390 results on '"Shin'ichi Nojiri"'
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2. Different Aspects of Entropic Cosmology
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Shin’ichi Nojiri, Sergei D. Odintsov, and Tanmoy Paul
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entropic cosmology ,inflation ,reheating ,primordial gravitational waves ,bounce ,Elementary particle physics ,QC793-793.5 - Abstract
We provide a short review of the recent developments in entropic cosmology based on two thermodynamic laws of the apparent horizon, namely the first and the second laws of thermodynamics. The first law essentially provides the change in entropy of the apparent horizon during the cosmic evolution of the universe; in particular, it is expressed by TdS=−d(ρV)+WdV (where W is the work density and other quantities have their usual meanings). In this way, the first law actually links various theories of gravity with the entropy of the apparent horizon. This leads to a natural question—“What is the form of the horizon entropy corresponding to a general modified theory of gravity?”. The second law of horizon thermodynamics states that the change in total entropy (the sum of horizon entropy + matter fields’ entropy) with respect to cosmic time must be positive, where the matter fields behave like an open system characterised by a non-zero chemical potential. The second law of horizon thermodynamics importantly provides model-independent constraints on entropic parameters. Finally, we discuss the standpoint of entropic cosmology on inflation (or bounce), reheating and primordial gravitational waves from the perspective of a generalised entropy function. more...
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- 2024
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3. Gravitational waves in f(Q) non-metric gravity via geodesic deviation
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Salvatore Capozziello, Maurizio Capriolo, and Shin'ichi Nojiri
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Physics ,QC1-999 - Abstract
We investigate gravitational waves in the f(Q) gravity, i.e., a geometric theory of gravity described by a non-metric compatible connection, free from torsion and curvature, known as symmetric-teleparallel gravity. We show that f(Q) gravity exhibits only two massless and tensor modes. Their polarizations are transverse with helicity equal to two, exactly reproducing the plus and cross tensor modes typical of General Relativity. In order to analyze these gravitational waves, we first obtain the deviation equation of two trajectories followed by nearby freely falling point-like particles and we find it to coincide with the geodesic deviation of General Relativity. This is because the energy-momentum tensor of matter and field equations are Levi-Civita covariantly conserved and, therefore, free structure-less particles follow, also in f(Q) gravity, the General Relativity geodesics. Equivalently, it is possible to show that the curves are solutions of a force equation, where an extra force term of geometric origin, due to non-metricity, modifies the autoparallel curves with respect to the non-metric connection. In summary, gravitational waves produced in non-metricity-based f(Q) gravity behave as those in torsion-based f(T) gravity and it is not possible to distinguish them from those of General Relativity only by wave polarization measurements. This shows that the situation is different with respect to the curvature-based f(R) gravity where an additional scalar mode is always present for f(R)≠R. more...
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- 2024
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4. Propagation speed of gravitational wave in scalar–Einstein–Gauss-Bonnet gravity
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Shin'ichi Nojiri and Sergei D. Odintsov
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Nuclear and particle physics. Atomic energy. Radioactivity ,QC770-798 - Abstract
The propagation speed of the gravitational wave in scalar–Einstein–Gauss-Bonnet (sEGB) gravity is generally different from that of light. Using differential equation conditions for the speed of gravitational waves to coincide with the light speed in the expanding universe, we constructed a general class of sEGB gravities where this condition is satisfied and realistic inflation occurs. It is demonstrated that the condition that the speed of gravitational wave coincides with that of the light in the Friedmann-Lemaître-Robertson-Walker (FRLW) universe is always different from the condition for gravitational wave speed in the sEGB black hole background. Moreover, it is shown that when gravitational wave speed in sEGB black hole is equal to the speed of light the black hole spacetime geometry is changing too so that formally there is no solution for such sEGB black hole. This may indicate that sEGB black holes hardly can be considered as realistic black holes unless some reasonable scenario to make gravitational wave speed to be equal to that of light is proposed, at least asymptotically. more...
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- 2024
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5. Microscopic interpretation of generalized entropy
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Shin'ichi Nojiri, Sergei D. Odintsov, and Tanmoy Paul
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Physics ,QC1-999 - Abstract
Generalized entropy, that has been recently proposed, puts all the known and apparently different entropies like The Tsallis, the Rényi, the Barrow, the Kaniadakis, the Sharma-Mittal and the loop quantum gravity entropy within a single umbrella. However, the microscopic origin of such generalized entropy as well as its relation to thermodynamic system(s) is not clear. In the present work, we will provide a microscopic thermodynamic explanation of generalized entropy(ies) from canonical and grand-canonical ensembles. It turns out that in both the canonical and grand-canonical descriptions, the generalized entropies can be interpreted as the statistical ensemble average of a series of microscopic quantity(ies) given by various powers of (−klnρ)n (with n being a positive integer and ρ symbolizes the phase space density of the respective ensemble), along with a term representing the fluctuation of Hamiltonian and number of particles of the system under consideration (in case of canonical ensemble, the fluctuation on the particle number vanishes). more...
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- 2023
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6. Micro-canonical and canonical description for generalised entropy
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Shin'ichi Nojiri and Sergei D. Odintsov
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Physics ,QC1-999 - Abstract
Few parameters dependent generalised entropy includes Tsallis entropy, Rényi entropy, Sharma-Mittal entropy, Barrow entropy, Kaniadakis entropy, etc as particular representatives. Its relation to physical systems is not always clear. In this paper, we propose the microscopic thermodynamic description for an arbitrary generalised entropy in terms of the particle system. It is shown that the change in the volume of the phase space of the particle system in the micro-canonical description or the difference in the integration measure in the phase space in the canonical description may lead to generalised entropy. Our consideration may help us understand the structure of quantum gravity. more...
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- 2023
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7. Isotropic compact stars in four-dimensional Einstein–Gauss–Bonnet gravity coupled with scalar field: reconstruction of model
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G. G. L. Nashed and Shin’ichi Nojiri
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Astrophysics ,QB460-466 ,Nuclear and particle physics. Atomic energy. Radioactivity ,QC770-798 - Abstract
Abstract It has been suggested that the Einstein–Gauss–Bonnet theory coupled with a scalar field (EGBS) may allow us to obtain physically viable models of celestial phenomena such that the scalar field effect is active in standard four dimensions. We consider the spherically symmetric and static configuration of the compact star and explain the consequences of the EGBS theory in the frame of stellar modeling. In our formulation, for any given static profile of energy density $$\rho $$ ρ with spherical symmetry and the arbitrary equation of state (EoS) of matter, we can construct a model which reproduces the profile. Because the profile of the energy density determines the mass M and the radius $$R_s$$ R s of the compact star, an arbitrary relation between the mass M and the radius $$R_s$$ R s of the compact star can be realized by adjusting the potential and the coefficient function of the Gauss–Bonnet term in the action of EGBS theory. This could be regarded as a degeneracy between the EoS and the functions characterizing the model, which indicates that the mass–radius relation alone is insufficient to constrain the model. Here, we investigate a novel class of analytic spherically symmetric interior solutions by the polytropic EoS. We discuss our model in detail and show that it is in agreement with the necessary physical conditions required for any realistic compact star, confirming that EGBS theory is consistent with observations. more...
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- 2023
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8. Holographic realization of constant roll inflation and dark energy: An unified scenario
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Shin'ichi Nojiri, Sergei D. Odintsov, and Tanmoy Paul
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Physics ,QC1-999 - Abstract
In the formalism of generalized holographic dark energy, the infrared cut-off LIR is generalized to the form, LIR=LIR(Lp,L˙p,L¨p,⋯,Lf,L˙f,L¨f,⋯,a,H,H˙,H¨,⋯), where Lp and Lf are the particle horizon and the future horizon, respectively (moreover, a is the scale factor and H is the Hubble parameter of the universe). Based on such formalism, we establish a holographic realization of constant roll inflation during the early universe, where the corresponding cut-off depends on the Hubble parameter and its derivatives (up to the second order). The viability of this holographic constant roll inflation with respect to the Planck data in turn puts a certain bound on the infrared cut-off at the time of horizon crossing. Such holographic correspondence of constant roll inflation is extended to the scenario where the infrared cut-off is corrected by the ultraviolet one, which may originate due to quantum effects. Besides the mere inflation, we further propose the holographic realization of an unified cosmic scenario from constant roll inflation (at the early time) to the dark energy era (at the late time) with an intermediate radiation dominated era followed by a Kamionkowski like reheating stage. In such a unified holographic scenario, the inflationary quantities (like the scalar spectral index and the tensor-to-scalar ratio) and the dark energy quantities (like the dark energy EoS parameter and the present Hubble rate) prove to be simultaneously compatible with observable constraints for suitable ranges of the infrared cut-off and the other model parameters. Moreover the curvature perturbations at super-Hubble scale prove to be a constant (with time) during the entire cosmic era, which in turn ensures the stability of the model under consideration. more...
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- 2023
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9. Modified cosmology from the thermodynamics of apparent horizon
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Shin'ichi Nojiri, Sergei D. Odintsov, and Tanmoy Paul
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Physics ,QC1-999 - Abstract
In the realm of the Bekenstein-Hawking entropy, the thermodynamics of apparent horizon bridges with the usual FLRW (Friedmann-Lemaître-Robertson-Walker) equation only for a special case where the matter field is given by a perfect fluid having equation of state (EoS) parameter =−1, i.e. p=−ρ with ρ and p representing the energy density and the pressure of the fluid, respectively. To include the case p≠−ρ, we consider the modification of the Bekenstein-Hawking entropy in the present work. In particular, we develop an entropy function that leads to the usual FLRW equations, for a general EoS of the matter fluid given by p=wρ, directly from the thermodynamics of the apparent horizon. The newly developed entropy acquires a correction over the Bekenstein-Hawking entropy and differs from the known entropies like the Tsallis, Rényi, Barrow, Sharma-Mittal, Kaniadakis, and Loop Quantum Gravity entropies proposed so far. Based on this finding, we examine how the Friedmann equations of the apparent horizon cosmology are accordingly modified if one starts with a general entropy depending on the Bekenstein-Hawking entropy. This results in some interesting cosmological consequences during the early and late stages of the universe. more...
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- 2022
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10. Early and late universe holographic cosmology from a new generalized entropy
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Shin'ichi Nojiri, Sergei D. Odintsov, and Tanmoy Paul
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Physics ,QC1-999 - Abstract
We propose a new four-parameter entropy function that generalizes the Tsallis, Rényi, Barrow, Sharma-Mittal, Kaniadakis and Loop Quantum Gravity entropies for suitable limits of the parameters. Consequently, we address the early and late universe cosmology corresponding to the proposed four-parameter entropy function. As a result, it turns out that the entropic cosmology from the generalized entropy function can unify the early inflation to the late dark energy era of the universe. In such a unified scenario, we find that – (1) the inflation era is described by a quasi de-Sitter evolution of the Hubble parameter, which has an exit at around 58 e-folding number, (2) the inflationary observable quantities like the spectral index for primordial scalar perturbation and the tensor-to-scalar ratio are simultaneously compatible with the recent Planck data, and (3) regarding the late time cosmology, the dark energy EoS parameter is found to be consistent with the Planck result for the same values of the entropy parameters that lead to the viable inflation during the early universe. Furthermore, we show that the entropic cosmology from the proposed entropy function is equivalent to holographic cosmology, where the respective holographic cut-offs are determined in terms of either particle horizon and its derivative or future horizon and its derivative. more...
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- 2022
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11. Consistency between black hole and mimetic gravity – Case of (2 + 1)-dimensional gravity
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Shin'ichi Nojiri and G.G.L. Nashed
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Physics ,QC1-999 - Abstract
We show that the mimetic theory with the constraint gρσ∂ρϕ∂σϕ=1 cannot realize the black hole geometry with the horizon(s). To overcome such issue, we may change the mimetic constraint a little bit by ω(ϕ)gρσ∂ρϕ∂σϕ=−1, where ω(ϕ) is a function of the scalar field ϕ. As an example, we consider (2+1)-dimensional mimetic gravity with the mimetic potential and construct black hole (BH) solutions by using this modified constraint. We study three different classes: In the first class, we assume the Lagrange multiplier and mimetic potential are vanishing and obtain a BH solution that fully matches the BH of GR despite the non-triviality of the mimetic field which ensures the study presented in Nashed et al. (2019) [42]. In the second class, we obtain a BH having constant mimetic potential and a non-trivial form of the Lagrange multiplier. In the third class, we obtain a new BH solution with non-vanishing values of the mimetic field, the Lagrange multiplier, and the mimetic potential. In any case, the solutions correspond to the space-time with only one horizon but we show that the formalism for the constraint works. more...
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- 2022
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12. Compact star in general F(R) gravity: Inevitable degeneracy problem and non-integer power correction
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Kota Numajiri, Taishi Katsuragawa, and Shin'ichi Nojiri
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Physics ,QC1-999 - Abstract
We investigate a compact star in the general F(R) gravity. Developing a novel formulation in the spherically symmetric and static space-time with the matter, we confirm that an arbitrary relation between the mass M and the radius Rs of the compact star can be realized by adjusting the functional form of F(R). Such a degeneracy with a choice of the equation of state (EOS) suggests that only mass-radius relation is insufficient to constrain the F(R) gravity. Furthermore, by solving the differential equation for dF(R)dR|R=R(r) near and inside the surface of the compact star with the polytropic EOS, the boundary condition demands a weak curvature correction to the Einstein gravity could be non-integer power of the scalar curvature, which gives a stringent constraint on the functional form of F(R). This consequence follows that the equation of motion in F(R) gravity includes the second-order derivative of Ricci scalar R, and thus, it is applicable to general F(R) models. more...
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- 2022
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13. Barrow entropic dark energy: A member of generalized holographic dark energy family
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Shin'ichi Nojiri, Sergei D. Odintsov, and Tanmoy Paul
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Physics ,QC1-999 - Abstract
The holographic cut-off, in the formalism of generalized holographic dark energy (HDE), is generalized to depend on LIR=LIR(Lp,L˙p,L¨p,⋯,Lf,L˙f,⋯,a), where Lp and Lf are the particle horizon and future horizon respectively, and a is the scale factor of the universe. Based on such formalism, we showed that the Barrow entropic dark energy (DE) model is equivalent to the generalized HDE where the respective holographic cut-off is determined by two ways – (1) in terms of particle horizon and its derivative and (2) in terms of future horizon and its derivative. Interestingly, such cut-off turns out to depend up-to first order derivative of Lp or Lf respectively. Such equivalence between the Barrow entropic dark energy and the generalized HDE is extended to the scenario where the exponent of the Barrow entropy allows to vary with the cosmological expansion of the universe. In both the cases (whether the Barrow exponent is a constant or varies with the cosmological evolution), we determine effective equation of state (EoS) parameter from the generalized holographic point of view, which, by comparing with the Barrow DE EoS parameter, further ensures the equivalence between the Barrow entropic dark energy and the generalized HDE. more...
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- 2022
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14. Statistical system based on p-adic numbers
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Mikoto Terasawa and Shin'ichi Nojiri
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Physics ,QC1-999 - Abstract
We propose statistical systems based on p-adic numbers. In the systems, the Hamiltonian is a standard real number which is given by a map from the p-adic numbers. Therefore we can introduce the temperature as a real number and calculate the thermodynamical quantities like free energy, thermodynamical energy, entropy, specific heat, etc. Although we consider a very simple system, which corresponds to a free particle moving in one dimensional space, we find that there appear the behaviors like phase transition in the system. Usually in order that a phase transition occurs, we need a system with an infinite number of degrees of freedom but in the system where the dynamical variable is given by p-adic number, even if degree of freedom is unity, there might occur the phase transition. more...
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- 2021
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15. Viable inflationary models in a ghost-free Gauss–Bonnet theory of gravity
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Shin’ichi Nojiri, S. D. Odintsov, V. K. Oikonomou, N. Chatzarakis, and Tanmoy Paul
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Astrophysics ,QB460-466 ,Nuclear and particle physics. Atomic energy. Radioactivity ,QC770-798 - Abstract
Abstract In this work we investigate the inflationary phenomenological implications of a recently developed ghost-free Gauss–Bonnet theory of gravity. The resulting theory can be viewed as a scalar Einstein–Gauss–Bonnet theory of gravity, so by employing the formalism for cosmological perturbations for the latter theory, we calculate the slow-roll indices and the observational indices, and we compare these with the latest observational data. Due to the presence of a freely chosen function in the model, in principle any cosmological evolution can be realized, so we specify the Hubble rate and the freely chosen function and we examine the phenomenology of the model. Specifically we focus on de Sitter, quasi-de Sitter and a cosmological evolution in which the Hubble rate evolves exponentially, with the last two being more realistic choices for describing inflation. As we demonstrate, the ghost-free model can produce inflationary phenomenology compatible with the observational data. We also briefly address the stability of first order scalar and tensor cosmological perturbations, for the exponential Hubble rate, and as we demonstrate, stability is achieved for the same range of values of the free parameters that guarantee the phenomenological viability of the models. more...
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- 2019
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16. Modified cosmology from extended entropy with varying exponent
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Shin’ichi Nojiri, Sergei D. Odintsov, and Emmanuel N. Saridakis
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Astrophysics ,QB460-466 ,Nuclear and particle physics. Atomic energy. Radioactivity ,QC770-798 - Abstract
Abstract We present a modified cosmological scenario that arises from the application of non-extensive thermodynamics with varying exponent. We extract the modified Friedmann equations, which contain new terms quantified by the non-extensive exponent, possessing standard $$\Lambda $$ Λ CDM cosmology as a subcase. Concerning the universe evolution at late times we obtain an effective dark energy sector, and we show that we can acquire the usual thermal history, with the successive sequence of matter and dark-energy epochs, with the effective dark-energy equation-of-state parameter being in the quintessence or in the phantom regime. The interesting feature of the scenario is that the above behaviors can be obtained even if the explicit cosmological constant is set to zero, namely they arise purely from the extra terms. Additionally, we confront the model with Supernovae type Ia and Hubble parameter observational data, and we show that the agreement is very good. Concerning the early-time universe we obtain inflationary de Sitter solutions, which are driven by an effective cosmological constant that includes the new terms of non-extensive thermodynamics. This effective screening can provide a description of both inflation and late-time acceleration with the same parameter choices, which is a significant advantage. more...
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- 2019
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17. Correspondence of cosmology from non-extensive thermodynamics with fluids of generalized equation of state
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Shin'ichi Nojiri, Sergei D. Odintsov, Emmanuel N. Saridakis, and R. Myrzakulov
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Nuclear and particle physics. Atomic energy. Radioactivity ,QC770-798 - Abstract
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 successful inflation without spoiling the correct late-time acceleration. more...
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- 2020
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18. Cosmological bound from the neutron star merger GW170817 in scalar–tensor and F(R) gravity theories
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Shin'ichi Nojiri and Sergei D. Odintsov
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Physics ,QC1-999 - Abstract
We consider the evolution of cosmological gravitational waves in scalar–tensor theory and F(R) gravity theory as typical models of the modified gravity. Although the propagation speed is not changed from the speed of light, the propagation phase changes when we compare the propagation in these modified gravity theories with the propagation in the ΛCDM model. The phase change might be detected in future observations. Keywords: Gravitational waves, Alternative theories of gravity, Cosmology more...
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- 2018
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19. Covariant generalized holographic dark energy and accelerating universe
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Shin’ichi Nojiri and S. D. Odintsov
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Astrophysics ,QB460-466 ,Nuclear and particle physics. Atomic energy. Radioactivity ,QC770-798 - Abstract
Abstract We propose the generalized holographic dark energy model where the infrared cutoff is identified with the combination of the FRW universe parameters: the Hubble rate, particle and future horizons, cosmological constant, the universe lifetime (if finite) and their derivatives. It is demonstrated that with the corresponding choice of the cutoff one can map such holographic dark energy to modified gravity or gravity with a general fluid. Explicitly, F(R) gravity and the general perfect fluid are worked out in detail and the corresponding infrared cutoff is found. Using this correspondence, we get realistic inflation or viable dark energy or a unified inflationary-dark energy universe in terms of covariant holographic dark energy. more...
- Published
- 2017
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20. Localization of vector field on dynamical domain wall
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Masafumi Higuchi and Shin'ichi Nojiri
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Physics ,QC1-999 - Abstract
In the previous works (arXiv:1202.5375 and arXiv:1402.1346), the dynamical domain wall, where the four dimensional FRW universe is embedded in the five dimensional space–time, has been realized by using two scalar fields. In this paper, we consider the localization of vector field in three formulations. The first formulation was investigated in the previous paper (arXiv:1510.01099) for the U(1) gauge field. In the second formulation, we investigate the Dvali–Shifman mechanism (arXiv:hep-th/9612128), where the non-abelian gauge field is confined in the bulk but the gauge symmetry is spontaneously broken on the domain wall. In the third formulation, we investigate the Kaluza–Klein modes coming from the five dimensional graviton. In the Randall–Sundrum model, the graviton was localized on the brane. We show that the (5,μ) components (μ=0,1,2,3) of the graviton are also localized on the domain wall and can be regarded as the vector field on the domain wall. There are, however, some corrections coming from the bulk extra dimension if the domain wall universe is expanding. more...
- Published
- 2017
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21. Holographic bounce
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Shin'ichi Nojiri, Sergei D. Odintsov, and Emmanuel N. Saridakis
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Nuclear and particle physics. Atomic energy. Radioactivity ,QC770-798 - Abstract
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. more...
- Published
- 2019
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22. Holographic inflation
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Shin'ichi Nojiri, Sergei D. Odintsov, and Emmanuel N. Saridakis
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Physics ,QC1-999 - Abstract
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 1σ region. more...
- Published
- 2019
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23. Different Faces of Generalized Holographic Dark Energy
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Shin’ichi Nojiri, Sergei D. Odintsov, and Tanmoy Paul
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holographic dark energy ,equivalence with generalized HDE ,Tsallis dark energy ,Rényi dark energy ,Sharma-Mittal dark energy ,quintessence dark energy ,Mathematics ,QA1-939 - Abstract
In the formalism of generalized holographic dark energy (HDE), the holographic cut-off is generalized to depend upon LIR=LIRLp,L˙p,L¨p,⋯,Lf,L˙f,⋯,a with Lp and Lf being 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 for 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 considered several entropic dark energy models—such as the Tsallis entropic DE, the Rényi entropic DE, and the Sharma–Mittal entropic DE—and found 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 was also established for the quintessence and for the Ricci DE model. In all the above cases, the effective equation of state (EoS) parameter corresponding to the holographic energy density was determined, by which the equivalence of various DE models with the respective generalized HDE models was further confirmed. The equivalent holographic cut-offs were 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. more...
- Published
- 2021
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24. Palatini–Born–Infeld gravity, bouncing universe, and black hole formation
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Meguru Komada, Shin'ichi Nojiri, and Taishi Katsuragawa
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Physics ,QC1-999 - Abstract
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 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 more...
- Published
- 2016
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25. Dynamical domain wall and localization
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Yuta Toyozato, Masafumi Higuchi, and Shin'ichi Nojiri
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Physics ,QC1-999 - Abstract
Based on the previous works (Toyozato et al., 2013 [24]; Higuchi and Nojiri, 2014 [25]), 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 George et al. (2009) [23]. more...
- Published
- 2016
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26. Reconstruction of scalar field theories realizing inflation consistent with the Planck and BICEP2 results
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Kazuharu Bamba, Shin'ichi Nojiri, and Sergei D. Odintsov
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Physics ,QC1-999 - Abstract
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. more...
- Published
- 2014
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27. Stability of Accelerating Cosmology in Two Scalar-Tensor Theory: Little Rip versus de Sitter
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Yusaku Ito, Shin’ichi Nojiri, and Sergei D. Odintsov
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dark energy ,scalar tensor theory ,little rip ,Science ,Astrophysics ,QB460-466 ,Physics ,QC1-999 - Abstract
We develop the general reconstruction scheme in two scalar model. The quintom-like theory which may describe (different) non-singular Little Rip or de Sitter cosmology is reconstructed. The number of scalar phantom dark energy models (with Little Rip cosmology or asymptotically de Sitter evolution) is presented. Stability issue of such dark energy cosmologies as well as the flow to fixed points are studied. The stability of Little Rip universe which leads to dissolution of bound objects sometime in future indicates that no classical transition to de Sitter space occurs. The possibility of unification of inflation with Little Rip dark energy in two scalar theory is briefly mentioned. more...
- Published
- 2012
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28. Topological Gravity Motivated by Renormalization Group
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Taisaku Mori and Shin’ichi Nojiri
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n/a ,Mathematics ,QA1-939 - Abstract
Recently, we have proposed models of topological field theory including gravity in Mod. Phys. Lett. A 2016, 31, 1650213 and Phys. Rev. D 2017, 96, 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. In particular, we construct a model with the solutions corresponding to the de Sitter space-time both in the ultraviolet and the infrared fixed points. more...
- Published
- 2018
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29. Cosmological Constant and Renormalization of Gravity
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Shin’ichi Nojiri
- Subjects
cosmological constant ,quantum gravity ,renormalization ,Astronomy ,QB1-991 - Abstract
In arXiv:1601.02203 and arXiv:1702.07063, we have proposed a topological model with a simple Lagrangian density and have tried to solve one of the cosmological constant problems. The Lagrangian density is the BRS exact and therefore the model can be regarded as a topological theory. In this model, the divergence of the vacuum energy coming from the quantum corrections from matters can be absorbed into the redefinition of the scalar field. In this paper, we consider the extension of the model in order to apply the mechanism to other kinds of divergences coming from the quantum correction and consider the cosmology in an extended model. more...
- Published
- 2018
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30. Different Faces of Generalized Holographic Dark Energy.
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Shin'ichi Nojiri, Sergei D. Odintsov 0001, and Tanmoy Paul
- Published
- 2021
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31. Revisiting compact star in F(R) gravity: Roles of chameleon potential and energy conditions
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Kota Numajiri, Yong-Xiang Cui, Taishi Katsuragawa, and Shin’ichi Nojiri
- Published
- 2023
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32. Slow-rotating charged black hole solution in dynamical Chern-Simons modified gravity
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G. G. L. Nashed and Shin’ichi Nojiri
- Subjects
High Energy Physics - Theory ,High Energy Physics - Theory (hep-th) ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology - Abstract
The Chern-Simons (CS) gravity is a modified theory of Einstein's general relativity (GR). The CS theory arises from the low energy limit of string theory which involves anomaly correction to the Einstein-Hilbert action. The CS term is given by the product of the Pontryagin density with a scalar field. In this study, we derive a charged slowly rotating black hole (BH) solution. The main incentives of this BH solution are axisymmetric and stationary and form distortion of the Kerr-Newman BH solution with a dipole scalar field. Additionally, we investigate the asymptotic correction of the metric with the inverse seventh power of the radial distance to the BH solution, This indicates that it will escape any meaningful constraints from weak-field experiments. To find this kind of BHs by observations, we investigate the propagation of the photon near the BH and we show that the difference between the left-rotated polarization and the right-handed one could be observed as stronger than the case of the Kerr-Newman BH. Finally, we derived the stability condition using the geodesic deviations., Comment: 20 pages 3 figures, will appear in Phys. Rev. D more...
- Published
- 2023
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33. Generalized black hole entropy in two dimensions
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Shin’ichi Nojiri, Sergei D. Odintsov, and Valerio Faraoni
- Subjects
High Energy Physics - Theory ,High Energy Physics - Theory (hep-th) ,Physics and Astronomy (miscellaneous) ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology - Abstract
The Bekenstein-Hawking entropy of a black hole is proportional to its horizon area, hence in $D=2$ spacetime dimensions it is constant because the horizon degenerates into two points. This fact is consistent with Einstein's gravity becoming topological in two dimensions. In $F(R)$ gravity, which is non-trivial even in $D=2$, we find that the entropy is constant, as for Bekenstein-Hawking. As shown in EPL 139 (2022) no.6, 69001 (arXiv:2208.10146), two-dimensional $F(R)$ gravity is equivalent to Jackiw-Teitelboim gravity, in turn equivalent to the Sachdev-Ye-Kitaev model where the entropy becomes constant in the large $N$ limit. Several recently proposed entropies are functions of the Bekenstein-Hawking entropy and become constant in $D=2$, but in two-dimensional dilaton gravity entropies are not always constant. We study general dilaton gravity and obtain arbitrary static black hole solutions for which the non-constant entropies depend on the mass, horizon radius, or Hawking temperature, and constitute new proposals for a generalized entropy., Comment: 12 pages, latex, to appear in Int. J. Geom. Meth. Mod. Phys more...
- Published
- 2023
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34. Unifying inflation with early and late dark energy in Einstein–Gauss–Bonnet gravity
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Shin’ichi Nojiri, Sergei D. Odintsov, and Diego Sáez-Chillón Gómez
- Subjects
Space and Planetary Science ,Astronomy and Astrophysics - Published
- 2023
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35. Multi-horizons black hole solutions, photon sphere and perihelion shift in weak ghost-free Gauss-Bonnet theory of gravity
- Author
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G. G. L. Nashed and Shin’ichi Nojiri
- Subjects
High Energy Physics - Theory ,Cosmology and Nongalactic Astrophysics (astro-ph.CO) ,High Energy Physics - Theory (hep-th) ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology ,Astrophysics - Cosmology and Nongalactic Astrophysics - Abstract
Among the modified gravitational theories, the ghost-free Gauss-Bonnet (GFGB) theory of gravity has been considered from the viewpoint of cosmology. The best way to check its applicability could be to elicit observable predicts which give guidelines or limitations on the theory, which could be contrasted with the actual observations. In the present study, we derive consistent field equations for GFGB and by applying the equations to a spherically symmetric space-time, we obtain new spherically symmetric black hole (BH) solutions. We study the physical properties of these BH solutions and show that the obtained space-time possesses multi-horizons and the Gauss-Bonnet invariants in the space-time are not trivial. We also investigate the thermodynamical quantities related to these BH solutions and we show that these quantities are consistent with what is known in the previous works. Finally, we study the geodesic equations of these solutions which give the photon spheres and we find the perihelion shift for weak GFGB. In addition, we calculate the first-order GFGB perturbations in the Schwarzschild solution and new BH solutions and show that we improve and extend existing results in the past literature on the spherically symmetric solutions., LaTeX 18 pages, 4 figures, to appear in Phys. Rev. D more...
- Published
- 2022
36. Alternative entropies and consistent black hole thermodynamics
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Shin’ichi Nojiri, Sergei D. Odintsov, Valerio Faraoni, Japan Society for the Promotion of Science, Agencia Estatal de Investigación (España), Ministerio de Economía y Competitividad (España), Ministerio de Ciencia, Innovación y Universidades (España), and Natural Sciences and Engineering Research Council of Canada more...
- Subjects
High Energy Physics::Theory ,General Relativity and Quantum Cosmology ,Physics and Astronomy (miscellaneous) ,Astrophysics::High Energy Astrophysical Phenomena ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) - Abstract
While the Bekenstein–Hawking entropy is the unique notion of entropy that makes classical black hole thermodynamics consistent, alternative entropy notions (Rényi, Tsallis, and generalized constructs) abound in the literature. We explore conditions under which they are part of a consistent horizon thermodynamics for certain classes of modified gravity black holes. We provide examples in which black hole masses and temperatures going hand-in-hand with these alternative entropies coincide with their usual counterparts associated with the Bekenstein–Hawking entropy., This work is partially supported by JSPS Grant-in-Aid for Scientific Research (C) No. 18K03615 (S. N.), by MINECO (Spain) project PID2019-104397GB-I00 (S. D. O), and by the Natural Sciences and Engineering Research Council of Canada grant 2016-03803 to V. F. more...
- Published
- 2022
37. Compact star in general ( ) gravity: Inevitable degeneracy problem and non-integer power correction
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Kota Numajiri, Shin'ichi Nojiri, and Taishi Katsuragawa
- Subjects
High Energy Physics - Theory ,Nuclear and High Energy Physics ,High Energy Physics - Theory (hep-th) ,Physics ,QC1-999 ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology - Abstract
We investigate a compact star in the general $F(R)$ gravity. Developing a novel formulation in the spherically symmetric and static space-time with the matter, we confirm that an arbitrary relation between the mass $M$ and the radius $R_s$ of the compact star can be realized by adjusting the functional form of $F(R)$. Such a degeneracy with a choice of the equation of state (EOS) suggests that only mass-radius relation is insufficient to constrain the $F(R)$ gravity. Furthermore, by solving the differential equation for $\left. \frac{dF(R)}{dR}\right|_{R=R(r)}$ near and inside the surface of the compact star with the polytropic EOS, the boundary condition demands a weak curvature correction to the Einstein gravity could be non-integer power of the scalar curvature, which gives a stringent constraint on the functional form of $F\left(R\right)$. This consequence follows that the equation of motion in $F(R)$ gravity includes the fourth-order derivative of metric, and thus, it is applicable to general $F(R)$ models., 14 pages. Typos are corrected more...
- Published
- 2022
38. Specific neutral and charged black holes in f(R) gravitational theory
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Shin'ichi Nojiri and Gamal Nashed
- Published
- 2021
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39. Dressed asymptotic states and QED infrared physics
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Shin'ichi Nojiri and Hideo Furugori
- Subjects
High Energy Physics - Theory ,High Energy Physics - Theory (hep-th) ,FOS: Physical sciences - Abstract
The dressed state formalisms, which incorporate interactions of soft particles into an asymptotic state, are known as the prescriptions expected to solve the problem of infrared (IR) divergence in the quantum field theory (QFT). A particularly famous example is the dressed state formalism proposed by Kulish and Faddeev in quantum electrodynamics (QED). As pointed out by Hirai and Sugishita, however, this formalism has problems in gauge invariance and the IR divergence. These problems are mainly caused by the existence of ghosts or unphysical photon modes. Therefore, we start by studying the asymptotic states in the Coulomb gauge, which excludes ghosts and/or unphysical photon modes. In this paper, we propose a formalism to construct the asymptotic states directly from the interaction of the theory by setting a sufficiently large time scale $T$. In this dressed state formalism, we define the asymptotic interaction remaining at $|t|>T$ in terms of some fixed order of $1/T$, and we are performing all calculations according to that order. We study the asymptotic states in QED specifically, but we can formally apply the dressed state formalism proposed in this paper to any perturbative QFT. We show that, at least in QED, we can construct divergence-free and unitary $S$-matrix using dressed states proposed in this paper. Furthermore, we discuss the transition rate to show that we can predict experimental results. We also show that the $\mathrm{U}(1)$ gauge symmetry of $S$-matrix leads to the QED large gauge symmetry, and deviation of the expectation values of the vector potential between initial and final spacelike hypersurfaces emerges as a QED memory effect. The dressed state formalism in this paper may give a unified and new insight into IR physics, including asymptotic symmetries, memory effects, and unitarity of the state evolution., 21 pages, two figure, title is changed, several revisions more...
- Published
- 2021
40. Area-law versus Rényi and Tsallis black hole entropies
- Author
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Sergei D. Odintsov, Valerio Faraoni, Shin'ichi Nojiri, Kazan Federal University, and National Research Council of Canada
- Subjects
Physics ,High Energy Physics - Theory ,Quantum aspects of black holes ,Physics::General Physics ,010308 nuclear & particles physics ,Gravitation, Cosmology and Astrophysics ,Altenrative gravity theories ,01 natural sciences ,General Relativity and Quantum Cosmology ,Black hole ,Entropy (classical thermodynamics) ,High Energy Physics::Theory ,General relativity ,0103 physical sciences ,Quantum gravity ,010306 general physics ,Black hole thermodynamics ,Mathematical physics - Abstract
The R\'enyi and Tsallis entropies are discussed as possible alternatives to the Bekenstein-Hawking area-law entropy. It is pointed out how replacing the entropy notion, but not the Hawking temperature and the thermodynamical energy may render the whole black hole thermodynamics inconsistent. The possibility to relate the R\'enyi and Tsallis entropies with the quantum gravity corrected Bekenstein-Hawking entropy is discussed., Comment: Explanation expanded, one reference added to the bibliography, matches the version to appear in Phys. Rev. D more...
- Published
- 2021
41. Viable inflationary models in a ghost-free Gauss–Bonnet theory of gravity
- Author
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N. Chatzarakis, Sergei D. Odintsov, Tanmoy Paul, V. K. Oikonomou, and Shin'ichi Nojiri
- Subjects
High Energy Physics - Theory ,Physics ,Cosmology and Nongalactic Astrophysics (astro-ph.CO) ,Physics and Astronomy (miscellaneous) ,FOS: Physical sciences ,lcsh:Astrophysics ,General Relativity and Quantum Cosmology (gr-qc) ,Astrophysics::Cosmology and Extragalactic Astrophysics ,First order ,General Relativity and Quantum Cosmology ,Exponential function ,Gravitation ,Formalism (philosophy of mathematics) ,Theoretical physics ,Exponential growth ,High Energy Physics - Theory (hep-th) ,Gauss–Bonnet theorem ,De Sitter universe ,lcsh:QB460-466 ,lcsh:QC770-798 ,lcsh:Nuclear and particle physics. Atomic energy. Radioactivity ,Engineering (miscellaneous) ,Astrophysics - Cosmology and Nongalactic Astrophysics ,Free parameter - Abstract
In this work we investigate the inflationary phenomenological implications of a recently developed ghost-free Gauss-Bonnet theory of gravity. The resulting theory can be viewed as a scalar Einstein-Gauss-Bonnet theory of gravity, so by employing the formalism for cosmological perturbations for the latter theory, we calculate the slow-roll indices and the observational indices, and we compare these with the latest observational data. Due to the presence of a freely chosen function in the model, in principle any cosmological evolution can be realized, so we specify the Hubble rate and the freely chosen function and we examine the phenomenology of the model. Specifically we focus on de Sitter, quasi-de Sitter and a cosmological evolution in which the Hubble rate evolves exponentially, with the last two being more realistic choices for describing inflation. As we demonstrate, the ghost-free model can produce inflationary phenomenology compatible with the observational data. We also briefly address the stability of first order scalar and tensor cosmological perturbations, for the exponential Hubble rate, and as we demonstrate, stability is achieved for the same range of values of the free parameters that guarantee the phenomenological viability of the models., Comment: EPJC Accepted more...
- Published
- 2019
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42. Modeling and testing the equation of state for (Early) dark energy
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Diego Sáez-Chillón Gómez, Sergei D. Odintsov, Shin'ichi Nojiri, German S. Sharov, Japan Society for the Promotion of Science, Ministerio de Ciencia, Innovación y Universidades (España), Universidad de Valladolid, and Kazan Federal University more...
- Subjects
Cosmology and Nongalactic Astrophysics (astro-ph.CO) ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,Astrophysics::Cosmology and Extragalactic Astrophysics ,01 natural sciences ,General Relativity and Quantum Cosmology ,Metric expansion of space ,symbols.namesake ,Hubble tension ,0103 physical sciences ,Dark energy ,Statistical physics ,010303 astronomy & astrophysics ,Physics ,Equation of state ,010308 nuclear & particles physics ,Equation of state (cosmology) ,Astronomy and Astrophysics ,Term (time) ,Space and Planetary Science ,symbols ,Gravitational singularity ,Singularities ,Equations for a falling body ,Hubble's law ,Free parameter ,Astrophysics - Cosmology and Nongalactic Astrophysics - Abstract
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., 19 pages, 2 figures, analysis extended, version accepted for publication in Phys. of the Dark Universe more...
- Published
- 2021
43. Different faces of generalized holographic dark energy
- Author
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Sergei D. Odintsov, Tanmoy Paul, Shin'ichi Nojiri, and Japan Society for the Promotion of Science
- Subjects
holographic dark energy ,ricci dark energy ,Sharma-Mittal dark energy ,Physics and Astronomy (miscellaneous) ,General Mathematics ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,01 natural sciences ,Particle horizon ,General Relativity and Quantum Cosmology ,Metric expansion of space ,Entropy (classical thermodynamics) ,0103 physical sciences ,Computer Science (miscellaneous) ,QA1-939 ,Astrophysics::Solar and Stellar Astrophysics ,Tsallis dark energy ,010306 general physics ,Scale factor (cosmology) ,Mathematical physics ,Physics ,equivalence with generalized HDE ,010308 nuclear & particles physics ,Quintessence dark energy ,Horizon ,Ricci dark energy ,Symmetry (physics) ,Holographic dark energy ,Chemistry (miscellaneous) ,quintessence dark energy ,Dark energy ,Equivalence with generalized HDE ,Rényi dark energy ,Mathematics ,Quintessence - Abstract
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 R\'{e}nyi 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., Comment: To appear in Symmetry more...
- Published
- 2021
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44. Thermal effects and scalar modes in the cosmological propagation of gravitational waves
- Author
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Shin'ichi Nojiri, Salvatore Capozziello, Sergei D. Odintsov, Japan Society for the Promotion of Science, Istituto Nazionale di Fisica Nucleare, Ministerio de Ciencia, Innovación y Universidades (España), Capozziello, S., Nojiri, S., and Odintsov, S. D. more...
- Subjects
Physics ,High Energy Physics - Theory ,Cosmology and Nongalactic Astrophysics (astro-ph.CO) ,Gravitational wave ,General relativity ,Scalar (mathematics) ,FOS: Physical sciences ,Astronomy and Astrophysics ,General Relativity and Quantum Cosmology (gr-qc) ,Cosmology ,General Relativity and Quantum Cosmology ,Gravitational waves ,Gravitation ,Classical mechanics ,Alternative theories of gravity ,High Energy Physics - Theory (hep-th) ,Space and Planetary Science ,Gravitational singularity ,Quantum ,Scalar field ,Astrophysics - Cosmology and Nongalactic Astrophysics - Abstract
We consider thermal effects in the propagation of gravitational waves on a cosmological background. In particular, we consider scalar field cosmologies and study gravitational modes near cosmological singularities. We point out that the contribution of thermal radiation can heavily affect the dynamics of gravitational waves giving enhancement or dissipation effects both at quantum and classical level.These effects are considered both in General Relativity and in modified theories like $F(R)$ gravity which can be easily reduced to scalar-tensor cosmology. The possible detection and disentanglement of standard and scalar gravitational modes on the stochastic background are also discussed., Comment: 21 pages, accepted for publication in Physics of the Dark Universe more...
- Published
- 2021
- Full Text
- View/download PDF
45. Mimetic Euler-Heisenberg theory, charged solutions and multi-horizon black holes
- Author
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Gamal G. L. Nashed and Shin'ichi Nojiri
- Subjects
High Energy Physics - Theory ,Physics ,Geodesic deviation ,General relativity ,FOS: Physical sciences ,Cosmological constant ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology ,Black hole ,Gravitation ,Singularity ,High Energy Physics - Theory (hep-th) ,Minkowski space ,Scalar field ,Mathematical physics - Abstract
We construct several new classes of black hole (BH) solutions in the context of the mimetic Euler-Heisenberg theory. We separately derive three differently charged BH solutions and their relevant mimetic forms. We show that the asymptotic form of all BH solutions behaves like a flat spacetime. These BHs, either with/without cosmological constant, have the non constant Ricci scalar, due to the contribution of the Euler-Heisenberg parameter, which means that they are not solution to standard or mimetic $f(R)$ gravitational theory without the Euler-Heisenberg non-linear electrodynamics and at the same time they are not equivalent to the solutions of the Einstein gravity with a massless scalar field. Moreover, we display that the effect of the Euler-Heisenberg theory makes the singularity of BH solutions stronger compared with that of BH solutions in general relativity. Furthermore, we show that the null and strong energy conditions of those BH solutions are violated, which is a general trend of mimetic gravitational theory. The thermodynamics of the BH solutions are satisfactory although there appears a negative Hawking temperature under some conditions. Additionally, these BHs obey the first law of thermodynamics. We also study the stability, using the geodesic deviation, and derive the stability condition analytically and graphically. Finally, for the first time and under some conditions, we derived multi-horizon BH solutions in the context of the mimetic Euler-Heisenberg theory and study their related physics., Comment: 19 pages, 9 figures, accepted for publication in Phys. Rev. D more...
- Published
- 2021
- Full Text
- View/download PDF
46. Statistical System based on $p$-adic numbers
- Author
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Mikoto Terasawa and Shin'ichi Nojiri
- Subjects
High Energy Physics - Theory ,Physics ,Nuclear and High Energy Physics ,Phase transition ,Free particle ,Statistical Mechanics (cond-mat.stat-mech) ,010308 nuclear & particles physics ,Entropy (statistical thermodynamics) ,QC1-999 ,One-dimensional space ,Degrees of freedom (statistics) ,FOS: Physical sciences ,01 natural sciences ,High Energy Physics - Theory (hep-th) ,0103 physical sciences ,Statistical physics ,010306 general physics ,Condensed Matter - Statistical Mechanics ,Real number ,Variable (mathematics) ,p-adic number - Abstract
We propose statistical systems based on $p$-adic numbers. In the systems, the Hamiltonian is a standard real number which is given by a map from the $p$-adic numbers. Therefore we can introduce the temperature as a real number and calculate the thermodynamical quantities like free energy, thermodynamical energy, entropy, specific heat, etc. Although we consider a very simple system, which corresponds to a free particle moving in one dimensional space, we find that there appear the behaviors like phase transition in the system. Usually in order that a phase transition occurs, we need a system with an infinite number of degrees of freedom but in the system where the dynamical variable is given by $p$-adic number, even if the degree of the freedom is unity, there might occur the phase transition., Comment: LaTeX 7 pages, 4 figures, several revisions, title is changed more...
- Published
- 2021
- Full Text
- View/download PDF
47. Nontrivial black hole solutions in f(R) gravitational theory
- Author
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Gamal G. L. Nashed and Shin'ichi Nojiri
- Subjects
Physics ,010308 nuclear & particles physics ,General relativity ,01 natural sciences ,Spherically symmetric spacetime ,Black hole ,Gravitation ,General Relativity and Quantum Cosmology ,0103 physical sciences ,Schwarzschild metric ,Gravitational singularity ,010306 general physics ,Mathematical physics ,Hawking radiation ,Scalar curvature - Abstract
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. more...
- Published
- 2020
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- View/download PDF
48. Considerations on gravitational waves in higher-order local and non-local gravity
- Author
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Salvatore Capozziello, Maurizio Capriolo, Shin'ichi Nojiri, Capozziello, S., Capriolo, M., and Nojiri, S.
- Subjects
High Energy Physics - Theory ,Nuclear and High Energy Physics ,General relativity ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,01 natural sciences ,Cosmology ,General Relativity and Quantum Cosmology ,Gravitational waves ,Gravitation ,Alternative theories of gravity ,0103 physical sciences ,010306 general physics ,Mathematical physics ,Physics ,High Energy Astrophysical Phenomena (astro-ph.HE) ,010308 nuclear & particles physics ,Gravitational wave ,Graviton ,Polarization (waves) ,lcsh:QC1-999 ,Massless particle ,Amplitude ,High Energy Physics - Theory (hep-th) ,Astrophysics - High Energy Astrophysical Phenomena ,lcsh:Physics - Abstract
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 $f \left( R, \Box R, \Box^2 R, \cdots, \Box^n R \right) = R + \sum_{i=1}^n \alpha_i R \Box^i R$ gravity, linear with respect to both $R$ and $\Box^i R$ and $ f \left( R, \Box R \right) = R + \alpha \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 \alpha_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 $\omega_{1}$, massless and with 2-helicity; $n+1$ further scalar modes of frequency $\omega_{2},\dots,\omega_{n+2}$, massive and with 0-helicity, each of which has the same mixed polarization, partly longitudinal and partly transverse., Comment: 15 pages, to appear in Physics. Lett. B more...
- Published
- 2020
49. Challenging matter creation models in the phantom divide
- Author
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Sergei D. Odintsov, Samuel Lepe, Miguel Cruz, Víctor H. Cárdenas, Shin'ichi Nojiri, Japan Society for the Promotion of Science, Consejo Nacional de Ciencia y Tecnología (México), Ministerio de Economía y Competitividad (España), Generalitat de Catalunya, and Ministry of Education, Culture, Sports, Science and Technology (Japan) more...
- Subjects
Physics ,010308 nuclear & particles physics ,Dark matter ,FOS: Physical sciences ,Lambda-CDM model ,General Relativity and Quantum Cosmology (gr-qc) ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Cosmological constant ,01 natural sciences ,General Relativity and Quantum Cosmology ,Physical cosmology ,Metric expansion of space ,symbols.namesake ,Theoretical physics ,0103 physical sciences ,Dark energy ,symbols ,010306 general physics ,Hubble's law ,Quintessence - Abstract
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 ΛCDM 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., This work has been supported by S. N. I. (CONACyTMexico) (M. C.) and by MINECO (Spain), FIS2016-76363-P, and by Project No. 2017 SGR247 (AGAUR, Catalonia) (S. D. O). This work is also supported by MEXT KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas “Cosmic Acceleration” No. 15H05890 (S. N.) and the JSPS Grant-in-Aid for Scientific Research (C) No. 18K03615 (S. N.). more...
- Published
- 2020
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- View/download PDF
50. 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
- Author
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Sergei D. Odintsov, Gamal G. L. Nashed, Emilio Elizalde, Shin'ichi Nojiri, Ministerio de Economía y Competitividad (España), and Ministry of Education, Culture, Sports, Science and Technology (Japan)
- Subjects
High Energy Physics - Theory ,Physics ,Geodesic deviation ,Physics and Astronomy (miscellaneous) ,General relativity ,Astrophysics::High Energy Astrophysical Phenomena ,Kretschmann scalar ,FOS: Physical sciences ,lcsh:Astrophysics ,Cosmological constant ,General Relativity and Quantum Cosmology (gr-qc) ,Computer Science::Digital Libraries ,General Relativity and Quantum Cosmology ,Black hole ,High Energy Physics - Theory (hep-th) ,lcsh:QB460-466 ,lcsh:QC770-798 ,Gravitational singularity ,lcsh:Nuclear and particle physics. Atomic energy. Radioactivity ,Engineering (miscellaneous) ,Ricci curvature ,Mathematical physics ,Scalar curvature - Abstract
Novel static black hole solutions with electric and magnetic charges are derived for the class of modified gravities: $f({\cal R})={\cal R}+2\beta\sqrt{{\cal R}}$, 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 $R=\frac{8r^2\Lambda+1}{r^2}$, 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 $\beta$ 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., Comment: 27 pages, 18 figures, will appear in EPJC more...
- Published
- 2020
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