Gravitation and Cosmology最新文献

The Trautman problem determines the conditions under which GWs transfer the information contained in them in an invariant manner. According to the analogy between plane gravitational and electromagnetic waves, the metric tensor of a plane gravitational wave is invariant under the five-dimensional group (G_{5}), which does not change the null hypersurface of the plane wave front. The theorems are proven on the equality to zero for the result of the action of the Lie derivative on the curvature 2-form of a plane GW in Riemann and Riemann–Cartan spaces in the direction determined by the vector generating the group (G_{5}). Thus the curvature tensor of a plane gravitational wave can invariantly transfer the information encoded in the source of the GW.

It is well known that space-time averaging is an operation that does not commute with building the Einstein tensor. In the framework of Macroscopic gravity (MG), a covariant averaging procedure, this noncommutativity gives averaged field equations with an additional correction term known as back-reaction. It is important to explore whether such a term, even if known to be small, may or may not cause any systematic effect for precision cosmology. In this work, we explore the application of the MG formalism to an almost Friedmann-Lemaître-Robertson-Walker (FLRW) model. Namely, we find solutions to the field equations of MG taking the averaged universe to be almost-FLRW modeled using a linearly perturbed FLRW metric. We study several solutions with different functional forms of the metric perturbations including plane-wave ansatzes. We find that back-reaction terms are present not only at the background level but also at the perturbed level, reflecting the nonlinear nature of the averaging process. Thus the averaging effect can extend to both the expansion and the growth of structure in the universe.

Based on 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 two-component statistical system of scalarly charged degenerate fermions, a numerical model of the cosmological evolution of gravitational-scalar perturbations in the presence of classical and phantom scalar fields is constructed and studied. The gravitational-scalar instability at early stages of expansion arises in the model under study at sufficiently large scalar charges, and the instability develops near unstable points of the vacuum doublet. Short-wave perturbations of the free phantom field turn out to be stable at stable singular points of the vacuum doublet. It is shown that for sufficiently large scalar charges, mass perturbations can grow to the values of masses black hole seeds (BHS).

The paper proposes and implements a method of obtaining a closed set of Vlasov–Maxwell–Einstein equations (and its weakly relativistic and nonrelativistic analogues) based on variation of the generalized Hilbert–Einstein–Pauli action. This technique also makes it possible to obtain the exact form of the energy-momentum tensor in terms of particle distribution functions. Using a hydrodynamic substitution in the Vlasov equation, the Euler–Lamb equations are obtained, which can be transformed to the form of Hamilton–Jacobi equations. Exact solutions of cosmological type of the hydrodynamic system are demonstrated, and their physical consequences are analyzed (including a generalization of the Milne–McCrea model).

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.