Gravitation and Cosmology最新文献

We study gravitational baryogenesis in an isotropic and homogeneous universe in the framework of general relativity. We investigate a new exact solution of Einstein’s field equations for the FRW metric. Our solution represents a transitioning model of the universe which was expanding in a decelerated mode and passes on to an accelerated mode after dominance of the cosmological constant (Lambda). We observe that gravitational baryogenesis occurs in the derived universe, and the derived baryon-entropy ratio is in good agreement with its observational value.

Nonlinear quantization of the domain wall (relativistic membrane of codimension 1) is considered. The membrane dust equation is considered as an analogue of the Hamilton–Jacobi equation, which allows us to construct its quantum analogue. The resulting equation has the form of a nonlinear Klein–Fock–Gordon equation. It can be interpreted as the mean field approximation for a quantum domain wall. Dispersion relations are obtained for small perturbations (in a linear approximation). The group speed of perturbations does not exceed the speed of light. For perturbations propagating along the domain wall, in addition to the massless mode (as in the classical case), a massive one appears. The result may be interesting in condensed matter theory and in membrane quantization in superstring and supergravity theories.

We consider the metric (f(R)) gravity model for the four-dimensional metric tensor depending on two variables, time and one spacelike coordinate. We obtain exact analytical vacuum solutions for different forms of the function (f(R)), which are solutions of cosmological type. These solutions are expressed in terms of arbitrary functions, which, under certain conditions, can be chosen as new variables.

In their Letter [Phys. Rev. Lett. 126, 101102 (2021); arXiv: 2010.07317], J.L. Blázquez-Salcedo, C. Knoll, and E. Radu have constructed a very interesting class of wormhole solutions in general relativity (GR), supported by a pair of classical charged spinor fields obeying the Dirac equation. The main new feature of these solutions is that such Dirac spinor fields can possess exotic properties, necessary for the existence of static wormhole configurations in GR. The present note contains a few remarks clarifying some points concerning this approach.

One of the most interesting proposals of dark energy model is Pilgrim Dark Energy (PDE). This model is based on the assumption that the rapid expansion of the Universe is due to a phantom-like repulsive force that is capable to prevent black hole formation. In this study, we explore this phenomenon in the framework of Loop Quantum Gravity motivated cosmology by taking the Generalized Ghost version of PDE. An interaction between the constructed DE model and cold dark matter has also been considered. We evaluate the cosmological parameters, such as the Hubble and deceleration parameters, the equation of state parameter (omega_{G}), the squared speed of sound and also cosmological planes (omega_{G})–(omega^{prime}_{G}) (where (omega^{prime}_{G}) is a derivative of (omega_{G}) with respect to the scale factor (a)) and the statefinder parameters (r) and (s) on the basis of the Generalized Ghost version of PDE, the parameter (u) and the interaction parameter (delta). The trajectories of these cosmological diagnostic parameters give evidence that our constructed model can explain the phantom regime as well as the quintessence regime in the late Universe. Also, from a stability analysis, we see that our model is classically stable.

Based on the mathematical model of a statistical system with scalar interaction of fermions, formulated earlier, a cosmological model based on a two-component statistical system of scalarly charged degenerate fermions interacting with an asymmetric scalar doublet of canonical and phantom scalar fields, is investigated. The asymptotic and limiting properties of the cosmological model are investigated, and it is shown that amon g all models there is a class of those with finite lifetime. The asymptotic behavior of the models near the corresponding singularities is investigated, and numerical implementations of such models are constructed.

We discuss vacuum static, spherically symmetric asymptotically flat solutions of the generalized hybrid metric-Palatini theory of gravity (generalized HMPG) suggested by Böhmer and Tamanini, involving both a metric (g_{munu}) and an independent connection (hat{Gamma}_{munu}^{alpha}); the gravitational field Lagrangian is an arbitrary function (f(R,P)) of two Ricci scalars, (R) obtained from (g_{munu}) and (P) obtained from ({hat{Gamma}}_{munu}^{alpha}). The theory admits a scalar-tensor representation with two scalars (phi) and (xi) and a potential (V(phi,xi)) whose form depends on (f(R,P)). Solutions are obtained in the Einstein frame and transferred back to the original Jordan frame for a proper interpretation. In the completely studied case (Vequiv 0), generic solutions contain naked singularities or describe traversable wormholes, and only some special cases represent black holes with extremal horizons. For (V(phi,xi)neq 0), some examples of analytical solutions are obtained and shown to possess naked singularities. Even in the cases where the Einstein-frame metric (g^{E}_{munu}) is found analytically, the scalar field equations need a numerical study, and if (g^{E}_{munu}) contains a horizon, in the Jordan frame it turns to a singularity due to the corresponding conformal factor.

The mechanism of generation of dark matter and dark radiation from the evaporation of primordial black holes is very interesting. We consider the case of Kerr black holes to generalize previous results obtained in the Schwarzschild case. For dark matter, the results do not change dramatically, and the bounds on warm dark matter apply similarly: in particular, the Kerr case cannot save the scenario of black hole domination for light dark matter. For dark radiation, the expectations for (Delta N_{textrm{eff}}) do not change significantly with respect to the Schwarzschild case, but for an enhancement in the case of spin 2 particles: in the massless case, however, the projected experimental sensitivity would be reached only for extremal black holes.

The flows of phase trajectories in cosmological models based on the asymmetric scalar Higgs doublet are investigated. It is shown that phase flows tend to split into several diverging flows. In this case, initially close phase trajectories can diverge significantly over time, which indicates the instability of the model with respect to finite perturbations.

We present exact solutions to the Friedmann equation in standard (Lambda)CDM cosmology in Weierstrass and Jacobi functions. The right-hand side of the Friedmann equation, describing various contributions of matter sources, is considered in a generic form. It is proved that the problem of integration of the Friedmann equation for simple equations of state of a medium is reduced to solving Abel integrals for algebraic functions.