Gravitation and Cosmology最新文献

We investigate the cosmic progression of the Universe in the framework of Barber’s second self-creation (BSC) theory of gravitation in a non-interacting scenario with dark energy and dark matter. Using a Bianchi Type I metric in the self-creation of gravitation, we obtain the field equations that account for the presence of a cosmological constant. By considering the relationship between the average scale factor and Barber’s scalar field as a power law, solutions to these equations have been successfully obtained. Two assumed cosmic dynamics in the form of exponential expansion and power-law expansion are used to get the dark energy equation-of-state parameter. The effect of a polytropic equation of state on the physical quantities is studied. The role of the deceleration parameter and the effect of cosmic anisotropy on the cosmic dynamics are discussed.

We study the role of (f(Q)) gravity in the context of homogeneous and isotropic space–time having flat spatial sections. We explore cosmological scenarios compatible with the observational data of the cosmic chronometer and supernovae Ia Pantheon sample in the (f(Q)) gravity framework. In particular, we study the constant deceleration and jerk parameter model with a generalized Hubble parameter ansatz to derive the Lagrangian of (f(Q)) gravity. We solve the field equations to reconstruct (f(Q)) gravity for a universe composed of matter and dark energy. In this way, the parameters of the present model are fully determined by the observations.

In the scientific literature of recent years, the processes of generation of axion-like particles by various configurations of electromagnetic fields and waves under astrophysical conditions have been studied. Much attention to these issues is due to the fact that in some cosmological models massive axion-like particles are considered as dark matter, and massless particles as dark energy. And although these particles have not yet been found experimentally, the study of the processes of emission of axion-like particles by electromagnetic fields and waves in astrophysical conditions is relevant to the construction of new cosmological models.

In this paper,photogeneration of massless axions, called arions, is studied during propagation of magnetodipolar electromagnetic radiation from a rotating pulsar or magnetar in the Coulomb field of a charged particle. It is shown that, as a result of the interaction of these electromagnetic fields, arions should arise, having a frequency that coincides with the frequency of the pulsar’s magnetodipole radiation.

An exact solution to the equation for the arion field is obtained, and the angular distribution is found for the arion fluxes emitted as a result of their photogeneration on Coulomb particles in the magnetospheres of pulsars and magnetars throughout space. It is shown that there is no forward emission of arions in the process under study.

The dynamics and physical structure of Saturn’s large inner moons (Rhea, Dione, Tethys, Enceladus, and Mimas) lead to conflicting evidence for determining their origin and age. The evolution of their orbits has been particularly difficult to understand. A theoretical approach based on tidal migration via resonances appears to have some success. However, the tidal paradigm, as the sole explanation for the evolution of orbits, leads to a situation where astronomical complexity increases far more rapidly than its ability to solve a problem. Suggestions that the moon system evolved through collisions in the distant past are little more than speculation. The local Hubble–Lemaître flow provides a simple explanation for the outward migration of these moons as well as the recession of Titan, Saturn’s largest moon. The Saturn moon system provides further evidence that the cosmological expansion operates on objects in the Solar System. The existing paradigm, which asserts that gravitationally bound systems cannot experience the Hubble–Lemaître flow, is called into question by the ever-increasing observational data.

We derive differential identities in the domain of Einstein nonsymmetric geometry. We introduce a local form of the nonsymmetric linear connection of a totally skew-symmetric torsion, which has been constructed and published in a global form in a previous work. The local form of this connection is expressed in terms of the symmetric and skew-symmetric parts of the nonsymmetric metric tensor as well as their derivatives. We apply the Dolan–McCrea variational scheme using a nonsymmetric metric tensor (G_{munu}). We demonstrate our rigorous proof via analyzing the generalized second-order metric tensor (G_{munu}) to its symmetric and skew-symmetric parts, respectively. The derived differential identities can be split into two differential identities such that one identity is expressed in terms of the symmetric part of (G_{munu}), and the second one in terms of the skew part of (G_{munu}). One of the demonstrated differential identities can be considered as a generalization of the second Bianchi identity. This identity is reduced to the conventional Riemannian one in the case of using the Ricci scalar.

We investigate a Bianchi-type VI cosmological model that integrates magnetized strange quark matter (MSQM) into the context of (f(R,L_{m})) gravity, where (R) denotes the Ricci scalar, and (L_{m}) signifies the matter Lagrangian. The model examines a nonlinear functional expression of (f(R,L_{m})=R/2+lambda R^{2}+alpha L_{m}), resolved by a linearly variable deceleration parameter. We derive cosmological parameters from redshift, including the Hubble parameter, and compare the model predictions with the empirical Hubble dataset values. We also examine the energy conditions within the model in detail, particularly, the effects of dark energy. This paper offers an extensive examination of the physical and geometrical characteristics of the universe while illustrating the significance of strange quark matter, magnetic fields, and dark energy in modified gravity theories. Some of the most important things we found were new information on how the deceleration parameter, anisotropy, and shear scalar behave. This helps us to understand how the universe is expanding in modified theories of gravity.

A group-theoretic method for constructing symmetric isometric embeddings is used to describe all possible four-dimensional surfaces in flat ((1,9))-dimensional space, whose induced metric is static and spherically symmetric. For such surfaces, we propose a classification related to the dimension of the elementary blocks forming the embedding function. All suitable 52 classes of embeddings are summarized in one table and analyzed for the unfolding property (which means that the surface does not belong locally to some subspace of the ambient space), as well as for the presence of smooth embeddings of the Minkowski metric. The obtained results are useful for the analysis of the equations of motion in the Regge–Teitelboim embedding gravity, where the presence of unfolded embeddings of the Minkowski metric is essential.

This study uses a specific ansatz within (f(Q)) gravity theory to examine the characteristics of spherically symmetric anisotropic compact stars. The research aims at enhance comprehension of these atypical entities by examining the physical properties of the compact star model using the Durgapal geometry within the context of (f(Q)) gravity. The (f(Q)) gravity field equations are resolved using the Karmarkar condition, and the physical characteristics such as density and pressure are computed for the resulting solution. The energy criteria are fulfilled, and the TOV equations perform the equilibrium analysis. The physical study indicates that our models meet the requirements for a well-behaved stellar system. This indicates that our solution is suitable for modeling astrophysical objects.

There are so many ideas that potentially explain the dark energy phenomenon, current research is focusing on a more in-depth analysis of the potential effects of modified gravity on both local and cosmic scales. In this paper we investigate some cosmic reconstructions in (f(Q)) cosmology, where (Q) is the nonmetricity corresponding to the evolution background in the Friedmann–Lemaître–Robertson–Walker (FLRW) universe. This allows us to determine how any FLRW cosmology can emerge from a particular (f(Q)) theory. We employ the reconstruction technique to generate explicit formulations of the (f(Q)) Lagrangian for several types of matter sources like a perfect fluid, a dustlike fluid, stiff fluid and a binary mixture of two fluids. Furthermore, we compute the field equations and the equation of state (EoS) parameter (omega) for two different reconstructed (f(Q)) models with variation of the involved constants, which gives a scenario of an accelerating universe, a quintessence region and the cosmological constant. We also observe that the time dependence of (omega) admits cosmic acceleration. These new (f(Q)) gravity inspired models may have an impact on gravitational phenomena at other cosmological scales.

We study an inhomogeneous cosmology in Kaluza–Klein space-time with a positive cosmological constant in a dust dominated era ((p=0)). Depending on an integration constant, we have derived two types of solutions. The dimensional reduction of the extra-dimensional scale factor is possible due to an inhomogeneity depending on the curvature of the metric for the positive cosmological constant in all solutions. The high value of entropy in the present observable universe and the possible matter leakage in 4D world due to reduction of the extra dimension are also discussed. Our solutions show an early decelerating and late accelerating nature of the universe. The findings are verified by the well-known Raychaudhuri equation.