Gravitation and Cosmology最新文献

We study (eta)-Ricci–Yamabe solitons and gradient (eta)-Ricci–Yamabe solitons in generalized Robertson–Walker space-times . At first, we provide an example of an (eta)-Ricci–Yamabe soliton. Next, we prove that if a generalized Robertson–Walker space-time admits an (eta)-Ricci–Yamabe soliton or a gradient (eta)-Ricci–Yamabe soliton, then it becomes a perfect fluid space-time. As a consequence, we obtain several interesting corollaries.

A new representation for the field strength of the quasiclassical gravitational field of a continuous medium is proposed, which makes it possible to describe the effect of hidden mass or dark matter in a new way. The connection of the new representation with the properties of a continuous medium and the role of these properties in the formation of the hidden mass effect are discussed. Based on the results obtained, a new model of the structure of disk galaxies and their own evolution under dynamic equilibrium conditions is being investigated. A general classification of possible types of spatial structures of disk galaxies is given. The existence conditions of disk galaxies with bulges and jets, as well as ring structures, are considered. A qualitative analysis of possible variants of the galaxies’ own evolution and the conditions of their spatial oscillations is carried out.

We present a cosmological model that incorporates a minimally coupled canonical scalar field, dark energy, and dark matter components of the cosmos within the context of general relativity (GR) based gravity. Using the first integral method, we solve the homogeneous scalar field equation with the Higgs potential and obtain some new analytical solutions for the scalar field (phi). These solutions are effective in explaining the Universe’s late-time acceleration phase. We use the (chi^{2})-minimization technique to compare our theoretical findings with a variety of observational data in order to evaluate the validity of this theoretical model. We make use of the observational data from three compilations of the Type Ia supernovae (SN Ia) dataset: the Union 2.1 compilation, the joint light-curve analysis (JLA), and the Pantheon sample. We calculate the current values of some significant cosmological parameters, such as the present values of the Hubble parameter (((H_{0})) and the deceleration parameter (((q_{0}))), and these values are in good agreement with the most recent observational evidence. As a function of redshift (z), we also look at the evolution of the Equation of State parameters ((w_{textrm{DE}}(z)) and (w_{phi}(z))), the density parameters, the potential ((V(z))), and the deceleration parameter ((q(z))).

We discuss a class of solutions of multidimensional gravity which are formally related to black-hole solutions but can observationally look like compact stars whose surface reflects back all particles or signals getting there. Some particular examples of such solutions are presented and studied, including those with a magnetic field in Maxwell or nonlinear electrodynamics (NED) in five dimensions. For NED as a possible source for magnetic mirror stars, we formulate a methodology of solving the 5D Einstein-NED equations and point out the conditions under which there always exist mirror star solutions. We also note that some of the Einstein–Maxwell solutions under consideration are discussed in the literature and called “topological stars” due to the circular topology of the fifth dimension.

The present paper focuses on relativistic magneto-fluid space–time in the framework of (F(R)) gravity theory. We characterize the relativistic magneto-fluid space–time stuffing in (F(R))-gravity and obtain the expression for the Ricci tensor, scalar curvature, and the equation of state. Also, by taking a Ricci soliton as its metric, we acquire the conditions under which the soliton shrinks, remains steady or grows when taking Killing and torse forming vector fields. We also establish the emergence of a black hole and a trapped surface outside the black hole and argue that the trapped surface is completely surrounded by the event horizon in the context of a relativistic magneto-fluid space–time stuffed in (F(R))-gravity when it admits a shrinking Ricci soliton by putting restrictions on the scalar function (omega) and the first derivative of (F(R)). It is also shown that when the space–time admits a gradient Ricci soliton as its metric, the gravitational dynamics is solely influenced by the magnetic field strength, magnetic permeability and density of the magnetic fluid, which further affects the total pressure on the considered space–time.

In this paper, I emphasize those features of the extended phase space approach to quantization of gravity that distinguish it among other approaches. First of all, it is the conjecture on a nontrivial topology of the Universe which was supported by Wheeler, Hawking, and other founders of quantum gravity. However, this conjecture appears to be in contradiction with the assumption on asymptotic states that is used in the path integral quantization of gauge theories. The presence of asymptotic states ensures gauge invariance of the theory, but, in the case of gravity, these states exist only in asymptotically flat space–times, which restricts possible topologies. Then we have two ways. The first way is to consider only asymptotically flat space–times. In fact, it reduces quantum gravity to quantum field theory in a given background. The second way is to reject the assumption on asymptotic states. In the case of a nontrivial topology, one cannot cover the whole space–time with a single coordinate system. One has to introduce various reference frames fixed by different gauge conditions in different space–time regions. The Hamiltonian describing a gravitating system will depend on gauge conditions. It leads to the conclusion that a unitary evolution may be broken down. This conclusion cannot be obtained in approaches based on the Wheeler–DeWitt equation or making use of the assumption on asymptotic states. The assessment of this conclusion is given.

We propose the theory of Master space-Teleparallel Supergravity ((widetilde{textrm{MS}}_{p})-TSG), subject to certain rules, as a local extension of the author’s recent theory of global MS({}_{p})-SUSY [1]. The latter reviews the physical processes underlying the standard Lorenz code of motion and its deformation tested in experiments for ultra-high energy cosmic ray and TeV-(gamma) photons observed. A local MS({}_{p})-SUSY theory was originally conceived as a theory of (widetilde{textrm{MS}}_{p})-supergravity (SG). The action of the simple (widetilde{textrm{MS}}_{p})-SG theory includes the Hilbert term for a fictitious graviton coexisting with a fictitious gravitino (sparticle) described by the Rarita–Scwinger kinetic term. Using Palatini’s formalism extended in a plausible fashion to this theory, we reinterpret the flat (widetilde{textrm{MS}}_{p})-SG theory with Weitzenböck torsion as the theory of (widetilde{textrm{MS}}_{p})-TSG having the gauge translation group in the tangent bundle. The Hilbert action here vanishes, and the gravitino action loses its spin connections, so that the accelerated reference frame has a Weitzenböck torsion induced by gravitinos. The action of (widetilde{textrm{MS}}_{p})-TSG is invariant under the Poincaré supergroup and under diffeomorphisms. The Weitzenböck connection defines the acceleration through force equation, with torsion (or contortion) playing the role of force. The accelerated particle mechanics in 4D Minkowski space–time is discussed. We develop a general deformation of the flat master space (MS({}_{p}towidetilde{textrm{MS}}_{p})), and show that the occurrence of inertial effects is clearly caused by that. We supplement the (widetilde{textrm{MS}}_{p})-TSG theory by considering the consequences for the Newtonian limit, the uniform acceleration field and the relativistic inertial force in Minkowski and semi-Riemannian spaces. The Weak Equivalence Principle (WEP) is a consequence of the theory.

In order to evaluate this work, cosmic matter in the form of diffusive barotropic fluid has been used to represent the flat FLRW Universe. The cosmic scalar field ((phi_{textrm{diffusion}})) is taken as a diffusion process, causing the diffusive fluid to dissipate. Also, three kinds of parametrizations of energy densities are introduced. A suitable hybrid scale factor is used to construct a four-parameter model. To approximate the cosmological parameters, the (chi^{2})-test is introduced. To find the nature of the expanding universe, the cosmological parameters are analyzed. The ultimate fate of the universe can be predicted through this model.

In the context of (F(T,T_{G})) gravity, we examine the reconstruction scenario of the Tsallis holographic dark energy (THDE) model where (T) stands for the torsion scalar, and (T_{G}) stands for the teleparallel Gauss–Bonnet term. The Hubble constant (H_{0}) and the deceleration value (q) are two crucial elements that govern the expansion history of the Universe in power-law cosmology. Using the Markov Chain Monte Carlo (MCMC) technique with the likelihood function, we obtain the sets of constraints as (H_{0}=70.93^{+0.0024}_{-0.0037}) km/s/Mpc, (q=-0.3384^{+0.0027}_{0.0026}) for the Pantheon data and (H_{0}=70.76732^{+0.0030}_{-0.00088}) km/s/Mpc, (q=-0.6478^{+0.0093}_{0.0075}) by using the joint data (OHD (+) BAO (+) Pantheon), respectively. We find that the considered data fit well with the power-law cosmology at late times. We also use the bulk-viscosity component to solve the modified Einstein’s field equations, (xi=xi_{0}+xi_{1}H+xi_{2}(dot{H}+H^{2})). We determine the universe’s current age to be (13.618) Gyr, which is in line with the WMAP data. We additionally discussed the (Om(z)) parameter of the derived model.

According to the Křížek–Somer hypothesis [New Astron. 17, 1 (2012); Grav. Cosmol. 21, 59 (2015)], a biological evolution of the Earth is possible only at certain values of the Hubble parameter, because the increasing luminosity of the Sun should be compensated by the increasing orbital radius of the Earth due to the local Hubble expansion, thereby keeping the Earth’s surface temperature sufficiently stable. Here, we examine this hypothesis in light of the recent data on the surface temperature on the early Earth, thereby imposing a few constraints on the admissible values of the local Hubble parameter. As follows from our analysis, the Křížek–Somer mechanism might be a valuable tool to resolve the important geophysical and paleontological puzzles, but the particular value of the local Hubble parameter is substantially affected by the current uncertainties in our knowledge about the temperature and other properties of the early Earth.