Article
Multidisciplinary Sciences
Vasilis K. Oikonomou, Fotis P. Fronimos, Olga Razina, Pyotr Tsyba
Summary: This work studies the phase space of f(R) gravity in the presence of a misalignment axion, and identifies four distinct and possibly unstable fixed points with physical significance. It demonstrates that the Chern-Simons terms do not affect the phase space.
Article
Physics, Mathematical
R. K. Tiwari, Bhupendra Kumar Shukla, A. Beesham, Sudha Agrawal
Summary: This paper investigates the dynamic behavior of the universe using the f(R,G) theory of gravity. The best fit values of model parameters are determined by solving the modified field equations and comparing with observational datasets. The study reveals a transition from deceleration to acceleration in the universe, and the f(R,G) cosmological model effectively describes late-time cosmic acceleration.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2023)
Article
Multidisciplinary Sciences
Rishi Kumar Tiwari, Bhupendra Kumar Shukla, Deger Sofuoglu, Dilay Kosem
Summary: In this study, a specific form of the time-dependent deceleration parameter is used to examine the accelerated expansion and phase transition in the flat FRW model of the universe within the context of f(R,T) gravity theory. The modified field equations are solved for a choice of f(R,T) = R+2f(T). The best fit values for the model parameters are determined using recent observational datasets, including 57 points from Cosmic Chronometers (CC) datasets and 1048 points from Pantheon supernovae datasets. Bayesian analysis and likelihood function, together with Markov Chain Monte Carlo (MCMC) method, are applied at 1s and 2s confidence levels. The physical behavior of parameters, such as density, pressure, and cosmographic parameters, corresponding to the constrained values of the model parameters, is then analyzed. Looking at the deceleration parameter, it is observed that the universe has transitioned from a decelerating expansion phase to an accelerating phase. Therefore, the cosmological model f(R,T) discussed in this study can successfully explain the accelerating expansion of the late universe without resorting to any dark energy component in the energy-momentum tensor.
Article
Astronomy & Astrophysics
Mariaveronica De Angelis, Laria Figurato, Giovanni Montani
Summary: In this study, the canonical quantum dynamics of the isotropic universe in metric f(R) gravity theory is analyzed, with investigations into Schrodinger evolution in both vacuum and scalar field cases. The results show nonspreading wave packet profiles in vacuum and increased localization of universe volume when an external scalar field is present. This suggests a spontaneous mechanism for Universe classicalization in metric f(R) gravity, with implications for the transition from a Planckian Universe to a classical de-Sitter phase.
Article
Physics, Multidisciplinary
A. I. Keskin
Summary: This study investigates the inflationary era of the Universe in the context of f(R) gravity, considering scalar perturbation equations and assuming specific parameter values to achieve inflation. The findings suggest that this inflation scenario for the early universe is possible, as observed parameters align with latest observational data.
EUROPEAN PHYSICAL JOURNAL PLUS
(2021)
Article
Physics, Particles & Fields
Vipin Kumar Sharma, Murli Manohar Verma
Summary: We explore the shifted f (R) model with delta as a distinguishing physical parameter for the study of constraints at local scales. This model seems to provide a unique f (R) at local scales and is suitable for the alternative explanation of dark matter-like effects.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Physics, Multidisciplinary
M. Sharif, M. Zeeshan Gul
Summary: This paper analyzes the stability of the closed Einstein static universe using linear homogeneous perturbations in the framework of energy-momentum squared gravity, showing that stable regions in this modified theory are large compared to other modified theories of gravity.
Article
Materials Science, Multidisciplinary
Avik De, Simran Arora, Uday Chand De, P. K. Sahoo
Summary: The article first investigates the geometrical structures of a pseudo-symmetric spacetime under the condition of conformal flatness, and then further studies the spacetime solutions of F(R)-gravity theory and their relationships with cosmological effects.
RESULTS IN PHYSICS
(2022)
Article
Physics, Mathematical
Bhupendra Kumar Shukla, R. K. Tiwari, D. Sofuoglu
Summary: In this article, the dynamical behavior of the universe is examined in the context of the f(R, G) theory of gravity, which incorporates the Ricci scalar and Gauss-Bonnet invariant. The best fit values of model parameters are predicted to be in agreement with recent observational datasets, using the CC, Pantheon and BAO datasets, Bayesian analysis, likelihood function, and MCMC method. The obtained f(R, G) model exhibits a transition from decelerating to accelerating expansion of the universe, explaining the late accelerating expansion without the need for dark energy.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2023)
Article
Physics, Multidisciplinary
Stefano Camera, Salvatore Capozziello, Lorenzo Fatibene, Andrea Orizzonte
Summary: We investigate how the cosmological equation of state can be used to scrutinize extended theories of gravity, specifically the Palatini f(R) gravity. The effective equation of state produced by a given model is studied, and the inverse problem of determining which models are compatible with a given effective equation of state is also considered. We find that power-law models are capable of transforming barotropic Equations of State into effective barotropic ones, and the form of equation of state is preserved only for f(R) = R. Additionally, quadratic and non-homogeneous effective Equations of State contain the Starobinsky model and other models.
EUROPEAN PHYSICAL JOURNAL PLUS
(2023)
Article
Physics, Particles & Fields
Beatrice Murorunkwere, Joseph Ntahompagaze, Edward Jurua
Summary: In this study, the 1+3 covariant approach and equivalence between f(R) gravity and scalar-tensor theory were applied to analyze cosmological perturbations. Focus was on dust dominated area with perturbation equations applied to a background solution of alpha R+beta R-n model, and the evolution of energy-density perturbations in a dust-dominated universe was found to decay with increasing redshift through numerical solutions.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Astronomy & Astrophysics
P. S. Ens, A. F. Santos
Summary: This work presents a method to consider two originally different corrections to the Friedmann equations, namely the Barrow entropy and the f (R) gravity. By combining these two models, a modified Friedmann equation is obtained. An application using a specific f (R) model is investigated, with calculations of the state parameter and density parameters for matter and dark energy, leading to a discussion on the dynamic evolution of the universe.
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
Multidisciplinary Sciences
Adam Z. Kaczmarek, Dominik Szczesniak
Summary: The study examines the f(R, R-μνR-μν) gravity theory within the mimetic approach, involving field equations, cosmological models, and inflation consistency. Reconstruction schemes are used to derive specific models and discuss dark energy density, indicating the potential of mimetic extension in describing diverse cosmological scenarios.
SCIENTIFIC REPORTS
(2021)
Article
Astronomy & Astrophysics
Ajay Kumar Sharma, Murli Manohar Verma
Summary: This study investigates a model with the form (R) = R-1+(delta)/R-c(delta) and examines its viability for inflation in the Jordan and Einstein frames. The model is extended to f (R) = R + R1+epsilon/R-c(delta) in an attempt to address the issues of the previous model. The analysis of slow roll parameters and power spectrum indices suggests that the model aligns closely with recent observational data and is consistent with standard inflationary approaches and observations.
ASTROPHYSICAL JOURNAL
(2022)
Article
Physics, Multidisciplinary
Salman Sajad Wani, James Q. Quach, Mir Faizal, Sebastian Bahamonde, Behnam Pourhassan
Summary: This study proposes that the Hamiltonian constraint in quantum gravity cannot probe change and therefore cannot provide a meaningful notion of time. It suggests that if the time-reparametrization symmetry is spontaneously broken, time can emerge in quantum gravity.
FOUNDATIONS OF PHYSICS
(2022)
Article
Astronomy & Astrophysics
Sebastian Bahamonde, Alexey Golovnev, Maria-Jose Guzman, Jackson Levi Said, Christian Pfeifer
Summary: This article discusses exact and perturbative spherically symmetric solutions in f(T, B)-gravity, and also presents general methods and strategies for finding spherically symmetric solutions in modified teleparallel theories of gravity using generalized Bianchi identities.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Sebastian Bahamonde, Jorge Gigante Valcarcel
Summary: We present new rotating vacuum configurations endowed with both dynamical torsion and nonmetricity fields in the framework of Metric-Affine gauge theory of gravity. By considering scalar-flat Weyl-Cartan geometries, we obtain an axisymmetric Kerr-Newman solution in the decoupling limit between the orbital and the spin angular momentum.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
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.
Review
Physics, Multidisciplinary
Sebastian Bahamonde, Konstantinos F. Dialektopoulos, Celia Escamilla-Rivera, Gabriel Farrugia, Viktor Gakis, Martin Hendry, Manuel Hohmann, Jackson Levi Said, Jurgen Mifsud, Eleonora Di Valentino
Summary: In this Review, the development of teleparallel gravity as a gauge theory of translations is comprehensively introduced. The construction of consistent teleparallel theories that respect physical conditions such as local Lorentz invariance is described. Various modified teleparallel theories of gravity are discussed and connected into general classes of covariant gravitational theories. The cosmological consequences and observational implications of teleparallel gravity are also explored, along with the application of machine learning to gravity research.
REPORTS ON PROGRESS IN PHYSICS
(2023)
Editorial Material
Multidisciplinary Sciences
Sergei D. D. Odintsov
Editorial Material
Multidisciplinary Sciences
Sergei D. Odintsov
Article
Astronomy & Astrophysics
Sebastian Bahamonde, Johann Chevrier, Jorge Gigante Valcarcel
Summary: In the framework of Metric-Affine Gravity, the correspondence between the Einstein tensor and the energy-momentum tensor of matter is extended towards a post-Riemannian description in terms of torsion and nonmetricity fields. The dynamical role of the traceless part of the nonmetricity tensor and its connection with shears are investigated, resulting in a new black hole solution with shear charges. The extension in the presence of dynamical torsion and Weyl vector leads to the broadest family of black hole solutions with spin, dilation, and shear charges in Metric-Affine Gravity so far.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
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.
Review
Astronomy & Astrophysics
Sebastian Bahamonde, Daniela D. Doneva, Ludovic Ducobu, Christian Pfeifer, Stoytcho S. Yazadjiev
Summary: In this paper, new scalarized black holes are discovered by coupling a scalar field with the Gauss-Bonnet invariant in teleparallel gravity. The usual Gauss-Bonnet term in four dimensions decays into two distinct boundary terms, the teleparallel Gauss-Bonnet invariants, when described in the language of torsion. These boundary terms can be coupled individually or in combinations to a scalar field, resulting in a teleparallel Gauss-Bonnet extension of the teleparallel equivalent of general relativity. The theory provides a natural extension with new phenomenology, where scalarization is triggered by torsion and exhibits distinct behavior compared to the usual Einstein-Gauss-Bonnet case.
Article
Astronomy & Astrophysics
Sebastian Bahamonde, Georg Trenkler, Leonardo G. Trombetta, Masahide Yamaguchi
Summary: In this paper, an analog version of Horndeski gravity is formulated in a symmetric teleparallel geometry assuming vanishing curvature and torsion. The theory can be recast as a sum of the Riemannian-Horndeski theory and purely teleparallel terms. The most general k-essence extension of symmetric teleparallel Horndeski gravity is found, as well as a novel theory containing higher-order derivatives acting on nonmetricity while respecting the second-order conditions. The FLRW cosmological equations for the model are also presented.
Article
Physics, Particles & Fields
Sebastian Bahamonde, Konstantinos F. Dialektopoulos, Manuel Hohmann, Jackson Levi Said, Christian Pfeifer, Emmanuel N. Saridakis
Summary: This study focuses on the cosmological perturbation theory in f(T) gravity, which is a simple extension of the teleparallel equivalent of general relativity. The authors examine the possibility of a non-flat FLRW background solution and perform perturbations for different spatial geometries. They determine the behavior of the perturbative modes in this non-flat FLRW setting for arbitrary f(T) models and identify propagating modes.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Particles & Fields
Kimet Jusufi, Salvatore Capozziello, Sebastian Bahamonde, Mubasher Jamil
Summary: We use observational data to constrain a black hole solution of extended teleparallel gravity models based on the S2 star orbiting the Galactic Center. We construct the shadow images of the Sgr A* black hole and find that the shadow radius increases with the parameter alpha. The observables, such as the deflection angle and angular diameter, are close to the Schwarzschild case. However, the electromagnetic intensity observed in the shadow images is smaller compared to General Relativity (GR), indicating differences in the Born-Infeld f(T) gravity.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Astronomy & Astrophysics
Sebastian Bahamonde, Shokoufe Faraji, Eva Hackmann, Christian Pfeifer
Summary: The paper investigates Thick accretion disks in the framework of Born-Infeld teleparallel gravity and compares them with the Schwarzschild solution in general relativity. An observational bound on the model parameter is found.
Article
Physics, Particles & Fields
Sebastian Bahamonde, Laur Jarv
Summary: This paper investigates the coincident gauge in static spherically symmetric spacetime configurations and provides the corresponding metrics. It is found that different connections result in different values for the boundary term, and simple arguments about the coincident gauge are not sufficient to uniquely determine the connection.
EUROPEAN PHYSICAL JOURNAL C
(2022)
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.
