Gravitation and Cosmology最新文献

The possibility of geometrization of the gravitational and electromagnetic fields in 4D Finsler space (the Model of Embedded Spaces—MES) is investigated. The model postulates a proper metric set of an element of distributed matter and asserts that space-time is a mutual physical embedding of such sets. The simplest MES geometry is constructed (its relativistic Finsler version) with a connection that depends on the properties of matter and its fields (torsion and nonmetricity are absent). The field hypothesis and the Least Action Principle of the matter-field system lead to Einstein-type and Maxwell-type equations, and their nonlinearity to the anisotropic field contribution to the seed mass of matter. It is shown that the seed matter plays the role of a physical vacuum of the Embedding and determines the cosmological constant. In the special case of a conformal metric, the Maxwell-type equations reduce to the Maxwell equations themselves and a negative electromagnetic contribution. A possible experimental verification of this result is evaluated. The “redshift” effect in an electric field is also mentioned as a method for studying the vacuum and a relic electric charge. A study of the gauge structure of the presented theory is postponed to the future.

The evolution of a quantized electromagnetic mode in a space-time toy model with nontrivial topology, allowing closed timelike and null world lines, is considered. The physical consequences of adopting an ontological or epistemological view on a quantum state are compared. It is done within a framework of two alternative interpretations of mode evolution—Deutsch’s D-CTC model (ontological) and S-CTC model (epistemological). The future states of the mode (with respect to the causal loop) are calculated for two types of interaction with the mode’s previous version coming from the future. The found differences of the predictions may be helpful for building a future fundamental theory unifying quantum physics and gravity.

We study embedding gravity, a modified theory of gravity in which our space-time is assumed to be a four-dimensional surface in flat ten-dimensional space. Based on a simple geometric idea, this theory can be reformulated as general relativity with additional degrees of freedom and a contribution to action which can be interpreted as describing dark matter. We study the canonical formalism for such a formulation of embedding gravity. After solving simple constraints, the Hamiltonian is reduced to a linear combination of four first-class constraints with Lagrange multipliers. There still remain six pairs of second-class constraints. Possible ways of taking these constraints into account are discussed. We show that one way of solving the constraints leads to a canonical system going into the previously known canonical formulation of the complete embedding theory with an implicitly defined constraint.

We present constraints on the deceleration ((q)) and jerk ((j)) parameters using the late-time integrated Sachs-Wolfe effect, type Ia supernovae, and (H(z)) data . We first directly measure the deceleration and jerk parameters using the cosmic chronometers data with the Taylor series expression of (H(z)).However, due to the unusual variations in the deceleration parameter with slight changes in other parameters like snap ((s)) and lerk ((l)), we found that direct measurements using the series expansion of (H(z)) is not a suitable method for non-(Lambda)CDM models, and so we will need to derive the deceleration parameter after constraining the density parameters and dark energy equation of state. Then we present the derived values of the deceleration parameter from the (Lambda)CDM, WCDM and CPL models. We also discuss the transition redshift (z_{t}) in relation with the deceleration parameter. Our best fit values for the deceleration parameter, after combining results from (H(z)), Union 2.1 and NVSS-ISW, are obtained as (-0.5808pm 0.025) for (Lambda)CDM, (-0.61pm 0.15) for both WCDM and CPL models. Our best fit for the combined jerk parameter for the (Lambda)CDM model is (1pm 3.971e-07), for WCDM it is (1.054pm 0.141), and for the CPL model it is (1.0654pm 0.1345). Also, the combined transition redshift is obtained as (0.724pm 0.047) for the (Lambda)CDM model.

A space experiment aimed at specifying the law of the Sun’s gravity more precisely is discussed. An extended “standard flight” scheme with two gravity assist maneuvers (slingshots) of a space probe near Venus and Earth is suggested, where the slingshots serve as amplifiers of small deflections caused by a deviation of the gravity law from a chosen ansatz. The deviation of the probe’s trajectory from its classic (Newtonian) pattern is calculated in detail using the isotropic Eddington–Robertson metric for the Sun’s gravity field and the “patched conic approximation” method for description of each slingshot. The trajectory deviation in the two-slingshots scheme is roughly assessed, the results indicating a principal possibility to detect the ultraweak distinctive feature of relativistic gravity.

We construct a special class of four-dimensional axisymmetric stationary space-times whose Ricci scalar is constant but are not Einstein space-times. We find that this solution has a ring singularity. At the end, we discuss some numerical results for these space-times.

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).