Gravitation and Cosmology最新文献

We first prove the existence of the gradient Ricci–Yamabe soliton (briefly GRYS) by constructing an explicit example endowed with the Robertson–Walker metric. Then we focus on the physical properties of the gradient Ricci–Yamabe solitons satisying Einstein’s field equations, under the assumptions of different subspaces of Gray’s decompositions. For instance, we prove that if a GRYS space-time satisfying Einstein’s field equations, in which the gradient of the potential function (psi) is a unit-timelike torse-forming vector field, belongs to the subspaces (mathcal{B}) and (mathcal{B}^{prime}), then it is a Robertson–Walker space-time with vanishing shear and vorticity. Moreover, its possible local cosmological structures are of Petrov types I, D, or O. Finally, we obtain the equations of state of a perfect-fluid space-time admitting the GRYS whose velocity field is a unit-timelike Killing vector field.

We study the question of whether the instability of the vertical perturbation mode of a self-gravitating disk can lead to the formation of a certain ring-shaped structure. Thus, we have discovered two modes of vertical perturbations. The gravitational instability of these perturbation modes is investigated below against the background of a radially pulsating disk model, which is a nonstationary generalization of the well-known Bisnovaty-Kogan–Zeldovich equilibrium model. The corresponding nonstationary analogues of the dispersion equation of these two modes of vertical perturbations are found. The results of the study are presented in the form of critical dependencies of the initial virial ratio on the disk rotation parameter. It is shown that with an increase in the disk’s degree of rotation, there decreases the range of the initial virial ratio at which ring structures can form. The initial physical conditions, criteria and the corresponding mechanisms for the formation of ring-shaped structures due to the instability of the bending mode of the vertical perturbation are determined. A comparative analysis of the gravitational instability increments of the annular horizontal and vertical perturbation modes is also carried out.

The Damour–Solodukhin wormhole (hereinafter DSWH) is known to mimic a Schwarzschild black hole (hereinafter SBH) horizon in some properties. To act as a mimicker, the DSWH parameter (lambda) by definition is required to be extremely tiny, i.e., (lambdasim 0). Our comparative analysis shows that such a requirement may be too restrictive at least as far as the Bondi accretion profiles of the two objects, the DSWH and SBH, are concerned. Intriguingly, it turns out that some profiles of a DSWH mimic those for the SBH near the horizon even at values of (lambda) considerably higher, i.e., (lambdasim 1).

We propose a theory of gravity in Minkowski space. The basic idea is that the presence of mass affects the world line of particles in flat Minkowski background. Although General Relativity is in complete agreement with observations so far, the theory discussed in this paper can solve some of the conceptual difficulties of GR, such as the energy problem, that is at the core of constructing this theory. We also discuss the cosmic expansion of the present universe and comment on the early universe. Furthermore, we demonstrate how the flatness problem and horizon problem are solved within this framework. As it will become clear, this is a theory of principle and not a theory of construction.

We study the evolution of a QCD ghost dark energy model under two branches of the DGP braneworld, and this model is distinguished from the (Lambda)CDM model by diagnostic methods of Statefinder hierarchy and Om((z)). Through the derivation of the evolution equation of the energy density parameters, the deceleration parameters and the equation-of-state (EoS) parameter, it can be proved that in both noninteractive and interactive scenarios (specifically, including (Q_{1}=3Hxirho_{textrm{de}},Q_{2}=3Hxirho_{textrm{dm}},Q_{3}=3Hxi(rho_{textrm{de}}+rho_{textrm{dm}}))), this model can well describe the evolution rule of today’s universe. In the later stage of the evolution of the universe, the main component of the universe changed from dark matter to dark energy, and the universe gradually transitioned from decelerating expansion to accelerating expansion, and it will not end up with a big rip in the future. And in the self-accelerating branch of the DGP braneworld, the accelerated expansion of the universe occurred earlier. In order to distinguish the QCD model from the (Lambda)CDM model, we adopted two diagnostic methods, namely, the Statefinder diagnostic and the Om diagnostic. From their respective diagnostic images, it can be seen that these two diagnostic methods cannot only effectively distinguish the QCD model from (Lambda)CDM, but also directly reflect that the coupling parameters (xi) have a certain impact on the dark energy model. It can also eliminate the degeneracy of different coupling parameters under the same interaction.

This article deals with an investigation of projective collineations in pseudo-symmetric type space-times. First we deduce that if a pseudo-Ricci symmetric space-time admits a projective collineation whose associated vector field is a special projective vector field, then the space-time represents a stiff matter fluid. Also, we establish that if a pseudo-projective symmetric space-time admits a projective collineation whose associated vector field is a projective vector field, then the space-time becomes an Einstein space-time. Finally, we illustrate that if a pseudo-projective symmetric space-time admits a non-affine projective vector field, then the space-time becomes a perfect fluid space-time.

We study the existence of a local classical solution to the Einstein-Scalar equations in higher dimensions. We reduce the problem to a single first-order integro-differential equation. Then, we employ the contraction mapping in the appropriate Banach space. Using the Banach fixed theorem, we show that there exists a unique fixed point, which is the solution to the main problem. Finally, for given initial data, we prove the existence of a local classical solution.

We investigate the implications of the modified gravity theory (f(R,L_{m})) on the cosmological evolution. By examining the nonlinear model (f(R,L_{m})={R}/{2}+(alpha R+1)L_{m}), we explore the impact of a nonminimal coupling between curvature and matter on the cosmic expansion. Using a parametrized deceleration parameter dependent on the redshift (z), we analyze the Friedmann–Lemaître–Robertson–Walker (FLRW) universe in the (f(R,L_{m})) framework. Through observational constraints derived from Cosmic Chronometers (CC), Type Ia Supernovae (SNIa), and Baryon Acoustic Oscillations (BAO), we perform a detailed comparison with the standard (Lambda)CDM model. Our results show that the (f(R,L_{m})) model is consistent with observational data, but deviations from the (Lambda)CDM model emerge in its geometric structure, highlighting the potential of (f(R,L_{m})) gravity in explaining the dark energy and cosmic acceleration.

Cosmological models based on an asymmetric scalar Higgs doublet (the canonical (Phi) and phantom (phi) fields) with potential interaction between the components are proposed. A qualitative analysis of the corresponding dynamic systems is performed, and their transformation properties with respect to similarity transformations of fundamental constants are revealed. The asymptotic behavior of this class of cosmological models near cosmological singularities is investigated. Numerical simulations reveal a number of interesting features of these models, in particular, the possibility of a fairly long “waiting phase,” during which the Universe is almost Euclidean, as well as the presence of bounce points, near which there occur strong oscillations of the scalar potentials.

A mathematical model of the evolution of the Universe, based on an asymmetric doublet of classical and phantom scalar Higgs fields with a kinetic connection between the components, is constructed and studied. A detailed qualitative analysis is carried out, the properties of the modelCs symmetry and invariance with respect to the similarity transformation of fundamental constants are proven. The principles of numerical modeling are formulated, and an example of numerical modeling of the model evolution is given for a specific set of fundamental constants and initial conditions. The asymptotic behavior of the model near cosmological singularities is studied, and It is shown that it then manifests itself as a perfect fluid with an extremely rigid equation of state.