possible evolution for the (Early) dark energy density and the corresponding dynamical equations, the equation of state is obtained, which allow us to analyze qualitatively the cosmological evolution and the dominance of each term in the equation of state along the cosmic expansion, which show some interesting consequences as the occurrence of (past) future singularities. Then, by considering two general models, their free parameters are fit with different sources of data, showing the goodness of the fits in comparison to more standard models. Results might be considered as a promising starting point to get a better understanding of the cosmological evolution as a whole.

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arXiv2021

Searching for dynamical black holes in various theories of gravity

We construct models of Einstein and $f(R)$ gravity with two scalar fields, which admit analytical solutions describing time-varying dynamical black holes. Their thermodynamics is investigated in the adiabatic approximation. In addition to the Misner-Sharp-Hernandez quasilocal mass, we provide time-dependent thermodynamical quantities, including the Hawking temperature, Helmholtz free energy, entropy, and thermodynamical energy. The latter does not always coincide with the Misner-Sharp-Hernandez mass at the horizon, although they coincide in the static limit. For Schwarzschild-type (i.e., $g_{tt}g_{rr}=-1$) black holes in Einstein gravity, one of the two scalars is always a ghost with negative kinetic energy. We show that this ghost can be avoided in $f(R)$ gravity.

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arXiv2020

Considerations on gravitational waves in higher-order local and non-local gravity

The detection of gravitational wave modes and polarizations could constitute an extremely important signature to discriminate among different theories of gravity. According to this statement, it is possible to prove that higher-order non-local gravity has formally the same gravitational spectrum of higher-order local gravity. In particular, we consider the cases of $f \left( R, \Box R, \Box^2 R, \cdots, \Box^n R \right) = R + \sum_{i=1}^n α_i R \Box^i R$ gravity, linear with respect to both $R$ and $\Box^i R$ and $ f \left( R, \Box R \right) = R + α\left(\Box R\right)^2 $ gravity, quadratic with respect to $\Box R$, where it is demonstrated the graviton amplitude changes if compared with General Relativity. We also obtain the gravitational spectrum of higher-order non-local gravity $ f \left( R, \Box^{-1} R, \Box^{-2} R, \cdots, \Box^{-n} R \right) = R + \sum_{i=1}^n α_i R \Box^{-i} R$. In this case, we have three state of polarization and $n+3$ oscillation modes. More in detail, it is possible to derive two transverse tensor $(+)$ and $(\times)$ standard polarization modes of frequency $ω_{1}$, massless and with 2-helicity; $n+1$ further scalar modes of frequency $ω_{2},\dots,ω_{n+2}$, massive and with 0-helicity, each of which has the same mixed polarization, partly longitudinal and partly transverse.

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arXiv2020

The dark universe future and singularities: the account of thermal and quantum effects

The knowledge of the universe future is of fundamental importance for any advanced civilization. We study the future of singular dark universe where thermal effects due to the Hawking radiation on the apparent horizon of the FRW universe are taken in consideration. It is shown that dark universe which ends up at finite-time Type I and Type III singularity or infinite-time Little Rip singularity transits to finite-time Type II singularity due to account of thermal effects. On the same time, the Type II and IV singular universe does not change its qualitative behavior. The combined account of quantum and thermal effects shows that depending on specific features of the universe only one of effects is dominant. When (conformal matter) quantum effects are dominant, the future singularity is usually removed while for dominant thermal effects the universe final state is the Type II singularity.

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arXiv2020

Unifying Inflation with Early and Late-time Dark Energy in $F(R)$ Gravity

In this work we shall present models of $F(R)$ gravity which realize in a unified way the inflationary era along with a post-inflationary early dark energy era, with the late-time dark energy era. We shall use two approach methods in order to realize the unified cosmological eras, firstly we specify a Hubble rate which may describe the three distinct acceleration eras, and then by using well known $F(R)$ gravity reconstruction techniques, we shall find the differential equation which may yield the $F(R)$ gravity that realizes the cosmologies. In our second approach, we shall present in a qualitative way, several $F(R)$ gravities which unify the inflationary era with the early and late-time dark energy eras, and we discuss several qualitative issues related to the terms that realize the post-inflationary early dark energy era. We quantify our analysis by numerically solving the Friedmann equation, using the redshift as the main variable, and expressing all the physical quantities as functions of the statefinder $y_\mathrm{H}(z)$, which depends on the redshift and the Hubble rate $H(z)$. For the model studied numerically, we present the behavior of some statefinder quantities, like the deceleration parameter, and we calculate the dark energy density parameter and the dark energy equation of state parameter at present time. After demonstrating that the dark energy era is viable, we investigate when the early dark energy term does not affect the late-time era, and this restricts the free parameters of the model.

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arXiv2020

Novel cosmological and black hole solutions in Einstein and higher-derivative gravity in two dimensions

We consider cosmological and black hole solutions in the Einstein and higher-derivative gravity in two dimensions where the theory is formulated first in $D$ dimensions. In the limit that $D$ tends to $2$ with simultaneous singular rescaling of the scalar curvature coupling constant as $1/(D-2)$, we get the novel Einstein and higher-derivative gravity. Due to non-trivial contribution of scalar curvature which is topological invariant in exactly two dimensions to gravitational equations in the two-dimensional limit one gets novel cosmological and black hole solutions.In particular, the de Sitter and radiation-dominated universe and the Schwarzschild-de Sitter and Schwarzschild-anti-de Sitter black hole solutions are obtained and their properties are discussed.

hep-th↗

arXiv2020

Challenging matter creation models in the phantom divide

We perform a study both statistical and theoretical for cosmological models of matter creation and their ability to describe effective phantom models of dark energy. Such models are beyond the $Λ$CDM model since the resulting cosmic expansion is not adiabatic. In fact, we show that this approach exhibits transient phantom/quintessence scenarios at present time and tends to the standard cosmological model at some stage of the cosmic evolution. We discuss some generalities of the thermodynamics properties for this type of cosmological model; we emphasize on the behavior of the temperature associated to dark matter, which keeps positive along cosmic evolution together with the entropy. The enrichment of this type of model by means of the incorporation of cosmological constant and dissipative effects in the fluid description to explore their cosmological consequences in the expansion of the universe is considered. Finally, a generalization for the matter production rate as an inhomogeneous expression of the Hubble parameter and its derivatives is discussed; as in all the cases examined, such election leads to an effective phantom/quintessence behavior.

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arXiv2020

Propagation of Gravitational Waves in Chern-Simons Axion $F(R)$ Gravity

In this paper we shall study the evolution of cosmological gravitational waves in the context of Chern-Simons axion $F(R)$ gravity. In the case of Chern-Simons axion $F(R)$ gravity there exist spin-0, spin-2 and spin-1 modes. As we demonstrate, from all the gravitational waves modes of the Chern-Simons axion $F(R)$ gravity, only the two tensor modes are affected, while the spin-0 and spin-1 modes are not affected at all. With regard to the two tensor modes, we show that these modes propagate in a non-equivalent way, so the resulting tensor modes are chiral. Notably, with regard to the propagation of the spin-2 graviton modes, the structure of the dispersion relations becomes more complicated in comparison with the Einstein gravity with the Chern-Simons axion, but the resulting qualitative features of the propagating modes are not changed. With regard to the spin-0 and spin-1 modes, the Chern-Simons axion $F(R)$ gravity contains two spin-0 modes and no vector spin-1 mode at all. We also find that for the very high energy mode, both the group velocity and the phase velocity are proportional to the inverse of the square root of the wave number, and therefore the velocities become smaller for larger wave numbers or even vanish in the limit that the wave number goes to infinity.

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arXiv2019

Correspondence of cosmology from non-extensive thermodynamics with fluids of generalized equation of state

We show that there is a correspondence between cosmology from non-extensive thermodynamics and cosmology with fluids of redefined and generalized equation of state. We first establish the correspondence in the case of basic non-extensive thermodynamics, and then we proceed by investigating the more consistent case, from the quantum field theoretical point of view, of varying exponent, namely depending on the scale. The obtained duality provides a way of explaining the complicated phenomenological forms of the effective fluid equation-of-state parameters that are being broadly used in the literature, since their microphysical origin may indeed lie in the non-extensive thermodynamics of spacetime. Finally, concerning the cosmological behavior, we show that at late times the effective fluid may drive the universe acceleration even in the absence of an explicit cosmological constant, and even if the initial fluid is the standard dust matter one. Similarly, at early times we obtain an effective cosmological constant which is enhanced through screening, and hence it can drive a a successful inflation without spoiling the correct late-time acceleration.

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arXiv2019

Ghost-free non-local $F(R)$ Gravity Cosmology

In this work we shall study ghost-free non-local $F(R)$ gravity models. Firstly we shall demonstrate how the ghost degrees of freedom may occur in the non-local $F(R)$ gravity models, and accordingly we shall modify appropriately the gravitational action of non-local $F(R)$ gravity models in order to eliminate the ghosts. Also we shall investigate how the (anti-)de Sitter and the Minkowski spacetime cosmological solutions may arise in the theory, and we investigate when these solutions are stable. Moreover, we shall examine the inflationary phenomenology of the Jordan frame ghost-free non-local $F(R)$ gravity. We shall study two $F(R)$ gravity models, the power law $F(R)$ gravity model $\sim R^n$ with $1<n<2$, $n\neq 2$ and the $R^2$ model, assuming that the slow-roll condition holds true for the Hubble rate during the inflationary era $\dot{H}\ll H^2$ and that the general constant-roll condition $\ddotϕ=3βH\dotϕ$ holds true for the evolution of the scalar field, which includes the slow-roll case for $β=0$. As we shall demonstrate, the power-law non-local $F(R)$ gravity case can produce a viable inflationary era, compatible with observations, in the constant-roll case, unlike for the $R^2$ model. It is conceivable that the results are model dependent, as in the ordinary vacuum $F(R)$ gravity.

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arXiv2019

Non-singular Bounce Cosmology from Lagrange Multiplier $F(R)$ Gravity

In this work we study non-singular{ bounce cosmology} in the context of the Lagrange multiplier generalized $F(R)$ gravity theory of gravity. We specify our study by using a specific variant form of the well known matter{ bounce cosmology}, with scale factor $a(t) = \left(a_0t^2 + 1 \right)^n$, and we demonstrate that for $n < 1/2$, the primordial curvature perturbations are generated deeply in the contraction era. Particularly, we show explicitly that the perturbation modes exit the horizon at a large negative time during the contraction era, which in turn makes the ``low-curvature'' regime, the era for which the calculations of observational indices related to the primordial power spectrum can be considered reliable. Using the reconstruction techniques for the Lagrange multiplier $F(R)$ gravity, we construct the form of effective $F(R)$ gravity that can realize such a cosmological evolution, and we determine the power spectrum of the primordial curvature perturbations. Accordingly, we calculate the spectral index of the primordial curvature perturbations and the tensor-to-scalar ratio, and we confront these with the latest observational data. We also address the issue of stability of the primordial metric perturbations, and to this end, we determine the form of $F(R)$ which realizes the non-singular cosmology for the whole range of cosmic time $-\infty < t < \infty$, by solving the Friedmann equations without the ``low-curvature'' approximation. This study is performed numerically though, due to the high complexity of the resulting differential equations. We also investigate the energy conditions in the present context. The phenomenology of the non-singular bounce is also studied in the context of a standard $F(R)$ gravity.

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arXiv2019

Holographic bounce

We investigate the bounce realization arising from the application of the holographic principle in the early universe, inspired by its well-studied late-time application. We first consider as Infrared cutoffs the particle and future event horizons, and we show that the decrease of the horizons at early times naturally increases holographic energy density at bouncing scales, while we additionally obtain the necessary null energy condition violation. Furthermore, adding a simple correction to the horizons due to the Ultraviolet cutoff we analytically obtain improved nonsingular bouncing solutions, in which the value of the minimum scale factor is controlled by the UV correction. Finally, we construct generalized scenarios, arisen from the use of extended Infrared cutoffs, and as specific examples we consider cutoffs that can reproduce $F(R)$ gravity, and the bounce realization within it.

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arXiv2019

Propagation of Gravitational Waves in Chern-Simons Axion Einstein Gravity

In this paper we shall investigate the propagation of gravitational waves in a flat Friedman-Robertson-Walker background, in the context of a string motivated corrected Einstein gravity. Particularly, we shall consider a misalignment axion Einstein gravity in the presence of a string originating Chern-Simons coupling of the axion field to the Chern-Pontryagin density in four dimensions. We shall focus our study on the propagation of the gravitational waves, and we shall investigate whether there exists any difference in the propagation of the polarization states of the gravitational waves. As we demonstrate, the dispersion relations are different in the Right-handed mode and the Left-handed mode. Finally, we compare the propagation of the axion Chern-Simons Einstein theory with that of standard $F(R)$ gravity.

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arXiv2019

Holographic inflation

We apply the holographic principle at the early universe, obtaining an inflation realization of holographic origin. Such a consideration has equal footing with its well-studied late-time application, and moreover the decrease of the horizons at early times naturally increases holographic energy density at inflationary scales. Taking as Infrared cutoff the particle or future event horizons, and adding a simple correction due to the Ultraviolet cutoff, whose role is non-negligible at the high energy scales of inflation, we result in a holographic inflation scenario that is very efficient in incorporating inflationary requirements and predictions. We first extract analytically the solution of the Hubble function in an implicit form, which gives a scale factor evolution of the desired e-foldings. Furthermore, we analytically calculate the Hubble slow-roll parameters and then the inflation-related observables, such as the scalar spectral index and its running, the tensor-to-scalar ratio, and the tensor spectral index. Confronting the predictions with Planck 2018 observations we show that the agreement is perfect and in particular deep inside the 1$σ$ region.

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arXiv2019

Gravitational Waves in the Presence of Viscosity

We analyze gravitational waves propagating in an isotropic cosmic fluid endowed with a bulk viscosity $ζ$ and a shear viscosity $η$, assuming these coefficients to vary with fluid density $ρ$ as $ρ^λ$, with $λ=1/2$ favored by experimental evidence. We give the general governing equation for the gravitational waves, and focus thereafter on two examples. The first concerns waves in the very late universe, close to the Big Rip, where the fate of the comic fluid is dependent highly on the values of the parameters. Our second example considers the very early universe, the lepton era; the motivation for this choice being that the microscopical bulk viscosity as calculated from statistical mechanics is then at maximum. We find that the gravitational waves on such an underlying medium are damped, having a decay constant equal to the inverse of the conformal Hubble parameter. Our results turn out to be in good agreement with other viscosity-based approaches.

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arXiv2019

Topological Gravity motivated by Renormalization Group

Recently we have proposed models of topological field theory including gravity in Mod. Phys. Lett. A 31 (2016) no.37, 1650213 and Phys. Rev. D 96 (2017) no.2, 024009, in order to solve the problem of the cosmological constant. The Lagrangian densities of the models are BRS (Becchi-Rouet-Stora) exact and therefore the models can be regarded as topological theories. In the models, the coupling constants, including the cosmological constant, look as if they run with the scale of the universe and its behavior is very similar to the renormalization group. Motivated by these models, we propose new models with an the infrared fixed point, which may correspond to the late time universe, and an ultraviolet fixed point, which may correspond to the early universe. Especially we construct a model with the solutions corresponding to the de Sitter space-time both in the ultraviolet and the infrared fixed points.

hep-th↗

arXiv2019

$F(R)$ Gravity with an Axion-like Particle: Dynamics, Gravity Waves, Late and Early-time Phenomenology

In this work we investigate several theoretical and phenomenological implications of a scalar -$F(R)$ gravity containing a non-minimal coupling to the scalar curvature. This kind of model is a generalization of axion-$F(R)$ gravity models, so we shall examine several implications of the latter theory. Firstly we study in detail the Einstein frame picture of the model, and also we discuss the dynamics of the cosmological system. By appropriately using the equations of motion, we demonstrate that an arbitrary cosmological evolution can be realized. Also we study the gravitational waves of the theory, and we demonstrate that the speed of their propagation is the same as in $F(R)$ gravity, but there is the possibility of enhancement or dissipation of the gravitational waves, an effect quite similar to the propagation of gravity waves in a viscous fluid. Finally, we examine the energy momentum tensor and we investigate which quantities related to it are conserved. We also present the constraints imposed by the radiation domination era on the non-minimal coupling of the axion scalar field to the scalar curvature.

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