arXiv subjects

S. D. Odintsov

Publications and source records attributed to S. D. Odintsov.

At least 73 records · Page 4Linked to original sources

arXiv2021

Novel Stellar Astrophysics from Extended Gravity

Novel implications on neutron stars come from extended gravity. Specifically, the GW190814 event indicated the probability of having large mass stars in the mass-gap region $2.5-5\, M_{\odot}$. If the secondary component of GW190814 is a neutron star, such large masses are marginally supported by General Relativity (GR), since a very stiff Equation of State (EoS) would be needed to describe such large mass neutron stars, which would be incompatible with the GW170817 event, without any modification of gravity. In view of the two groundbreaking gravitational wave observations, we critically discuss the elevated role of extensions of GR towards the successful description of the GW190814 event, and we also speculate in a quantitative way on the important issue of the largest allowed neutron star mass.

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arXiv2021

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

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

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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

Causal Limit of Neutron Star Maximum Mass in $f(R)$ Gravity in View of GW190814

We investigate the causal limit of maximum mass for stars in the framework of$f(R)$ gravity. We choose a causal equation of state, with variable speed of sound, and with the transition density and pressure corresponding to the SLy equation of state. The transition density is chosen to be equal to twice the saturation density $ρ_t\sim 2ρ_0$, and also the analysis is performed for the transition density, chosen to be equal to the saturation density $ρ_t\sim ρ_0$. We examine numerically the combined effect of the stiff causal equation of state and of the sound speed on the maximum mass of static neutron stars, in the context of Jordan frame of $f(R)$ gravity. This yields the most extreme upper bound for neutron star masses in the context of extended gravity. As we will evince for the case of $R^2$ model, the upper causal mass limit lies within, but not deeply in, the mass-gap region, and is marginally the same with the general relativistic causal maximum mass, indicating that the $\sim 3M_{\odot}$ general relativistic limit is respected. In view of the modified gravity perspective for the secondary component of the GW190814 event, we also discuss the strange star possibility. Using several well established facts for neutron star physics and the Occam's razor approach, although the strange star is exciting, for the moment, it remains a possibility for describing the secondary component of GW190814. We underpin the fact that the secondary component of the compact binary GW190814 is probably a neutron star, a black hole or even a rapidly rotating neutron star, but not a strange star. We also discuss, in general, the potential role of the extended gravity description for the binary merging.

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arXiv2021

Neutron Stars Phenomenology with Scalar-tensor Inflationary Attractors

In this work we shall study the implications of a subclass of $E$-models cosmological attractors, namely of $a$-attractors, on hydrodynamically stable slowly rotating neutron stars. Specifically, we shall present the Jordan frame theory of the $a$-attractors, and by using a conformal transformation we shall derive the Einstein frame theory. We discuss the inflationary context of $a$-attractors in order to specify the allowed range of values for the free parameters of the model based on the latest cosmic-microwave-background-based Planck 2018 data. Accordingly, using the notation and physical units frequently used in theoretical astrophysics contexts, we shall derive the Tolman-Oppenheimer-Volkoff equations in the Einstein frame. Assuming a piecewise polytropic equation of state, the lowest density part of which shall be chosen to be the WFF1, or APR or the SLy EoS, we numerically solve the Tolman-Oppenheimer-Volkoff equations using a double shooting python-based "LSODA" numerical code. The resulting picture depends on the value of the parameter $a$ characterizing the $a$-attractors. As we show, for large values of $a$, which do not produce a viable inflationary era, the $M-R$ graphs are nearly identical to the general relativistic result, and these two are discriminated at large central densities values. Also, for large $a$-values, the WFF1 equation of state is excluded, due to the GW170817 constraints. In addition, the small $a$ cases produce larger masses and radii compared to the general relativistic case and are compatible with the GW170817 constraints on the radii of neutron stars. Our results indicate deep and not yet completely understood connections between non-minimal inflationary attractors and neutron stars phenomenology in scalar-tensor theory.

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arXiv2021

Inflationary magnetogenesis with reheating phase from higher curvature coupling

We investigate the generation of magnetic fields from inflation, which occurs via breakdown of the conformal invariance of the electromagnetic (EM) field, when coupled with the Ricci scalar and the Gauss-Bonnet invariant. For the case of instantaneous reheating, the resulting strength of the magnetic field at present is too small and violates the observational constraints. However, the problem is solved provided there is a reheating phase with a non-zero e-fold number. During reheating, the energy density of the magnetic field is seen to evolve as $(a^3H)^{-2}$ and, after that, as $a^{-4}$ up to the present epoch (here $a$ is the scale factor and $H$ the Hubble parameter). It is found that this reheating phase -- characterized by a certain e-fold number, a constant value of the equation of state parameter, and a given reheating temperature -- renders the magnetogenesis model compatible with the observational constraints. The model provides, in turn, a viable way of constraining the reheating equation of state parameter, from data analysis of the cosmic microwave background radiation. Moreover we discuss the Schwinger backreaction in the present context and determine the necessary constraints on the reheating equation of state parameter.

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arXiv2021

Late-Time Cosmology of Scalar-Coupled $f(R, \mathcal{G})$ Gravity

In this work by using a numerical analysis, we investigate in a quantitative way the late-time dynamics of scalar coupled $f(R,\mathcal{G})$ gravity. Particularly, we consider a Gauss-Bonnet term coupled to the scalar field coupling function $ξ(ϕ)$, and we study three types of models, one with $f(R)$ terms that are known to provide a viable late-time phenomenology, and two Einstein-Gauss-Bonnet types of models. Our aim is to write the Friedmann equation in terms of appropriate statefinder quantities frequently used in the literature, and we numerically solve it by using physically motivated initial conditions. In the case that $f(R)$ gravity terms are present, the contribution of the Gauss-Bonnet related terms is minor, as we actually expected. This result is robust against changes in the initial conditions of the scalar field, and the reason is the dominating parts of the $f(R)$ gravity sector at late times. In the Einstein-Gauss-Bonnet type of models, we examine two distinct scenarios, firstly by choosing freely the scalar potential and the scalar Gauss-Bonnet coupling $ξ(ϕ)$, in which case the resulting phenomenology is compatible with the latest Planck data and mimics the $Λ$-Cold-Dark-Matter model. In the second case, since there is no fundamental particle physics reason for the graviton to change its mass, we assume that primordially the tensor perturbations propagate with the speed equal to that of light's, and thus this constraint restricts the functional form of the scalar coupling function $ξ(ϕ)$, which must satisfy the differential equation $\ddotξ=H\dotξ$.

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arXiv2021

$k$-Inflation-corrected Einstein-Gauss-Bonnet Gravity with Massless Primordial Gravitons

In the present paper, we study the inflationary phenomenology of a $k$-inflation corrected Einstein-Gauss-Bonnet theory. Non-canonical kinetic terms are known for producing Jean instabilities or superluminal sound wave velocities in the aforementioned era, but we demonstrate in this work that by adding Gauss-Bonnet string corrections and assuming that the non-canonical kinetic term $ωX^γ$ is in quadratic, one can obtain a ghost free description. Demanding compatibility with the recent GW170817 event forces one to accept that the relation $\ddotξ=H\dotξ$ for the scalar coupling function $ξ(ϕ)$. As a result, the scalar functions of the theory are revealed to be interconnected and by assuming a specific form for one of them, specifies immediately the other. Here, we shall assume that the scalar potential is directly derivable from the equations of motion, once the Gauss-Bonnet coupling is appropriately chosen, but obviously the opposite is feasible as well. As a result, each term entering the equations of motion, can be written in terms of the scalar field and a relatively tractable phenomenology is produced. For quadratic kinetic terms, the resulting scalar potential is quite elegant functionally. Different exponents, which lead to either a more perplexed solution for the scalar potential, are still a possibility which was not further studied. We also discuss in brief the non-Gaussianities issue under the slow-roll and constant-roll conditions holding true, and we demonstrate that the predicted amount of non-Gaussianities is significantly enhanced in comparison to the $k$-inflation free Einstein-Gauss-Bonnet theory.

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arXiv2020

Canonical Scalar Field Inflation with String and $R^2$-Corrections

Assuming that a scalar field controls the inflationary era, we examine the combined effects of string and $f(R)$ gravity corrections on the inflationary dynamics of canonical scalar field inflation, imposing the constraint that the speed of the primordial gravitational waves is equal to that of light's. Particularly, we study the inflationary dynamics of an Einstein-Gauss-Bonnet gravity in the presence of $αR^2$ corrections, where $α$ is a free coupling parameter. As it was the case in the pure Einstein-Gauss-Bonnet gravity, the realization that the gravitational waves propagate through spacetime with the velocity of light, imposes the constraint that the Gauss-Bonnet coupling function $ξ(ϕ)$ obeys the differential equation $\ddotξ=H\dotξ$, where $H$ is the Hubble rate. Subsequently, a relation for the time derivative of the scalar field is extracted which implies that the scalar functions of the model, which are the Gauss-Bonnet coupling and the scalar potential, are interconnected and simply designating one of them specifies the other immediately. In this framework, it is useful to freely designate $ξ(ϕ)$ and extract the corresponding scalar potential from the equations of motion but the opposite is still feasible. We demonstrate that the model can produce a viable inflationary phenomenology and for a wide range of the free parameters. Also, a mentionable issue is that when the coupling parameter $α$ of the $R^2$ correction term is $α<10^{-3}$ in Planck Units, the $R^2$ term is practically negligible and one obtains the same equations of motion as in the pure Einstein-Gauss-Bonnet theory, however the dynamics still change, since now the time derivative of $\frac{\partial f}{\partial R}$ is nonzero.

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arXiv2020

Unification of a Bounce with a Viable Dark Energy Era in Gauss-Bonnet Gravity

In this work we shall demonstrate that it is possible to describe in a unified way a primordial bounce with the dark energy era, in the context of Gauss-Bonnet modified gravity. Particularly, the early time bounce has a nearly scale invariant power spectrum of primordial scalar curvature perturbations, while the dark energy era is a viable one, meaning that it mimics the $Λ$-Cold-Dark-Matter model and also is compatible with the Planck 2018 data on cosmological parameters. In addition, our analysis indicates that the dark energy era is free from dark energy oscillations, which occur in the context of $f(R)$ gravity. We further addressed the later issue by examining $f(R)$ extensions of Gauss-Bonnet models, and we showed that the $f(R)$ gravity part of the action actually produces the dark energy oscillations at redshifts $z\sim 4$.

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arXiv2020

From a Bounce to the Dark Energy Era with $F(R)$ Gravity

In this work we consider a cosmological scenario in which the Universe contracts initially having a bouncing-like behavior, and accordingly after it bounces off, it decelerates following a matter dominated like evolution and at very large positive times it undergoes through an accelerating stage. Our aim is to study such evolution in the context of $F(R)$ gravity theory, and confront quantitatively the model with the recent observations. Using several reconstruction techniques, we analytically obtain the form of $F(R)$ gravity in two extreme stages of the universe, particularly near the bounce and at the late time era respectively. With such analytic results and in addition by employing appropriate boundary conditions, we numerically solve the $F(R)$ gravitational equation to determine the form of the $F(R)$ for a wide range of values of the cosmic time. The numerically solved $F(R)$ gravity realizes an unification of certain cosmological epochs of the universe, in particular, from a non-singular bounce to a matter dominated epoch and from the matter dominated to a late time dark energy epoch. Correspondingly, the Hubble parameter and the effective equation of state parameter of the Universe are found and several qualitative features of the model are discussed. The Hubble radius goes to zero asymptotically in both sides of the bounce, which leads to the generation of the primordial curvature perturbation modes near the bouncing point. Correspondingly, we calculate the scalar and tensor perturbations power spectra near the bouncing point, and accordingly we determine the observable quantities like the spectral index of the scalar curvature perturbations, the tensor-to-scalar ratio, and as a result, we directly confront the present model with the latest Planck observations. Furthermore the $F(R)$ gravity dark energy epoch is confronted with the Sne-Ia+BAO+H(z)+CMB data.

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arXiv2020

GW170817-compatible Constant-roll Einstein-Gauss-Bonnet Inflation and Non-Gaussianities

In this paper we investigate the inflationary phenomenology of an Einstein-Gauss-Bonnet theory compatible with the GW170817 event, by imposing the constant-roll evolution on the scalar field. We develop the constant-roll GW170817-compatible Einstein-Gauss-Bonnet formalism, and we calculate the slow-roll indices and the observational indices of inflation, for several models of interest. As we demonstrate, the phenomenological viability of the models we study is achieved for a wide range of the free parameters. In addition, for the same values of the free parameters that guarantee the inflationary phenomenological viability of the models, we also make predictions for the non-Gaussianities of the models, since the constant-roll evolution is known to enhance non-Gaussianities. As we show the non-Gaussianities are of the same order for the slow-roll and constant-roll case, and in fact in some cases, the amount of the non-Gaussianities is smaller in the constant-roll case.

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arXiv2020

Bottom-up reconstruction of non-singular bounce in F(R) gravity from observational indices

We apply the bottom-up reconstruction technique in the context of bouncing cosmology in F(R) gravity, where the starting point is a suitable ansatz of observable quantity (like spectral index or tensor to scalar ratio) rather than a priori form of Hubble parameter. In inflationary scenario, the slow roll conditions are assumed to hold true, and thus the observational indices have general expressions in terms of the slow-roll parameters, as for example the tensor to scalar ratio in F(R) inflation can be expressed as $r = 48ε_F^2$ with $ε_F = -\frac{1}{H_F^2}\frac{dH_F}{dt_F}$ and $H_F$, $t_F$ are the Hubble parameter, cosmic time respectively. However, in the bouncing cosmology (say in F(R) gravity theory), the slow-roll conditions are not satisfied, in general, and thus the observable quantities do not have any general expressions that will hold true irrespective of the form of F(R). Thus, in order to apply the bottom-up reconstruction procedure in F(R) bouncing model, we use the conformal correspondence between F(R) and scalar-tensor model where the conformal factor in the present context is chosen in a way such that it leads to an inflationary scenario in the scalar-tensor frame. Due to the reason that the scalar and tensor perturbations remain invariant under conformal transformation, the observable viability of the scalar-tensor inflationary model confirms the viability of the conformally connected F(R) bouncing model. Motivated by these arguments, here we construct a viable non-singular bounce in F(R) gravity directly from the observable indices of the corresponding scalar-tensor inflationary model.

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arXiv2020

Rectifying Einstein-Gauss-Bonnet Inflation in View of GW170817

In this work we introduce a new theoretical framework for Einstein-Gauss-Bonnet theories of gravity, which results to particularly elegant, functionally simple and transparent gravitational equations of motion, slow-roll indices and the corresponding observational indices. The main requirement is that the Einstein-Gauss-Bonnet theory has to be compatible with the GW170817 event, so the gravitational wave speed $c_T^2$ is required to be $c_T^2\simeq 1$ in natural units. This assumption was also made in a previous work of ours, but in this work we express all the related quantities as functions of the scalar field. The constraint $c_T^2\simeq 1$ restricts the functional form of the scalar Gauss-Bonnet coupling function $ξ(ϕ)$ and of the scalar potential $V(ϕ)$, which must satisfy a differential equation. However, by also assuming that the slow-roll conditions hold true, the resulting equations of motion and the slow-roll indices acquire particularly simple forms, and also the relation that yields the $e$-foldings number is $N=\int_{ϕ_i}^{ϕ_f}ξ''/ξ'd ϕ$, a fact that enables us to perform particularly simple calculations in order to study the inflationary phenomenological implications of several models. As it proves, the models we presented are compatible with the observational data, and also satisfy all the assumptions made during the process of extracting the gravitational equations of motion. More interestingly, we also investigated the phenomenological implications of an additional condition $ξ'/ξ''\ll 1$, which is motivated by the slow-roll conditions that are imposed on the scalar field evolution and on the Hubble rate, in which case the study is easier. Our approach opens a new window in viable Einstein-Gauss-Bonnet theories of gravity.

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arXiv2020

Unifying Holographic Inflation with Holographic Dark Energy: a Covariant Approach

In the present paper, we use the holographic approach to describe the early-time acceleration and the late-time acceleration eras of our Universe in a unified manner. Such ``holographic unification'' is found to have a correspondence with various higher curvature cosmological models with or without matter fields. The corresponding holographic cut-offs are determined in terms of the particle horizon and its derivatives, or the future horizon and its derivatives. As a result, the holographic energy density we propose is able to merge various cosmological epochs of the Universe from a holographic point of view. We find the holographic correspondence of several $F(R)$ gravity models, including axion-$F(R)$ gravity models, of several Gauss-Bonnet $F(G)$ models and finally of $F(T)$ models, and in each case we demonstrate that it is possible to describe in a unified way inflation and late-time acceleration in the context of the same holographic model.

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arXiv2020

Non-Minimally Coupled Einstein Gauss Bonnet Inflation Phenomenology in View of GW170817

We study the inflationary phenomenology of a non-minimally coupled Einstein Gauss-Bonnet gravity theory, in the presence of a scalar potential, under the condition that the gravitational wave speed of the primordial gravitational waves is equal to unity, that is $c_T^2=1$, in natural units. The equations of motion, which are derived directly from the gravitational action, form a system of differential equations with respect to Hubble's parameter and the inflaton field which are very complicated and cannot be solved analytically, even in the minimal coupling case. In this paper, we present a variety of different approximations which could be used, along with the constraint $c_T^2=1$, in order to produce an inflationary phenomenology compatible with recent observations. All the different approaches are able to lead to viable results if the model coupling functions obey simple relations, however, different approaches contain different approximations which must be obeyed during the first horizon crossing, in order for the model to be rendered correct. Models which may lead to a non-viable phenomenology are presented as well in order to understand better the inner framework of this theory. Furthermore, since the velocity of the gravitational waves is set equal to $c_T^2=1$, as stated by the striking event of GW170817 recently, the non-minimal coupling function, the Gauss-Bonnet scalar coupling and the scalar potential are related to each other. Here, we shall assume no particular form of the scalar potential and we choose freely the scalar functions coupled to the Ricci scalar and the Gauss-Bonnet invariant. Certain models are also studied in order to assess the phenomenological validity of the theory, but we need to note that all approximations must hold true in order for a particular model to be valid.

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arXiv2020

Inflationary Attractors in $F(R)$ Gravity

In this letter we shall demonstrate that the viable $F(R)$ gravities can be classified mainly into two classes of inflationary attractors, either the $R^2$ attractors or the $α$-attractors. To show this, we shall derive the most general relation between the tensor-to-scalar ratio $r$ and the spectral index of primordial curvature perturbations $n_s$, namely the $r-n_s$ relation, by assuming that the slow-roll condition constrains the values of the slow-roll indices. As we show, the relation between the tensor-to-scalar ratio and the spectral index of the primordial curvature perturbations has the form $r\simeq \frac{48 (1-n_s)^2}{(4-x)^2}$, where the dimensionless parameter $x$ contains higher derivatives of the $F(R)$ gravity function with respect to the Ricci scalar, and it is a function of the $e$-foldings number $N$ and may also be a function of the free parameters of the various $F(R)$ gravity models. For $F(R)$ gravities which have a spectral index compatible with the observational data and also yield $x\ll 1$, these belong to the $R^2$-type of attractors, with $r\sim 3 (1-n_s)^2$, and these are viable theories. Moreover, in the case that $x$ takes larger values in specific ranges and is constant for a given $F(R)$ gravity, the resulting $r-n_s$ relation has the form $r\sim 3 α(1-n_s)^2$, where $α$ is a constant. Thus we conclude that the viable $F(R)$ gravities may be classified into two limiting types of $r-n_s$ relations, one identical to the $R^2$ model at leading order in $x$, and one similar to the $α$-attractors $r-n_s$ relation, for the $F(R)$ gravity models that yield $x$ constant. Finally, we also discuss the case that $x$ is not constant.

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arXiv2020

$f(R)$ Gravity $k$-Essence Late-time Phenomenology

In this work we shall study the late-time behavior of $k$-Essence $f(R)$ gravity without scalar potential, in the presence of matter and radiation perfect fluids. We quantify the late-time study by using the statefinder function $Y_H(z)=\frac{ρ_{DE}}{ρ_m^{(0)}}$, which is a function of the redshift and of the Hubble rate. By appropriately rewriting the Friedmann equation in terms of the redshift and of the function $Y_H(z)$, we numerically solve it using appropriate initial conditions, and we critically examine the effects of the $k$-Essence higher order kinetic terms. As we demonstrate, the effect of the higher order scalar field kinetic terms on the late-time dynamics is radical, since the dark energy oscillations are absent, and in addition, the cosmological physical quantities are compatible with the latest Planck data and also the model is almost indistinguishable from the $Λ$ Cold Dark Matter model. This is in contrast to the standard $f(R)$ gravity case, where the oscillations are present. Furthermore, by choosing a different set of values of two of the free parameters of the model, and specifically the coefficient of the higher order kinetic term and of the exponent of $R^δ$ appearing in the $f(R)$ action, we demonstrate that it is possible to obtain $ρ_{DE}<0$ for redshifts $z\sim 2-3.8$, which complies phenomenologically with, and seems to explain, the observational data for the same redshifts, and also to obtain a viable cosmological evolution at $z\sim 0$, at least when the dark energy equation of state parameter and the dark energy density parameters are considered.

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