Gravitation and Cosmology最新文献

The 4D gravitational model with a real scalar field (varphi), Einstein and Gauss–Bonnet terms is considered. The action contains the potential (U(varphi)) and the Gauss–Bonnet coupling function (f(varphi)). For a special static spherically symmetric metric (ds^{2}=(A(u))^{-1}du^{2}-A(u)dt^{2}+u^{2}dOmega^{2}), with (A(u)>0) ((u>0) is a radial coordinate), we verify the so-called reconstruction procedure suggested by Nojiri and Nashed. This procedure presents certain implicit relations for (U(varphi)) and (f(varphi)) which lead to exact solutions to the equations of motion for a given metric governed by (A(u)). We confirm that all relations in the approach of Nojiri and Nashed for (f(varphi(u))) and (varphi(u)) are correct, but the relation for (U(varphi(u))) contains a typo which is eliminated in this paper. Here we apply the procedure to the (external) Schwarzschild metric with the gravitational radius (2mu) and (u>2mu). Using the “no-ghost” restriction (i.e., reality of (varphi(u))), we find two families of ((U(varphi),f(varphi))). The first one gives us the Schwarzschild metric defined for (u>3mu), while the second one describes the Schwarzschild metric defined for (2mu<u<3mu) ((3mu) is the radius of the photon sphere). In both cases the potential (U(varphi)) is negative.

We describe the possible forms of black hole images viewed by a distant observer (or a telescope) on the celestial sphere. These images are numerically calculated based on general relativity and the equations of motion in the Kerr–Newman metric. A black hole image is a gravitationally lensed image of the black hole event horizon. It may be viewed as a black spot on the celestial sphere, projected inside the position of a classical black hole shadow. In the nearest future it will be possible to verify modified gravity theories by observations of astrophysical black holes with Space Observatory Millimetron.

The widely discussed “black-bounce” mechanism of removing a singularity at (r=0) in a spherically symmetric space-time, proposed by Simpson and Visser, consists in removing the point (r=0) and its close neighborhood, resulting in emergence of a regular minimum of the spherical radius that can be a wormhole throat or a regular bounce. Instead, it has been recently proposed to make (r=0) a regular center by properly modifying the metric, still preserving its form in regions far from (r=0). Different algorithms of such modifications have been formulated for a few classes of singularities. The previous paper considered space-times whose Ricci tensor satisfies the condition (R^{t}_{t}=R^{r}_{r}), and regular modifications were obtained for the Schwarzschild, Reissner-Nordström metrics, and two examples of solutions with magnetic fields obeying nonlinear electrodynamics (NED). The present paper considers regular modifications of more general space-times, and as examples, modifications with a regular center have been obtained for the Fisher (also known as JNW) solution with a naked singularity and a family of dilatonic black holes. Possible field sources of the new regular metrics are considered in the framework of general relativity (GR), using the fact that any static, spherically symmetric metric can be presented as a solution with a combined source involving NED and a scalar field with some self-interaction potential. This scalar field is, in general, not required to be of phantom nature (unlike the sources for black bounces), but in the examples discussed here, the possible scalar sources are phantom in a close neighborhood of (r=0) and are canonical outside it.

We investigate quasinormal ringing in both time and frequency domains for scalar and neutrino perturbations around black hole solutions that simultaneously describe regular and extreme configurations in the framework of nonlinear electrodynamics. Two types of solutions are considered: those with de Sitter and Minkowski cores. The quasinormal frequencies obtained from two independent methods exhibit excellent agreement. Furthermore, we derive an analytical expression in the eikonal limit and discuss the validity of the correspondence between the eikonal quasinormal modes and null geodesics.

A coupled model of variable-modified Chaplygin gas (as a candidate of dark energy) and dark matter is assumed in the background of the Friedmann–Lemaitre–Robertson–Walker universe. Firstly, a two-dimensional autonomous dynamical system is considered for a flat universe after determining some suitable dimensionless parameters. The evolution of those parameters along with the deceleration parameter is observed. Next, the evolution of matter and energy density parameters is studied for variational coupling parameters. Phase portrait analysis is performed to explain the present as well as future expansion of the universe. Secondly, both three- and four-dimensional dynamic systems are constructed when the evolution of spatially homogeneous and isotropic model of the universe is considered, which includes the cosmological constant and three-curvature symmetry surfaces. For the three-dimensional system, updates of dimensionless parameters along with deceleration parameters are analyzed for variational coupling parameters with respect to progression of the universe. Again, the evolution of the deceleration parameter is studied for different values of (n). Stability analysis is carried out for the concerned dynamical system with the help of eigenvalues. The four-dimensional dynamical system is remarkable as it helps to study the evolution of (Lambda) density along with other parameters. Lastly, for the concerned dynamical system, the evolution of matter and energy density parameters is analyzed for different values of (c) to study whether or not the coupling parameters affect the ultimate evolution of the universe.

Gravitational quantum theory applied to the weak gravity regions of deep gravitational wells predicts that photon-particle interaction cross sections can vary significantly, depending on the eigenspectral composition of the particle’s wave function. These often-reduced cross sections can potentially enable the nature and origin of dark matter to be understood without recourse to new particles or new physics, and without compromising the observations from nucleosynthesis and the cosmic microwave background. The present work extends the calculations of the Einstein-(A) coefficients relevant to these photon interactions (previously carried out for (1/r) central point-mass (CPM) potentials) to potentials derived from Navarro–Frenk–White (NFW) radial density profiles, which more realistically describe galaxy halos. The Wentzel–Kramers–Brillouin (WKB) and Modified Airy Function (MAF) approximation strategies were used to find the eigenfunctions appropriate to these potentials, and hence obtain the relevant Einstein-(A) coefficients. The results show that states with high principal and angular quantum number in NFW potentials have a significantly low transition rate. The results are also compared to those in the CPM potentials published in an earlier work.

The current study investigates dark energy cosmological models using a boundary term and a non-coincident gauge formulation of nonmetricity gravity. To obtain the modified field equations from the action, we considered the function (f(Q,C)=Q+lambda C^{m}), where (Q) is the nonmetricity scalar, (C) is the boundary term given by (C=mathring{R}-Q), and (lambda,m) are model parameters. The scale factor that we acquired, (a(t)=[sinh(k_{0}t)]^{1/n}), is determined by taking into account the time-dependent deceleration parameter. The constants (n) and (k_{0}) are used in this calculation. By comparing the Hubble function with (H(z)) datasets, we were able to use likelihood analysis to determine the model parameters that best fit the data. We have performed our result analysis and a discussion using the cosmological parameters, including the effective equation-of-state parameter, energy density, energy conditions, deceleration parameter, OM diagnostic analysis, and age of the universe, using these best match values of the model parameters.

We consider the time-symmetric initial data problem for GR minimally coupled to a phantom scalar field and a Maxwell field. The main focus is on initial data sets describing two interacting mouths of the same traversable wormhole. These data sets are similar in many respects to the Misner initial data with two black holes. More precisely, the corresponding solutions of the constraint equations determine the initial geometries which are topologically equivalent to the manifold (mathbb{S}^{2}timesmathbb{S}^{1})-{point} (i.e., ({mathbb{R}^{3}}) with a “handle”) and therefore describe initial states of intra-universe wormholes. Thus the results of the paper can be considered as an input for numerical simulation of such wormholes.

Following Einstein’s idea of representing particles as solitons, i.e., clots of some nonlinear universal field, the Brioschi 16-spinors are introduced since they prove to be well suited for the role of this fundamental field. Taking into account the principle of spontaneous symmetry breaking as the foundation for the stability of particles as topological solitons, the 16-spinor realization of the Skyrme–Faddeev chiral model is suggested. Within the scope of this model, it is possible to describe photons as solitons, the interaction with electromagnetic and gravitational fields being included. The existence of asymptotically exact soliton solutions to the equations of motion is proven, with the special large parameter (tau) being introduced.

In the context of the self-creation theory of gravitation and its counterpart, the relativity theory, we examine their associated Bianchi-type (VI_{0}) cosmological models by considering the existence of electromagnetic fields. The solution of the Einstein equations is presented by assuming that the cosmological model leads to a constant deceleration parameter ((q=textrm{const})). There is no restriction on the pressure and density for the solution derived (i.e., the equation of state is not used). The technique of obtaining the scalar field (phi) is different from that used in the previous investigation. A law that shows the effect of the electromagnetic field on the entropy of the universe is derived. The entropy introduced in the two theories is a consequence of the second law of thermodynamics. Consequently, the well-known thermodynamic functions of the universe are revisited. The scalar field introduced in the self-creation theory give a good explanation for the entropy and other thermodynamics functions of the universe as compared to general relativity. Moreover, in the absence of the electromagnetic field, the solution obtained in the self-creation theory and in general relativity indicate a radiation model, provided that the obtained models, whether expressed geometrically or physically are displayed.