arXiv subjects

Sergei D. Odintsov

Publications and source records attributed to Sergei D. Odintsov.

At least 55 records · Page 3Linked to original sources

arXiv2023

Unifying inflation with early and late dark energy in Einstein-Gauss-Bonnet gravity

In the era of precision cosmology, different observational data has led to precise measurements of the Hubble constant that differ significantly, what has been called the Hubble tension problem. In order to solve such a discrepancy, many different solutions have been proposed, from systematic errors on the observational data to theoretical proposals that assume an early dark energy that might affect the universe expansion at the time of recombination. In this paper, a model of varying cosmological constant is proposed in the framework of Einstein-Gauss-Bonnet gravity. The corresponding gravitational action is reconstructed and such a model is shown to reproduce well the inflationary era together with dark energy epoch and at the same time to provide an explanation for the discrepancy on the Hubble constant predictions. The transition to a phantom epoch is also realised, avoiding the usual instability problems of ordinary scalar field models.

gr-qc↗

arXiv2023

Generalized black hole entropy in two dimensions

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.

hep-th↗

arXiv2023

Generalised (non-singular) entropy functions with applications to cosmology and black holes

The growing interest of different entropy functions proposed so far (like the Bekenstein-Hawking, Tsallis, Rényi, Barrow, Sharma-Mittal, Kaniadakis and Loop Quantum Gravity entropies) towards black hole thermodynamics as well as towards cosmology lead to the natural question that whether there exists a generalized entropy function that can generalize all these known entropies. With this spirit, we propose a new 4-parameter entropy function that seems to converge to the aforementioned known entropies for certain limits of the entropic parameters. The proposal of generalized entropy is extended to non-singular case, in which case, the entropy proves to be singular-free during the entire cosmological evolution of the universe. The hallmark of such generalized entropies is that it helps us to fundamentally understand one of the important physical quantities namely ``entropy''. Consequently we address the implications of the generalized entropies on black hole thermodynamics as well as on cosmology, and discuss various constraints of the entropic parameters from different perspectives.

gr-qc↗

arXiv2022

A non-singular generalized entropy and its implications on bounce cosmology

We propose a new five-parameter entropy function that proves to be singular-free during the entire cosmic evolution of the universe, and at the same time, also generalizes the Tsallis, Barrow, Rényi, Sharma-Mittal, Kaniadakis and Loop Quantum Gravity entropies for suitable limits of the parameters. In particular, all the above mentioned known entropies become singular (or diverge) when the Hubble parameter vanishes in course of the universe's evolution (for instance, in bounce cosmology at the instant of bounce), while the newly proposed entropy function seems to be singular-free even at $H = 0$ (where $H$ represents the Hubble parameter of the universe). Such non-singular behaviour of the entropy function becomes useful in describing bouncing scenario, in which case, the universe undergoes through $H = 0$ at the instant of bounce. It turns out that the entropic cosmology corresponding to the singular-free generalized entropy naturally allows symmetric bounce scenarios, such as -- exponential bounce and quasi-matter bounce scenario respectively. In the case of exponential bounce, the perturbation modes are in the super-Hubble domain at the distant past, while for the quasi-matter bounce, the perturbation modes generate in the deep sub-Hubble regime far before the bounce and hence resolves the ``horizon issue''. Based on this fact, we perform a detailed perturbation analysis for the quasi-matter bounce in the present context of singular-free entropic cosmology. Furthermore the entropic cosmology in the present context is shown to be equivalent with the generalized holographic cosmology where the holographic cut-offs are determined in terms of either future horizon and its derivative or the particle horizon and its derivative.

gr-qc↗

arXiv2022

Modified cosmology from the thermodynamics of apparent horizon

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 $p$ represent the energy density and the pressure of the fluid, respectively. To include the case $p \neq -ρ$, 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.

gr-qc↗

arXiv2022

New entropies, black holes, and holographic dark energy

The Bekenstein-Hawking entropy is a cornerstone of horizon thermodynamics but quantum effects correct it, while inequivalent entropies arise also in non-extensive thermodynamics. Reviewing our previous work, we advocate for a new entropy construct that comprises recent and older proposals and satisfies four minimal key properties. The new proposal is then applied to black holes and to holographic dark energy and shown to have the potential to cause early universe inflation or to alleviate the current Hubble tension. We then analyze black hole temperatures and masses consistent with alternative entropies.

gr-qc↗

arXiv2022

Testing viable extensions of Einstein-Gauss-Bonnet gravity

Some models within the framework of Gauss-Bonnet gravities are considered in the presence of a non-minimally coupled scalar field. By imposing a particular constraint on the scalar field coupling, an extension of the called Einstein-Gauss-Bonnet gravity that keeps the correct speed of propagation for gravitational waves, is considered. The cosmological evolution for this viable class of models is studied and compared with observational data (BAO, CMB, Sne Ia,..), where we obtain the corresponding bounds for these theories and show that such theories fit well the data and provide a well-behaved cosmological evolution in comparison to the standard model of cosmology. Some statistical parameters show that the goodness of the fits are slightly better than those for $Λ$CDM model.

gr-qc↗

arXiv2022

Alternative entropies and consistent black hole thermodynamics

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.

gr-qc↗

arXiv2022

Bounce universe with finite-time singularity

This work explains how the presence of a Type-IV singularity (a mild singularity) can influence the dynamics of a bouncing universe. In particular, we examine bounce cosmology that appears with a Type-IV singularity in the context of a ghost free Gauss-Bonnet theory of gravity. Depending on the time of occurrence of the Type-IV singularity, three different cases may arise -- when the singularity occurs before the bounce or after the bounce or at the instant of the bounce respectively. However in all of these cases, we find that in the case when the singularity "globally" affects the spacetime, the scalar power spectrum becomes red tilted and the tensor-to-scalar ratio is too large to be consistent with the observational data. Based on these findings, we investigate a different bouncing scenario which also appears with a Type-IV singularity, and the singularity affects the spacetime "locally" around the time when it occurs. As a result, and unlike to the previous scenario, the perturbation modes in the second bouncing scenario are likely to generate far away from the bounce in the deep contracting phase. This finally results to the simultaneous compatibility of the observable quantities with the Planck data, and ensures the viability of the bounce model where the Type-IV singularity has local effects on the spacetime around the time of the singularity.

gr-qc↗

arXiv2022

Early and late universe holographic cosmology from a new generalized entropy

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.

gr-qc↗

arXiv2022

Helical magnetogenesis with reheating phase from higher curvature coupling and baryogenesis

We investigate the generation of helical magnetic fields and address the baryon asymmetry of the universe from an inflationary magnetogenesis scenario, in which the conformal and parity symmetries of the electromagnetic field are broken through its coupling to the Ricci scalar and to the Gauss-Bonnet invariant via the dual field tensor, so that the generated magnetic field can have a helical nature. Depending on the reheating mechanism, we consider two different cases - (1) instantaneous reheating scenario, in which case the reheating e-fold number is zero, and (2) Kamionkowski reheating scenario which is parametrized by a non-zero e-fold number, a reheating equation of state parameter and a given reheating temperature. It is demonstrated that for both the reheating mechanisms, the generated magnetic fields can be compatible with the observations for suitable range of the model parameter present in the non-minimal coupling of the electromagnetic field. Actually the present magnetogenesis model does not produce sufficient hierarchy between the electric and magnetic fields at the end of inflation, and thus the electric field is not able to sufficiently induce (or enhance) the magnetic field during the Kamionkowski reheating stage. This in turn makes both the reheating cases almost similar from the perspective of magnetic field's evolution. Furthermore we find that the magnetic fields at the galactic scale with strength $\sim 10^{-13}\mathrm{G}$ can lead to the resultant value of the ratio of the baryonic number density to the entropy density as large as $\sim 10^{-10}$, which is consistent with the observational data.

gr-qc↗

arXiv2022

Towards a smooth unification from an ekpyrotic bounce to the dark energy era

In the context of a ghost free $f(R,\mathcal{G})$ model, we present a non-singular cosmological scenario in which the universe initially contracts through an ekpyrotic phase having a bouncing like behaviour, and following the bounce, it smoothly transits to a matter or radiation like deceleration era which is further smoothly connected to the dark energy era at present epoch. We consider the Gauss-Bonnet (GB) coupling function in such a way that it gets compatible with the event GW170817. Using suitable reconstruction technique, we obtain the non-trivial scalar field potential as well as the GB coupling function. Such scalar potential and GB coupling function source a smooth unified scenario from an ekpyrotic bounce to the dark energy era with an intermediate deceleration era. The occurrence of ekpyrotic contraction phase justifies the resolution of the anisotropic problem (also known as BKL instability) in the background evolution. The Hubble radius shows an asymmetric behaviour around the bounce, in particular, the evolution of the Hubble radius leads to the generation era of the primordial perturbation modes far before the bounce in the deep sub-Hubble regime. Accordingly we perform the scalar and tensor perturbation evolution in the present context, and as a result, the scalar power spectrum at large scale modes is found to be problematic. Thus an extended scenario is proposed where we consider a pre-ekpyrotic phase having the equation of state parameter is less than unity, and re-examine the scalar and tensor power spectra, on large scales that cross the Hubble radius during the pre-ekpyrotic stage. In this regard, the GB coupling function shows considerable effects in reducing the tensor to scalar ratio compared to the case where the GB coupling is absent. Furthermore the dark energy epoch is consistent with the Planck+SNe+BAO data.

gr-qc↗

arXiv2022

How fundamental is entropy? From non-extensive statistics and black hole physics to the holographic dark universe

We propose a new entropy construct that generalizes the Tsallis, Rényi, Sharma-Mittal, Barrow, Kaniadakis, and Loop Quantum Gravity entropies and reduces to the Bekenstein-Hawking entropy in a certain limit. This proposal is applied to the Schwarzschild black hole and to spatially homogeneous and isotropic cosmology, where it is shown that it can potentially describe inflation and/or holographic dark energy.

gr-qc↗

arXiv2021

Barrow entropic dark energy: A member of generalized holographic dark energy family

The holographic cut-off, in the formalism of generalized holographic dark energy (HDE), is generalized to depend on $L_\mathrm{IR} = L_\mathrm{IR} \left( L_\mathrm{p}, \dot L_\mathrm{p}, \ddot L_\mathrm{p}, \cdots, L_\mathrm{f}, \dot L_\mathrm{f}, \cdots, a\right)$, where $L_\mathrm{p}$ and $L_\mathrm{f}$ 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 $L_\mathrm{p}$ or $L_\mathrm{f}$ 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.

gr-qc↗

arXiv2021

Area-law versus Rényi and Tsallis black hole entropies

The Rényi 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 Rényi and Tsallis entropies with the quantum gravity corrected Bekenstein-Hawking entropy is discussed.

gr-qc↗

arXiv2021

Unifying an asymmetric bounce to the dark energy in Chern-Simons F(R) gravity

We propose a cosmological scenario in which the universe undergoes through a non-singular bounce, and after the bounce, it decelerates having a matter-like dominated evolution during some regime of the deceleration era, and finally at the present epoch it evolves through an accelerating stage. Our aim is to study such evolution in the context of Chern-Simons corrected F(R) gravity theory and confront the model with various observational data. Using the reconstruction technique, and in addition by employing suitable boundary conditions, we determine the form of F(R) for the entire possible range of the cosmic time. The form of F(R) seems to unify a non-singular bounce with a dark energy epoch, in particular, from a non-singular bounce to a deceleration epoch and from a deceleration epoch to a late time acceleration era. It is important to mention that the bouncing scenario in the present context is an asymmetric bounce, in particular, the Hubble radius monotonically increases and asymptotically diverges at the late contracting era, while it seems to decrease with time at the present epoch. Such evolution of the Hubble radius leads to the primordial perturbation modes generate at the deep contracting era when all the perturbation modes lie within the horizon. We calculate the scalar and tensor power spectra, and as a result, the primordial observables are found to be in agreement with the latest Planck 2018 constraints. In this regard, the Chern-Simons term seems to have considerable effects on the tensor perturbation evolution, however keeping intact the scalar part of the perturbation with that of in the case of a vacuum F(R) model, and as a result, the Chern-Simons term proves to play an important role in making the observable quantities consistent with the Planck results. Furthermore the theoretical expectation of the dark energy observables are confronted with the Planck+SNe+BAO data.

gr-qc↗

arXiv2021

Thermal effects and scalar modes in the cosmological propagation of gravitational waves

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.

gr-qc↗

arXiv2021

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

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

gr-qc↗