Gravitation and Cosmology最新文献

Using the general solution that we recently obtained for the coordinate-independent definition of a relative velocity of a luminous source as measured along the observer’s line of sight in generic pseudo-Riemannian space-time, in the present article we invoke important implications for test particles and observers in several instructive cases. We consider a test particle as a luminous object, otherwise, if it is not, we assume that a luminous source is attached to it, which has neither mass nor volume. We calculate the relative velocities in special metrics: the Minkowski metric, the test particle and observer at rest in an arbitrary stationary metric, a uniform gravitational field, a rotating reference frame, the Schwarzschild metric, a Kerr-type metrics, and the spatially homogeneous and isotropic Robertson–Walker space-time of the standard cosmological model. In the last case, it leads to a remarkable cosmological consequence that the resulting, so-called, kinetic recession velocity of an astronomical object is always subluminal even for large redshifts of order one or more, so that it does not violate the fundamental physical principle of causality. We also calculate the carrying-away measure of a galaxy at redshift (z) by the expansion of space, which proves, in particular, that the cosmological expansion of a flat 3D space is fundamentally different from the kinematics of galaxies moving in a nonexpanding flat 3D space.

In general relativity, coordinate transformations are often made to simplify calculations, and theoretical predictions are calculated in some specific coordinates. We take the test particle’s motion in Schwarzschild space-time as an example, to illustrate that the solutions for orbit and velocity as well as the time from perihelion to aphelion depend on the coordinates employed for the calculations, even if they are formulated in terms of orbital energy and angular momentum. The aim of this work is to demonstrate that coordinate transformations may change the solutions, and solutions achieved in specific coordinates may not be the final answer and should be mapped into the observer’s reference frame for being compared with observations.

The BSW effect implies that the energy (E_{textrm{c.m.}}) in the center of mass frame of two particles colliding near a black hole can become unbounded. Usually, it is assumed that the particles move along geodesics or electrogeodesics. Instead, we consider another version of this effect. One particle is situated at rest near a static, generally speaking, distorted black hole. If another particle (say, coming from infinity) collides with it, the collision energy (E_{textrm{c.m.}}) in the center of mass frame grows unboundedly (the BSW effect). The force required to keep such a particle near a black hole diverges for nonextremal horizons but remains finite and nonzero for an extremal one and vanishes in the horizon limit for ultraextremal black holes. A generalization to the rotating case implies that a particle corotates with the black hole but does not have a radial velocity. At that, the energy (Eto 0), provided the angular momentum (L) is zero. This condition replaces that of fine tuning of the parameters in the standard version of the BSW effect.

It is shown that under a set of straightforward propositions there exists, at the event horizon and at nonzero radii inside the event horizon of a nonrotating, uncharged, spherically symmetric black hole (BH), under reasonable curvature constraints, a nonempty set of virtual exchange particle modes which can propagate to the black hole’s exterior. This finding reveals that a BH event horizon is not a one-way membrane, but instead a limited two-way membrane. The paper’s technology also permits presentation of what is called virtual cosmic censorship, which requires that the aforesaid virtual exchange particle mode propagation tend to zero at the singularity limit.

We study the properties of roots of a polynomial system of equations which define a set of identical point particles located on a Unique Worldline (UW), in the spirit of the Wheeler–Feynman’s old conception. As a consequence of Vieta’s formulas, a great number of conservation laws are fulfilled for collective algebraic dynamics on the UW. These, besides the canonical ones, include the laws with higher derivatives and those containing multiparticle correlation terms as well. On the other hand, such a “super-conservative” dynamics turns out to be manifestly Lorentz invariant and quite nontrivial. At great values of “cosmic time” (t), the roots-particles demonstrate universal recession (resembling that in the Milne’s cosmology and simulating “expansion” of the Universe), for which the Hubble’s law holds true, with the Hubble parameter inversely proportional to (t).

The main properties of the logamediate inflation driven by a non-canonical scalar field in the framework of DGP braneworld gravity are investigated. Considering high energy conditions, we analytically calculate the slow-roll parameters. Then, we deal with perturbation theory and calculate the most important respective parameters, such as the scalar spectral index and the tensor-to-scalar ratio. We find that the spectrum of scalar fluctuations is always red-tilted. Also, we understand that the running in the scalar spectral index is nearly zero. Finally, we compare this inflationary scenario with the latest observational results from Planck 2018.

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).