Article
Astronomy & Astrophysics
Yannan Qi, Weiqiang Yang, Yitao Wang, Chenchen Zhao, TianYi Han, Yabo Wu
Summary: This study investigates the late-time dynamics of the F(R) gravity model, focusing on the cosmological evolution and the implications from supernovae observations. Numerical solutions of the field equations reveal a phantom evolution and physically viable cosmology at late times with fine-tuned parameters. It is noteworthy that dark energy oscillations exist in this model, and simple F(R) gravity models can realize phantom, nearly de-Sitter, and quintessential dark energy eras without the need for phantom scalar fields or relativistic fluids.
PHYSICS OF THE DARK UNIVERSE
(2023)
Article
Astronomy & Astrophysics
S. D. Odintsov, V. K. Oikonomou
Summary: The deformed R-2 model closely approximates the R-2 model during inflation, but exhibits stronger gravity and can incorporate an early dark energy term to control late-time dynamics.
Article
Physics, Multidisciplinary
Abdelghani Errehymy, Sunil Kumar Maurya, Sudan Hansraj, Mohammed Daoud, Haifa I. Alrebdi, Abdel-Haleem Abdel-Aty
Summary: In this paper, generalized Ellis-Bronikov traversable wormhole geometries for static and spherically symmetric spacetime in the background of f(R) gravity are explored. It is found that these wormholes are generated by dark matter galactic halos and violate the energy conditions, supporting their existence and traversability.
ANNALEN DER PHYSIK
(2023)
Article
Astronomy & Astrophysics
Bita Farsi, Ahmad Sheykhi
Summary: This study reveals the effects of an extra longitudinal degree of freedom on the evolution of perturbations in the context of mimetic gravity. By analyzing linear perturbations, it is found that the mimetic potential can act as dark energy and influence the dynamics of matter perturbations and cosmological parameters.
Article
Physics, Multidisciplinary
Amir H. Rezaei, T. Azizi, N. Rashidi
Summary: In this paper, an exponential form of modified gravity is considered. The theory is consistent with local tests and various aspects of the model are investigated, including the static solution, scalar field potential, and stability of critical points. The realization of inflation and its graceful exit in this model are also studied.
Article
Astronomy & Astrophysics
V. K. Oikonomou, F. P. Fronimos, Pyotr Tsyba, Olga Razina
Summary: Under the assumption that the axion scalar field forms the dark matter in the Universe, this paper extends the formalism of kinetic axion R2 gravity to include non-minimally coupled Gauss-Bonnet terms. It is shown that this Gauss-Bonnet term has significant effects on inflationary phenomenology and the kinetic axion scenario. Unlike in kinetic axion R2 gravity, the Gauss-Bonnet corrected kinetic axion R2 gravity leads to a non-extended inflationary era and immediate commencement of reheating driven by R2 fluctuations.
PHYSICS OF THE DARK UNIVERSE
(2023)
Article
Physics, Mathematical
Abdul Jawad, Sabir Hussain
Summary: This paper investigates the Tsallis holographic dark energy model in different cosmological contexts, analyzing various cosmological parameters and planes, and comparing them with the Planck data to draw conclusions about the consistency of cosmic expansion.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2021)
Article
Physics, Multidisciplinary
M. Zubair, Lala Rukh Durrani
Summary: This paper analyzes Tsallis holographic dark energy in a flat Friedmann-Robertson Walker model under the framework of f(R, T) gravity, studying the effects in a non interactive universe by taking different IR cut-offs. Different cosmological tools are used to study the cosmic evolution, and the impact of various parameters is analyzed. The models exhibit appropriate behavior for the system parameters and support accelerated expansion, showing stability for event horizon partially based on the speed of sound. Various diagnostic mechanisms are used to further examine the behavior of the model, with Om parameter supporting quintessence era and statefinder hierarchy effectively distinguishing the model from the ΛCDM model.
CHINESE JOURNAL OF PHYSICS
(2021)
Article
Physics, Multidisciplinary
J. A. Astorga-Moreno, E. A. Mena-Barboza
Summary: Modified gravity theories have gained significant attention in the past decade. This study utilizes deformed phase space in f(R) gravity to derive the Wheeler-DeWitt equation in the Quantum Cosmology scenario for a Friedmann-Lemaitre-Robertson-Walker model. Furthermore, a function f(R) that corresponds to the -Cold Dark Matter model is also considered.
Article
Physics, Multidisciplinary
J. K. Singh, Akanksha Singh, G. K. Goswami, J. Jena
Summary: We investigate a flat FLRW-model in f (R, T)-gravity, which shows accelerated expansion in the late-time Universe and is consistent with the standard cosmology Lambda CDM.
Article
Physics, Particles & Fields
Himanshu Chaudhary, Amine Bouali, Niyaz Uddin Molla, Ujjal Debnath, G. Mustafa
Summary: This paper investigates the observational signatures of f(R,G,T) gravity in the FRW universe and proposes a new cosmological model. By using the Markov Chain Monte Carlo technique, the model parameters are constrained, and various diagnostic tests and statistical analyses are conducted to evaluate the model's behavior and consistency with observational data. The findings highlight the potential and validity of the f(R,G,T) framework in understanding the dynamics and evolution of the universe.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Astronomy & Astrophysics
Joao C. Lobato, Isabela S. Matos, Mauricio O. Calvao, Ioav Waga
Summary: The study introduces an alternative choice of variables for treating f(R) models in their Palatini formulation, leading to the derivation of general results and a complete description of the phase space for exponential gravity. It is shown that Palatini exponential gravity theories have a final attractor critical point with specific effective equation of state parameters. Comparison of analytical results with numerical solutions of the field equations is also conducted.
Article
Physics, Multidisciplinary
M. Zubair, Lala Rukh Durrani, Saira Waheed
Summary: In this paper, a reconstruction of the f(T, B) gravity model is conducted, showing that all reconstructed models favor the current accelerated expansion regime by representing phantom cosmic epoch or de-Sitter model for different scenarios. The validity of the null energy condition is only observed for one case, while it remains invalid for other cases. Additionally, the stability of the reconstructed models is explored, revealing that both power law and de-Sitter solutions exhibit stable behavior against introduced perturbations.
EUROPEAN PHYSICAL JOURNAL PLUS
(2021)
Article
Astronomy & Astrophysics
A. Oliveros
Summary: In this study, a reparametrization method for a viable f(R) gravity model is proposed to represent it as a perturbation of the Lambda CDM model. The results show that the proposed model is practically indistinguishable from the Lambda CDM model at the background level and exhibits excellent agreement with the numerical solution. By appropriately choosing the model parameters, the obtained cosmological parameters are consistent with observations and the predictions of the Lambda CDM model. Additionally, the anomalous behavior at high redshifts is avoided by utilizing an approximate analytical solution.
INTERNATIONAL JOURNAL OF MODERN PHYSICS D
(2023)
Article
Physics, Multidisciplinary
Jay Solanki
Summary: This article develops a physically realistic model of compact stars undergoing gravitational collapse in f(R) gravity and finds analytical solutions describing the interior space-time metric, energy density, pressures, and heat flux density. The study also explores the physical properties of collapsing stars, providing insights into their inherent nature during gravitational collapse.
EUROPEAN PHYSICAL JOURNAL PLUS
(2022)
Article
Physics, Multidisciplinary
S. D. Odintsov, V. K. Oikonomou, F. P. Fronimos
Summary: This study explores the combined effects of string and f (R) gravity corrections on canonical scalar field inflation during the inflationary era controlled by a scalar field. By imposing the constraint that the primordial gravitational waves propagate at the speed of light, the dynamics of Einstein-Gauss-Bonnet gravity with alpha R-2 corrections are investigated. The research demonstrates the interconnectedness of Gauss-Bonnet coupling and scalar potential in the model, as well as the feasibility of extracting scalar potential from equations of motion when freely designating Gauss-Bonnet coupling xi(phi).
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
Multidisciplinary Sciences
Asmaa G. Shalaby, Vasilis K. Oikonomou, Gamal G. L. Nashed
Summary: The study explores modified cosmological models based on f(T) gravitational theory and non-extensive thermodynamics framework, with effects captured by the parameter delta. Detailed examination of cosmological evolution with collisionless non-relativistic matter was conducted, and constraints on the non-extensive thermodynamics parameter delta were imposed using Planck 2018 data.
Article
Physics, Particles & Fields
S. D. Odintsov, V. K. Oikonomou, F. P. Fronimos
Summary: In this paper, we study the inflationary phenomenology of a k-inflation corrected Einstein-Gauss Bonnet theory by adding Gauss Bonnet string corrections to achieve a ghost free description. The scalar functions of the theory are revealed to be interconnected, and assuming a specific form for one immediately specifies the other. By deriving the scalar potential from the equations of motion, a relatively tractable phenomenology is produced for each term entering the equations of motion. The predicted amount of non-Gaussianities is significantly enhanced compared to the k-inflation free Einstein-Gauss-Bonnet theory under certain conditions.
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)
Article
Physics, Particles & Fields
V. K. Oikonomou, F. P. Fronimos
Summary: The theoretical framework introduced in this work consists of non-minimal coupled canonical scalar field, non-minimal coupling to the Gauss-Bonnet invariant, and non-minimal kinetic coupling. Constraints are imposed on the propagation speed of primordial tensor perturbations to comply with the GW170817 event, which affects the choice of scalar potential and non-minimal couplings. The theoretical framework, belonging to the larger class of Horndeski theories, cannot yield viable results due to the mismatch in gravitational wave speed with that of light.
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
Artyom Astashenok, Sergei D. Odintsov, V. K. Oikonomou
Summary: By considering the thermal effects induced from Hawking radiation on the apparent horizon of the Universe, various dark energy models were investigated. The study explored the evolution of dark energy density as the Universe expands, leading to different types of singularities, such as sudden future singularities and big rip singularities. The research also looked into the transition between deceleration and acceleration in quintessence dark energy, as well as the potential remedies for crushing-type singularities.
