Gravitation and Cosmology最新文献

A gravitational potential has the spherical property when the field outside any uniform spherical shell is indistinguishable from that of a point mass at the center. We present the general potentials that possess this property on constant curvature spaces, using the Euler–Poisson–Darboux identity for spherical means. Our results are consistent with known findings in flat three-dimensional space and reduce to Gurzadyan’s cosmological theorem when the rescaling factor is exactly 1. Our approach naturally extends to nontrivial spatial topologies.

A compact Hamiltonian formulation of metric gravity is developed, investigated and applied to some relevant problems. This new Hamiltonian formulation is closely related (by some canonical transformations of the dynamical variables) to the two known and only successful Hamiltonian formulations of the metric gravity proposed earlier (by Dirac and by the author’s group). In reality, our set of natural and canonical dynamical variables in metric gravity allows one to reduce both canonical and total Hamiltonians to very compact expressions, each being a pure quadratic form in the spatial momenta (p^{mn}). Analytical formulas for the primary and secondary constraints are also compact and physically transparent. In general, our new approach transforms the traditional Hamiltonian formulations of metric gravity into a simple procedure which allows one to solve a number of long-standing problems.

The main topic of this paper is to characterize gradient (eta)-Ricci–Yamabe solitons on warped product manifolds. At first, we establish necessary and sufficient conditions under which a warped product manifold admits gradient (eta)-Ricci–Yamabe solitons. Next, we deduce the condition under which a warped product manifold is conformal to a sphere in Euclidean space. Finally, we obtain some results in GRW space-times.

We consider static, spherically symmetric configurations of nonlinear electromagnetic fields with Lagrangians (L(f)), where (f=F_{munu}F^{munu}), in general relativity (GR) and other metric theories of gravity. The corresponding exact solutions are well known in the framework of GR in cases where only an electric charge ((q_{e})) or a magnetic charge ((q_{m})) are present, but only a few solutions in particular examples of (L(f)) are known for dyonic systems with both nonzero (q_{e}) and (q_{m}). We study the properties of such systems in the special case of equal electric and magnetic charges and, assuming a correct Maxwell limit of (L(f)), show that there always exists such a configuration of the electromagnetic field that the invariant (f) is zero in the whole space. It leads to the existence of the corresponding families of solutions both in GR in the presence of other sources of gravity (like fluids or scalar fields) and in a wide range of extended theories of gravity (e.g., scalar-tensor and (f(R)) gravity), in which the nonlinear electromagnetic field behaves in an especially simple manner.

We investigate the growth of cosmic structures within the framework of modified gravity described by the functional form (f(R,L_{m})), where (R) is the Ricci scalar and (L_{m}) the matter Lagrangian. This class of models introduces a nonminimal coupling between matter and curvature, leading to modifications in both the background dynamics and the evolution of linear density perturbations. We derive the modified Friedmann equations, obtain an explicit expression for the Hubble parameter (H(z)), and formulate a corrected linear growth equation with properly defined effective gravitational coupling (G_{text{eff}}). The observable quantity (fsigma_{8}(z)) is then computed and compared with 23 redshift-space distortion (RSD) measurements from major surveys including 6dFGS, BOSS, eBOSS, WiggleZ, VIPERS, and FastSound. The model parameters are constrained using a combined analysis of Hubble parameter data, Pantheon({}^{+}) supernovae, DESI BAO, and the Planck 2018 CMB shift parameter, employing chi-square minimization and MCMC sampling. The results show that the (f(R,L_{m})) model fits the growth data slightly better than (Lambda)CDM, particularly at intermediate and high redshifts, while naturally predicting a mild suppression of late-time structure growth. This feature helps alleviate the (sigma_{8}) tension between low- and high-redshift observations. Our findings highlight the consistency of (f(R,L_{m})) gravity with current cosmological datasets and underscore its potential as a viable alternative to (Lambda)CDM. We also discuss prospects for constraining such matter-curvature coupling models with future high-precision surveys such as Euclid and LSST.

This research paper studies almost Riemann solitons and almost gradient Riemann solitons on space-times obeying the (f(r,T))-gravity field equations. First, we obtain the divergence of the potential vector field of an almost Riemann soliton in terms of the scalar curvature on a perfect-fluid space-time in (f(r,T))-gravity. We then discuss almost Riemann solitons on special perfect fluids (dark fluid, dust, radiation). As a natural generalization, we extend the framework to anisotropic-fluid and field-based matter sources (e.g., scalar and electromagnetic fields), indicating how the soliton structure equations adapt in (f(r,T))-gravity beyond the perfect-fluid ansatz.

We present a higher-dimensional cosmological model with a generalized Chaplygin-type gas to explain the late-time acceleration of the Universe. The model admits dimensional reduction and reproduces the standard 4D Chaplygin cosmology at (d=0). Using Hubble-57 data, we obtain observational constraints on the model parameters. Due to the highly nonlinear nature of the field equations, the model successfully describes both matter-dominated and accelerating phases of the Universe. By adopting a first-order approximation, we derive exact time-dependent solutions for the scale factors in both ordinary and extra dimensions, along with an exact expression for the flip time. The model approaches the (Lambda)CDM scenario at large scale factors and exhibits the desired transition from deceleration to acceleration. The evolution of the deceleration parameter, effective EoS parameter, and jerk parameter is also discussed. The constrained value of the Chaplygin parameter (alphall 1) disfavors the pure Chaplygin gas model ((alpha=1)) and suggests that smaller values of (alpha) are favored at late times.

We perform a comprehensive analysis of thermodynamic fluid space-time. First we describe a 4D relativistic thermodynamic fluid space-time. Next we explore a new class of semi-Riemannian manifolds which are Ricci-symmetric and derive Codazzi type Ricci tensor on sequential warped product thermodynamic fluid space-time. Furthermore, we show that the warping functions for a sequential warped product thermodynamic fluid space-time are constants under specific conditions. In the latter part, specific curvature aspects in relativistic thermodynamic fluid space-time are studied. We conclude with an elucidation of 4D relativistic thermodynamic fluid space-time through an example.

Bażański’s approach is extended and applied in the framework of Parametrized Absolute Parallelism (PAP) geometry and Einstein–Maxwell geometry. Two novel path deviation equations are deduced. The first equation is derived in the PAP geometry, and it incorporates two parameters. It could be used to investigate the deviation from charged spinning test particles’ trajectories in unified field theories. Three special cases are considered to confirm the viability of this equation. The second equation is deduced in Einstein–Maxwell geometry. It can be employed to investigate the deviation of charged particles from their trajectories in Einstein–Maxwell theory. Both new path deviation equations tend to the geodesic deviation equation under certain conditions.

This article explores the dynamics and stability of cosmological models within the framework of Parameterized Absolute Parallelism (PAP) geometry, with particular emphasis on the effects of spin-torsion interactions. Utilizing the Parameterized Raychaudhuri Equation, we derive stability conditions for fixed points and examine their influence on the evolution of the universe’s expansion scalar. Our findings indicate that the parameter (b) associated with spin-torsion coupling plays a pivotal role: when it fosters stability, it prevents singularities and facilitates smooth exponential expansion reminiscent of inflation. Conversely, when (b<1), instability and divergence occur. This framework also addresses singularity avoidance through nonsingular bounces and establishes a connection between spin-torsion effects and accelerated expansion, effectively mimicking dark energy. Numerical simulations support these conclusions, illustrating evolution of the scale factor, the expansion scalar, and stability conditions. The results highlight the potential of the PAP geometry in tackling significant cosmological challenges, including singularity resolution and the mechanisms driving accelerated expansion. We apply these findings to a comprehensive model encompassing the Big Bang, Big Rip, and Big Crunch scenarios.