Gravitation and Cosmology最新文献

Three physically reasonable static Weyssenhoff fluid sphere models have been obtained by solving the relevant field equations of the Einstein–Cartan theory of gravitation, when Weyssenhoff fluid is the source of spin and gravitation. The spin of the gravitating matter influences the fields of these fluid sphere models. The gravitational field of two of the models is proved to be of Petrov type (D), while the interpretation of the gravitational field of the remaining model fails due to the influence of the spin component (s_{0}). One of the fluid sphere models is accelerating and rotating, while the other two are only rotating. Gravity in each of these models repels and prevents the collapse.

We study modified gravity theory known as the Regge-Teitelboim approach, in which gravity is represented by the dynamics of a surface isometrically embedded in a flat bulk. We obtain some particular solutions of Regge-Teitelboim equations corresponding to a circularly symmetric vacuum 2+1-dimensional space-time. In contrast to GR, this vacuum space-time is not flat, so it is possible for the gravitational field to exist even without matter or a cosmological constant.

The process of the Joule–Thomson adiabatic expansion within rational NED (RNED)-AdS spacetime is investigated. The isenthalpic (P{-}T) diagrams and the inversion temperature are depicted. The inversion temperature depends on the magnetic charge and RNED coupling constant of black holes. When the Joule–Thomson coefficient vanishes, a cooling-heating phase transition occurs. We consider the cosmological constant as a thermodynamic pressure, and the black hole mass is treated as chemical enthalpy.

It is shown that an arbitrary static, spherically symmetric metric can be presented as an exact solution of a scalar-tensor theory (STT) of gravity with certain nonminimal coupling function (f(phi)) and potential (U(phi)). The scalar field in this representation can change its nature from canonical to phantom on certain coordinate spheres. This representation, however, is valid in general not in the full range of the radial coordinate but only piecewise. Two examples of STT representations are discussed: for the Reissner–Nordström metric and for the Simpson–Visser regularization of the Schwarzschild metric (the so-called black bounce space-time).

The dynamics of particles described by the Dirac equation is considered in a homogeneous stationary rotating cosmological model, which is the closest generalization of Goedel’s cosmological model and admits the existence of unclosed timelike lines. It is shown that, in the space-time of the rotating cosmological model, the intrinsic angular momentum of a spinor particle precesses around the axis of rotation, and the angular velocity of rotation of the cosmological model affects the mass of the spinor particle, while the spin magnetic moment of the particles can generate electromagnetic radiation called “spin light.”

We used the absolute parallelism geometry to obtain a new formula for the Ricci scalar. We consider (f(R,Sigma,T)) modified theories of gravity, where the gravitational Lagrangian is given by three arbitrary functions of the Ricci scalar (R), Ricci torsion scalar (Sigma), and the trace of the stress-energy tensor (T). We obtain the gravitational field equations in the metric formalism. The evolution of the function (f(R)) withr time is studied, and we discuss the parameters that make up the function and impose constraints on these parameters. The solution of the (f(R,Sigma,T)) gravity equations are obtained under a varying polynomial deceleration parameter. The effect of torsion on cosmological models is also discussed. Physical aspects of the energy density, pressure, and energy conditions of the cosmological models proposed in this article are studied, and the evolution of the physical parameters is shown in figures. Evolution of the fluid pressure and energy density parameter as a function of redshift has been obtained. The (f(R)) gravity and (f(R,T)) gravity theories as special cases could be inferred from (f(R,Sigma,T)) gravity. Several special cases have been studied, with illustrations for each case.

In terms of the relational approach to space-time geometry and physical interactions, we show that the Dirac equation for a free fermion in the momentum representation can be obtained starting from a binary system of complex relations (BSCR) between elements of two abstract sets. With the derivation performed, we show that the 4-dimensional pseudo-Euclidean momentum space is not needed a priori but naturally emerges from considerations of rather general nature (2-spinor algebra). A bispinor wave function is constructed for a fermion with positive energy and an arbitrary distribution of momenta. Special attention is paid to physical assumptions that should be made to enable the construction.

We consider ((1+8))- and ((1+10))-dimensional Einstein–Gauss–Bonnet models with a cosmological constant. Some new examples of exact solutions are obtained, governed by three non-coinciding constant Hubble-like parameters (Hneq 0), (h_{1}) and (h_{2}), obeying the condition (mH+k_{1}h_{1}+k_{2}h_{2}neq 0), corresponding to factor spaces of dimensions (mgeqslant 3), (k_{1}>1), and (k_{2}geqslant 1). In this case, the multidimensional cosmological model deals with four factor spaces: the external 3D (“our”) world and internal subspaces with dimensions (m-3), (k_{1}), and (k_{2}).

We consider the gravity assist maneuver, that is, a correction of spacecraft motion at its passing near a planet, as a tool for evaluating the Eddington post-Newtonian parameters (beta) and (gamma), characterizing vacuum spherically symmetric gravitation fields in metric theories of gravity. We estimate the effect of variation in (beta) and (gamma) on a particular trajectory of a probe launched from the Earth’s orbit and passing closely near Venus, where relativistic corrections slightly change the impact parameter of probe scattering in Venus’s gravitational field. It is shown, in particular, that a change of (10^{-4}) in (beta) or (gamma) leads to a shift of about 50 km in the probe’s aphelion position.

We use the single-mode coherent and squeezed number state formalism and analyze the nature of a massive homogeneous scalar field minimally coupled to gravity in the framework of semiclassical gravity in the Friedmann-Robertson-Walker (FRW) universe. We have obtained an estimate leading solution to the semiclassical Einstein equation for the FRW universe which shows the scale factor (t^{2/3}) power-law expansion. The mechanism of the particle production and quantum fluctuations are also analyzed in the FRW universe.