Article
Astronomy & Astrophysics
J. Kluson, B. Matous
Summary: This note demonstrates the application of the Weyl-De Donder formalism, also known as covariant Hamiltonian formalism, to F(R)-gravity, deriving covariant Hamiltonians and corresponding equations of motion.
GENERAL RELATIVITY AND GRAVITATION
(2021)
Article
Physics, Multidisciplinary
Mithun Bairagi
Summary: This paper investigates the impacts of Rmu nu Rmu nu and R-4 gravity corrections on the general relativity-based cosmological model, and applies perturbation theory to analyze these effects. The study finds that these corrections have significant influences on the observed results, and also validates the effectiveness of the generalized second law of thermodynamics under these gravity theories.
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
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
Bofeng Wu
Summary: A signature invariant geometric algebra framework for spacetime physics is formulated in this paper, including techniques related to relative vector and spacetime split, as well as rotor techniques for Lorentz transformations and relativistic dynamics of a massive particle in curved spacetime. The paper also presents a three-dimensional analogue of Newton's second law for a particle in curved spacetime, and provides a geometric algebra approach to gyroscopic precession in Lense-Thirring spacetime for a gyroscope moving.
SCIENTIFIC REPORTS
(2022)
Article
Mechanics
Paolo Pezzutto, Victor I. Shrira
Summary: This paper discusses the weakly nonlinear water-wave evolution over a horizontal bottom and introduces the advantages of using the integro-differential Zakharov equation, which utilizes the Hamiltonian structure, over the direct use of the Euler equations. However, applying the Zakharov equation to finite-depth situations is challenging due to the singular kernels governing the four-wave interactions and the canonical transformation. The paper shows that the absence of a limit of the kernels at the singularities is inconsequential and demonstrates that the integrals involving trivial manifold singularities can be evaluated uniquely.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Astronomy & Astrophysics
Adrian Casado-Turrion, Antonio Dobado, Alvaro de la Cruz-Dombriz
Summary: This study investigates gravitational collapse in the context of f(R) theories of gravity. By generalizing the Oppenheimer-Snyder model to fit within metric f(R) gravity, novel exterior solutions are found, and it is verified that certain vacuum metrics cannot represent spacetime outside a collapsing dust star in metric f(R) gravity. Additionally, it is discovered that the Oppenheimer-Snyder model is incompatible with the junction conditions of the Palatini formulation of f(R) gravity.
Article
Astronomy & Astrophysics
Yonadav Barry Ginat
Summary: This paper discusses the consideration of small-scale inhomogeneities in modeling the universe on a cosmic scale, introducing a multiple-scales method based on singular perturbation theory and deriving an effective Einstein equation. Conditions are established to ensure that deviations from the large-scale space-time metric do not grow unboundedly, and it is shown that these conditions are satisfied in the case of non-relativistic matter.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Yoelsy Leyva, Giovanni Otalora
Summary: This article studies inflation and the generation of primordial fluctuations in f (R) gravity's rainbow. The cosmological perturbations and the scalar and tensor primordial power spectrum are calculated. The predictions of the model are contrasted with current observational data from PLANCK and BICEP/Keck. New results for the scalar spectral index ns and the tensor-to-scalar ratio r are found, along with new observational constraints on the rainbow functions.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Physics, Multidisciplinary
Uday Chand De, Sameh Shenawy, H. M. Abu-Donia, Nasser Bin Turki, Suliman Alsaeed, Abdallah Abdelhameed Syied
Summary: The main objective of this paper is to investigate spacetimes that admit concircular curvature tensor in f(R) gravity theory. Firstly, concircularly flat and concircularly flat perfect fluid spacetimes in f R gravity are studied, obtaining the expressions of the isotropic pressure and the energy density. Next, some energy conditions are considered. Finally, perfect fluid spacetimes with divergence free concircular curvature tensor in f(R) gravity are studied, and several results are obtained.
FRONTIERS IN PHYSICS
(2022)
Article
Physics, Multidisciplinary
M. Farasat Shamir, Adnan Malik, G. Mustafa
Summary: This manuscript discusses the wormhole geometry in the f(R) theory of gravity, exploring stability and behavior using various distributions and equations, confirming the existence of Gaussian and Lorentzian non-commutative wormholes in the theory.
CHINESE JOURNAL OF PHYSICS
(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
Lakhan V. Jaybhaye, Snehasish Bhattacharjee, P. K. Sahoo
Summary: This paper aims to recreate the gravitational baryogenesis epoch in the framework of the f (R, Lm) theory of gravity, where R and Lm are the curvature scalar and the matter Lagrangian, respectively. We examine the model, f (R, Lm) = R2 + L & alpha;m + & zeta;, under the supposition that the universe is saturated with dark energy and perfect fluid, with a non-zero baryon to entropy ratio during a radiation dominance era. Our study found that f (R, Lm) gravity can considerably and steadily make a contribution to the phenomenon of gravitational baryogenesis. & COPY; 2023 Elsevier B.V. All rights reserved.
PHYSICS OF THE DARK UNIVERSE
(2023)
Article
Astronomy & Astrophysics
Ping Li, Rui Jiang, Jian Lv, Xianghua Zhai
Summary: In this paper, the perturbations of charged static spherically symmetric black holes in the f(R)=R-2 alpha root R model by a scalar field are studied. The results show the stability of the black hole and the properties of the superradiant modes.
Article
Astronomy & Astrophysics
G. Fanizza, G. Marozzi, M. Medeiros, G. Schiaffino
Summary: Inspired by the fully non-linear Geodesic Light-Cone (GLC) gauge, a new cosmological perturbation theory is built and the connection with standard cosmological perturbation theory is explored through gauge transformations related to a new set of perturbations. In addition, various definitions for gauge invariant combinations are utilized, with emphasis on the relationships with linearized GLC gauge perturbations. The GLC gauge is shown to offer advantages in terms of providing simple expressions for light-like observables and matching the proper time of a free-falling observer.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
