Article
Astronomy & Astrophysics
V. K. Oikonomou, P. D. Katzanis, Ilias C. Papadimitriou
Summary: In this work, a bottom-up approach is used to obtain viable inflationary Einstein-Gauss-Bonnet models compatible with the GW170817 event. A recently developed theoretical framework is employed to specify the tensor-to-scalar ratio in terms of the e-foldings number. Using this ratio, the Einstein-Gauss-Bonnet theory is reconstructed, finding the scalar potential and Gauss-Bonnet coupling scalar function as functions of the e-foldings number. The calculation of the spectral index of the primordial scalar perturbations and tensor spectral index is greatly simplified, allowing for easy determination of these observational indices.
CLASSICAL AND QUANTUM GRAVITY
(2022)
Article
Astronomy & Astrophysics
Mayukh R. Gangopadhyay, Hussain Ahmed Khan, Yogesh
Summary: In this study, the researchers investigated the application of Natural inflation and Mutated Hilltop inflation, two theoretically promising models of inflation, in the Einstein-Gauss Bonnet (EGB) gravity framework. By considering the observations from GW170817 that the speed of gravitational wave is equal to the speed of light, they explored the new features in the effective potential due to the non-minimal coupling to the Gauss-Bonnet invariant in the action. The analysis not only focused on the inflationary dynamics, but also examined the reheating dynamics and the energy spectrum of the gravitational wave.
PHYSICS OF THE DARK UNIVERSE
(2023)
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: In this work, we calculate the effect of GW170817-compatible Einstein-Gauss-Bonnet theory on the energy spectrum of primordial gravitational waves. We present a formalism for studying the inflationary and post-inflationary dynamics of GW170817-compatible Einstein-Gauss-Bonnet theories. The results show that these models primarily affect the energy spectrum through their tensor spectral index and tensor-to-scalar ratio, which can be detected in future gravitational wave experiments.
ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Miguel A. Garcia-Aspeitia, A. Hernandez-Almada
Summary: The study revisits the Einstein-Gauss-Bonnet model, providing additional evidence for its pros and cons from a cosmological perspective. Results suggest that the acceleration parameter alpha of the EGB model is constrained in background cosmology, and the universe evolution seems to be always in an accelerated state.
PHYSICS OF THE DARK UNIVERSE
(2021)
Article
Astronomy & Astrophysics
S. D. Odintsov, V. K. Oikonomou, F. P. Fronimos
Summary: In this study, we propose an alternative formalism for the inflationary phenomenology of rescaled Einstein-Gauss-Bonnet models. By constraining the propagation velocity of primordial tensor perturbations, we extract an approximate form for the time derivative of the scalar field coupled to the Gauss-Bonnet density. We show that the slow-roll indices can be written in a closed form as functions of three dimensionless parameters, and prove that the Einstein-Gauss-Bonnet model can produce a blue-tilted tensor spectral index under certain conditions. We also discuss the implications of our models on the preheating and reheating era, as well as issues related to the energy spectrum of primordial gravitational waves.
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: In this work, a refined Einstein-Gauss-Bonnet inflationary theoretical framework is provided, which is compatible with observational constraints on the gravitational wave speed. By introducing new assumptions and conditions, much simpler expressions for the slow-roll indices, tensor and scalar spectral indices, and tensor-to-scalar ratio are successfully derived. A particular model with a blue-tilted tensor spectral index is exemplified, showing compatibility with the latest Planck data and potential to describe the NANOGrav result according to recent literature.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Physics, Multidisciplinary
S. A. Venikoudis, F. P. Fronimos
Summary: This paper investigates the inflationary phenomenology in a specific gravitational theory with logarithmic modifications, comparing the viability of different models and considering non-Gaussianities and observational indices in the calculations. The results show that the models are phenomenologically viable for a wide range of parameters, with the logarithmic term making a minor contribution to numerical calculations as expected.
EUROPEAN PHYSICAL JOURNAL PLUS
(2021)
Article
Astronomy & Astrophysics
Konstantinos F. Dialektopoulos, Jackson Levi Said, Zinovia Oikonomopoulou
Summary: In this work, the dynamical system phase space of Einstein-Gauss-Bonnet theory in cosmological minisuperspace is explored in the context of GW170817. The main features of the theory are bound together through a system of autonomous differential equations in the background of a flat Friedmann-Lemaitre-Robertson-Walker spacetime. The stability criteria of the critical points in this system are analyzed. The phase space of this scalar-tensor gravity is rich due to the fourth-order contributions of the Gauss-Bonnet invariant and the second order contribution of the scalar field together with their coupling dynamics. Additional critical points are found compared to previous works, which may be important for understanding the larger evolution of standard background cosmology within this class of gravitational models.
PHYSICS OF THE DARK UNIVERSE
(2023)
Article
Astronomy & Astrophysics
V. K. Oikonomou, F. P. Fronimos
Summary: Researchers demonstrate how to modify cosmological theories to produce primordial gravitational waves consistent with the GW170817 event by introducing a kinetic coupling correction in the Einstein-Gauss-Bonnet theory. By studying the conditions on the scalar field dependent coupling function, they prove the viability of this theoretical framework.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
Eugeny Babichev, William T. Emond, Sabir Ramazanov
Summary: We study black holes in a modified gravity scenario involving a scalar field quadratically coupled to the Gauss-Bonnet invariant. The nonminimal coupling to gravity leads to symmetry restoration near the black hole horizon, prompting the development of the scalar wall in its vicinity. The model is fully consistent with the existence of an inflationary stage and predicts the speed of gravitational waves to be extremely close to unity.
Article
Astronomy & Astrophysics
M. Bousder, M. Bennai
Summary: This study investigates the charge of the 4D-Einstein-Gauss-Bonnet black hole by examining the charge of a particle-antiparticle pair at r(-) and r(+) horizons, showing the existence of two types of Schwarzschild black holes and quantified values for the Einstein-Gauss-Bonnet black hole charge. The study also reveals the Hawking-Bekenstein formula with two logarithmic corrections, where the second correction depends on the cosmological constant and the black hole charge. Finally, the thermodynamics of the EGB-AdS black hole is explored.
Article
Physics, Particles & Fields
S. N. Sajadi
Summary: In this paper, we extend the AdS/QCD model to quadratic gravity to study the influence of gravity. We find an anisotropic black brane solution in a 5D Einstein-Gauss-Bonnet-two Maxwell-dilaton system. The system is characterized by an arbitrary exponent, a dilaton field, a time component of the first Maxwell field, and a magnetic component of the second Maxwell field. We investigate three cases and consider the effects of the theory parameter and the anisotropic parameter. The blackening function exhibits a thermodynamic phase transition between small/large and AdS/large black brane with suitable chemical potential and other parameters.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Particles & Fields
Li-Ming Cao, Liang-Bi Wu
Summary: This article discusses that in the four-dimensional Einstein-Gauss-Bonnet gravity, to ensure the existence of a well defined linearized gravitational wave equation, the spacetime must be locally conformally flat.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Physics, Particles & Fields
Dmitry Chirkov, Alex Giacomini, Sergey A. Pavluchenko, Alexey Toporensky
Summary: The paper systematically investigates solutions with static compact extra dimensions and expanding three-dimensional subspace, providing a scheme for constructing solutions with stability analysis. It finds that solutions with negative spatial curvature of extra dimensions are always stable, while those with positive curvature are stable for a narrow range of parameters. Differences between positive and negative curvature cases, including the coexistence with maximally-symmetric solutions, are noted and discussed.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Physics, Particles & Fields
Yi-Li Wang, Xian-Hui Ge
Summary: This paper investigates the Einstein-Maxwell-Gauss-Bonnet-axion theory in 4-dimensional spacetime through a Kaluza-Klein-like process. By obtaining the dyonic black hole solution coupled with higher derivative terms, the behavior of shear viscosity to entropy density ratio at different temperatures is studied. It is found that the ratio of uncharged black holes violates a specific bound at low temperatures, and the main feature of this ratio remains almost unchanged in 4 dimensions.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Astronomy & Astrophysics
S. D. Odintsov, V. K. Oikonomou
Summary: This work examines the implications of a subclass of E-models cosmological attractors, known as a-attractors, on hydrodynamically stable slowly rotating neutron stars. By numerically solving the Tolman-Oppenheimer-Volkoff equations in the Einstein frame, the study finds that the masses and radii of neutron stars vary with the parameter a characterizing the a-attractors. Results indicate a complex relationship between non-minimal inflationary attractors and neutron star phenomenology in scalar-tensor theory.
PHYSICS OF THE DARK UNIVERSE
(2021)
Article
Physics, Multidisciplinary
V. K. Oikonomou
Summary: This study introduces a new theoretical framework of power-law f(R) gravity corrected canonical scalar field inflation and examines the inflationary dynamics of this new framework. The main characteristic is the dominance of a power-law f(R) gravity term, compared to the Einstein-Hilbert term, with good agreement with the latest Planck data for a wide range of free parameters. The resulting theory allows for viable inflationary theories which may otherwise be non-viable.
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: In this work, a refined Einstein-Gauss-Bonnet inflationary theoretical framework is provided, which is compatible with observational constraints on the gravitational wave speed. By introducing new assumptions and conditions, much simpler expressions for the slow-roll indices, tensor and scalar spectral indices, and tensor-to-scalar ratio are successfully derived. A particular model with a blue-tilted tensor spectral index is exemplified, showing compatibility with the latest Planck data and potential to describe the NANOGrav result according to recent literature.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: This study focuses on static neutron stars within a specific class of non-minimally coupled inflationary potentials, known as universal attractors. The analysis reveals that all studied cases predict larger maximum masses for neutron stars and are consistent with the GW170817 constraints on neutron star radii.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Physics, Multidisciplinary
A. Astashenok, S. Capozziello, S. D. Odintsov, V. K. Oikonomou
Summary: Novel implications on neutron stars come from extended gravity. Specifically, new research suggests the possibility of large mass stars in the mass gap region, which may require extensions of General Relativity to successfully describe this phenomenon and speculate on the largest allowed neutron star mass in a quantitative manner.
Article
Physics, Particles & Fields
G. G. L. Nashed, S. D. Odintsov, V. K. Oikonomou
Summary: This paper investigates spherically symmetric spacetime solutions that describe the interior of compact stellar objects within the context of higher-order curvature theory of the f(R) type. By deriving the non-vacuum field equations and solving the system of nonlinear differential equations, the study successfully identifies a specific solution for compact stars. Furthermore, the model is shown to be more stable and different from those in general relativity, with specific implications for the considered compact star Her X-1.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Physics, Mathematical
V. K. Oikonomou, Achilles Gitsis, Maria Mitrou
Summary: This study focuses on the phase space singularities of interactive quintessence model in the presence of matter fluid. The presence of matter fluid affects the dynamical system, making the singularities depend on initial conditions rather than occurring for general initial conditions.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2021)
Review
Multidisciplinary Sciences
Sergei D. Odintsov, Vasilis K. Oikonomou, Ratbay Myrzakulov
Summary: In this work, the authors exhaustively study the effects of modified gravity on the energy spectrum of the primordial gravitational waves background. They provide calculations for various cases of modified gravity, focusing on specific forms of interest. They also present the exact form of a parameter involved in the calculation of the effects of modified gravity on the energy spectrum for the first time.
Article
Astronomy & Astrophysics
Sergei D. Odintsov, Tanmoy Paul
Summary: This work explores the influence of a Type-IV singularity on the dynamics of a bouncing universe in the context of a ghost-free Gauss-Bonnet theory of gravity. It is found that when the singularity has a global effect on spacetime, the scalar power spectrum becomes red-tilted and the tensor-to-scalar ratio is incompatible with observational data. However, if the singularity only locally affects spacetime around the time of occurrence, the observable quantities are compatible with Planck data.
Editorial Material
Multidisciplinary Sciences
Sergei D. D. Odintsov
Editorial Material
Multidisciplinary Sciences
Sergei D. Odintsov
Article
Multidisciplinary Sciences
Artyom V. Astashenok, Sergey D. Odintsov, Vasilis K. Oikonomou
Summary: This study investigates the Chandrasekhar mass limit of white dwarfs in various models of f (R) gravity. Two equations of state for stellar matter are used: the simple relativistic polytropic equation and the realistic Chandrasekhar equation of state. The solution for any model of f (R) = R + beta R-m gravity shows that the stellar mass decreases compared to standard General Relativity. For realistic equations of state, there is a value of the central density for which the mass of a white dwarf peaks. This implies that in modified gravity, there exists a minimum radius for stable white dwarfs, which is greater than in General Relativity. The behavior of the Chandrasekhar mass limit in f (R) gravity is also investigated.
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: In this study, we analyze the scenario where the effective gravitational Lagrangian of a minimally coupled scalar field becomes a rescaled Einstein-Hilbert gravity at large curvatures, leading to changes in the inflationary phenomenology and satisfaction of swampland criteria. Two inflation models, fiber inflation and supergravity alpha-attractor models, are discussed to show their viability and satisfaction of swampland criteria. The late-time phenomenology of the fiber inflation potential in the presence of full f(R) gravity is examined, showing the production of a viable dark energy era similar to the Lambda-cold-dark-matter model. The modified gravity model presents a universe described by a rescaled Einstein-Hilbert gravity at early times, with the scalar field mainly controlling dynamics synergistically with f(R) gravity at late times.
Article
Astronomy & Astrophysics
S. D. Odintsov, V. K. Oikonomou, F. P. Fronimos
Summary: This paper investigates the late-time dynamics of scalar coupled f(R, G) gravity through numerical analysis, exploring various models in modified gravity theories. It is found that the contribution of Gauss-Bonnet related terms is minor when f(R) gravity terms are present, with the f(R) sector dominating at late times. The Einstein-Gauss-Bonnet models considered produce a dark energy era free from oscillations, with constraints on scalar coupling functions for compatibility with observational data.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: This article introduces a theoretical model of F(R) gravity that can describe inflation, early and late dark energy eras in the presence of a light axion particle as the dark matter component of the Universe. The model exhibits early and late-time dark energy eras, with an early dark energy era followed by a deceleration era and then a late-time dark energy era. A notable feature is the absence of dark energy oscillations in the dark energy era within the redshift interval z = [0, 10]. The model also shows a sudden jump in the value of the Hubble rate within the redshift interval z ≈ [2, 2.6], increasing and then decreasing until z ≈ 0.
Article
Astronomy & Astrophysics
Nelson R. F. Braga, Octavio C. Junqueira
Summary: This study investigates the influence of rotation on the transition temperature of strongly interacting matter produced in non-central heavy ion collisions. By using a holographic description of an AdS black hole, the authors extend the analysis to the more realistic case where the matter spreads over a region around the rotational axis. The results show the coexistence of confined and deconfined phases and are consistent with the concept of local temperature in rotating frames developed by Tolman and Ehrenfest.
Article
Astronomy & Astrophysics
Bing Sun, Jiachen An, Zhoujian Cao
Summary: This paper investigates the effect of gravitational constant variation on the propagation of gravitational waves. By employing two analytical methods, the study finds that variations in the gravitational constant result in amplitude and phase corrections for gravitational waves, and the time variation of the gravitational constant can be constrained through the propagation of gravitational waves.
Article
Astronomy & Astrophysics
Abdellah Touati, Zaim Slimane
Summary: This letter presents the first study of Hawking radiation as a tunneling process within the framework of non-commutative gauge theory of gravity. The non-commutative Schwarzschild black hole is reconstructed using the Seiberg-Witten map and the star product. The emission spectrum of outgoing massless particles is computed using the quantum tunneling mechanism. The results reveal pure thermal radiation in the low-frequency scenario, but a deviation from pure thermal radiation in the high-frequency scenario due to energy conservation. It is also found that noncommutativity enhances the correlations between successively emitted particles.
Article
Astronomy & Astrophysics
Shahar Hod
Summary: The travel times of light signals between two antipodal points on a compact star's surface are calculated for two different trajectories. It is shown that, for highly dense stars, the longer trajectory along the surface may have a shorter travel time as measured by asymptotic observers. A critical value of the dimensionless density-area parameter is determined for constant density stars to distinguish cases where crossing through the star's center or following a semi-circular trajectory on the surface has a shorter travel time as measured by asymptotic observers.