Gravitation and Cosmology最新文献

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.

We make some considerations regarding the time delay of two light signals around a closed line, the well-known Sagnac effect. In our opinion, this experiment in flat space-time is simply a further test of Einstein’s equivalence principle. The revolving observer is physically equivalent to an observer at rest in a Lense-Thirring-like geometry.

We consider the C-metric as a gravitational field configuration that describes an accelerating black hole in the presence of a semi-infinite cosmic string, along the accelerating direction. We adopt the expression for the gravitational energy-momentum developed in the teleparallel equivalent of general relativity (TEGR) and obtain an explanation for the acceleration of the black hole. The gravitational energy enclosed by surfaces of constant radius around the black hole is evaluated, and in particular, the energy contained within the gravitational horizon is obtained. This energy turns out to be proportional to the square root of the area of the horizon. We find that the gravitational energy of the semi-infinite cosmic string is negative and dominant for large values of the radius of integration. This negative energy explains the acceleration of the black hole that moves towards regions of lower gravitational energy along the string.

The previously formulated mathematical model of a statistical system with scalar interaction of fermions and the theory of gravitational-scalar instability of a cosmological model based on a one-component statistical system of scalarly charged degenerate fermions ((mathfrak{M}_{1}^{c}) models), has led to the possibility of black hole formation in the early Universe using the mechanism of gravitational-scalar instability, which ensures the exponential growth of perturbations. The evolution of spherical masses in the (mathfrak{M}_{1}^{c}) model, as well as the evolution of black holes with allowance for their evaporation, is studied. Arguments in favor of the possibility of black hole formation in the early Universe with the help of the proposed mechanism is given, and a numerical model is constructed that confirms this reasoning. The range of parameters of the (mathfrak{M}_{1}^{c}) model, which ensures the growth of black hole masses in the early Universe up to (10^{4}{-}10^{6}M_{odot}), is identified.

We demonstrate that the latest constraints on inflationary observables, namely, the tensor-to-scalar ratio (r) and the scalar spectral index (n_{S}) from the Cosmic Background Radiation (CMB) observations are already strong enough to rule out the model of a scalar field with a power law potential even in the presence of kinetic coupling to gravity with a positive coupling constant. The case for a negative coupling constant needs a special treatment.