Gravitation and Cosmology最新文献

We study the evolution of the universe using several Rip cosmologies within the framework of (f(R,Sigma,T)) gravity, utilizing spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) models. There are three types of Rip models. Employing the Little Rip (LR) scenario, we derive exact expressions for the pressure, energy density, and the equation of state parameter and investigate their time evolution. Particular emphasis is placed on examining the physical acceptability of the model through the behavior of key quantities such as the energy conditions (NEC, DEC, and SEC) and the transition of pressure from positive to negative values, which naturally explains the shift from decelerated to accelerated expansion. The analysis demonstrates that the energy density remains positive and decreases with cosmic time, while the pressure evolves from a radiation-like regime to a dark-energy-dominated phase. Moreover, violation of the strong energy condition, a necessary feature for late-time cosmic acceleration, is explicitly realized. Our findings indicate that the the LR scenario in (f(R,Sigma,T)) gravity provides a viable description of the late-time universe, avoids finite-time singularities, and offers an alternative explanation to standard dark energy models.

In this study, we characterize a Lorentzian manifold ((M^{n},g)) of dimension (geq 3) satisfying Gray’s (mathcal{C}^{bot}) condition. First, we prove that if a Lorentzian manifold (M) satisfies Gray’s (mathcal{C}^{bot}) condition and whose Ricci curvature annihilates the curvature transformation, then in the neighborhood of a point where (|nabla,r_{g}|neq 0), (M) is a generalized Robertson–Walker (GRW) space-time. Next, it is established that if a quasi-Einstein Lorentzian manifold satisfies Gray’s (mathcal{C}^{bot}) condition, then in the neighborhood of a point where (betaneq 0), (M) is a GRW space-time. Furthermore, it is confirmed that any perfect fluid GRW space-time is Bach-flat.

This paper considers the statistical theory of cosmogonical bodies formation in order to derive equations for the gravitational field of a condensing cosmogonical body. Starting the conception for forming a spheroidal body inside a protoplanetary nebula, this theory solves the problem of gravitational condensation of such a cloud in a view to planetary formation in its own gravitational field. Within the framework of the statistical theory, new models and evolutionary equations of statistical mechanics have been obtained. It is shown that the presence of a dark matter (DM) (or an ethereal medium) around a gravitationally interacting mass serves as a basis for their gravitational interactions and gravitational wave propagation. First, the equation of motion of a “liquid” particle in a “quasi-solid” approximation inside a spheroidal body in gravitational and inertial fields is considered. Using the gravimagnetic vector potential introduced, the equation of motion for a solid particle with DM surrounding in gravitational and gravimagnetic fields is obtained. Taking into account this equation, the first system of equations of the gravitational/gravimagnetic field is derived. Using both the Lagrange function of a moving “liquid” gaseous–ethereal particle (with a “solid” core) in the gravitational/gravimagnetic field and the Lagrange function of the same field, the expression of a total action for the gravitational/gravimagnetic field and matter is obtained. Applying the least action principle and varying only the gravitational potentials together with the anti-diffusion velocity (but not the coordinates), the second system of equations of the gravitational/gravimagnetic field of a condensing spheroidal body is derived. Using these equations, the total energy of the gravitational/gravimagnetic field and gravitating matter as well as the density and gravitational energy flow are estimated. We show that the Newtonian approximation (following from Einstein’s GR) does not fully describe the gravitational field of gravitating masses, while in the framework of the proposed statistical theory the mentioned equation of particle motion in an arbitrary non-inertial frame of reference can be obtained.

In Phys. Rev. D 92, 103004 (2015), simple analytical solutions of the Friedman equations were obtained for a universe having a stiff matter component at early times together with dark matter (DM) and dark energy (DE) components. In this analysis, the universe is considered to be made of a dark fluid which behaves as stiff matter in the early phase of the universe (when the internal energy dominates). It is also more logical to consider quantum gravitational effects in the early phase of the cosmological evolution. In this analysis, following Phys. Rev. D 65, 043508 (2002), we consider renormalization group (RG) improved modified Friedmann equations, where Newton’s gravitational constant (G) and the cosmological constant (Lambda) flow with the momentum scale (k) of the universe. We obtain the momentum scale in terms of the scale factor of the universe when a stiff-matter era is present in the early time regime ((t) being smaller than the Planck time). It is observed that for a universe undergoing a stiff matter era, radiation era, and matter era, inflation is absent in the early time regime when the flow of (G) and (Lambda) is under consideration. Considering the total equation of state as a combination of linear and polytropic equations of state, we observe that after the Planck time the universe can undergo an inflationary phase, and we find out that the inflation is enhanced by quantum gravitational effects. Finally, we propose a new energy density that shows a transition between a primordial stiff matter era followed by an inflation era just after the Planck time, and then a transition from inflation to a radiation dominated reheating phase around a second transition point in this RG improved cosmological model.

The role of horizon area quantization in black hole (BH) thermodynamics is investigated. The coefficient appearing in the quantization of area is fixed by an appeal to the saturated form of Landauer’s principle. Then, by considering a transition between discrete states of the event horizon area, which is in turn equivalent to transitions between discrete mass states of the BH, the change in mass can be obtained. The change in mass is then equated to a product of the Hawking temperature and the change in BH entropy between two consecutive discrete states, applying the first law of BH thermodynamics. This gives the corrected Hawking temperature. In particular, we apply this technique to the Schwarzschild BH, the quantum corrected Schwarzschild BH, the Reissner–Nordström charged BH, and the Kerr rotating Kerr BH geometry, and obtain a corrected Hawking temperature in each of these cases. We then take a step forward by inserting this corrected Hawking temperature in the first law of BH thermodynamics once again to calculate the BH entropy in terms of the BH horizon area. This leads to logarithmic and inverse corrections to the BH entropy.

This paper is dedicated to Professor Ts.I. Gutsunaev, who recently passed away. In our previous paper [1] we constructed a nonlinear superposition of a stationary euclidon solution with an arbitrary axially symmetric stationary gravitational field using the method of variation of parameters. In the same work [1], a stationary soliton solution to the Einstein equations was generalized to the case of a stationary seed metric. The formulas obtained in [1, 2] have a simple, compact structure that enables an effective non-linear “addition” of solutions. In this framework, euclidon solutions serve as building blocks that generate almost all known vacuum, static, axially symmetric solutions to the Einstein equations, including the important Kerr–NUT family. Moreover, the stationary euclidon solution admits a clear physical interpretation as a relativistically accelerated, noninertial reference frame, providing an alternative perspective on the physical meaning of celebrated solutions such as Kerr’s [3].

Quasinormal frequencies in the Einstein–Aether theory have been extensively investigated with the help of numerical methods. Here we derive analytic expressions for quasinormal modes of test scalar, electromagnetic and Dirac fields, using the higher-order WKB method and expansion in terms of (1/ell), where (ell) is the multipole number. The obtained analytic formulas are surprisingly precise and in agreement with previously published numerical data. In addition, we check the validity of the correspondence between null geodesics and eikonal quasinormal modes.

We investigate how the rotation of celestial bodies influences the gravitational redshift of light, utilizing the Kerr metric. We refine this approach by considering rotational effects which include such phenomena as frame dragging that alters the space-time fabric around these objects. We derive an expression for gravitational redshift using the Kerr metric and analyze its variation with respect to rotation and the point of light emission, particularly, between equatorial and polar regions. Our findings, derived from simplified relativistic mechanics about angular momentum, show that gravitational redshift is significantly affected by both the object’s rotation and the position of light emission. By applying our results to celestial bodies such as the Sun and millisecond pulsars, we demonstrate that the gravitational redshift is not uniform across the surface of a rotating body, allowing us to differentiate the redshift originating from different locations on the surface. This variation provides new insights into the role of rotation in gravitational redshift.

We study the cosmic expansion scenario of the universe based on Tsallis, Rényi and Sharma–Mittal holographic dark energy models in the framework of (f(R,L_{m})) gravity theory. A numerical solution of our (f(R,L_{m})) dominated model is obtained, and then combined data sets are utilized to estimate the best-fit values of the free parameters of the model. The behavior of matter-energy density, the equation of state (EoS) parameter, and several other cosmological functions are calculated and plotted against cosmological time. For specific values of the numerical parameters of the models, it is found that our results are in good agreement with recent observations.

We complement the recent theory of Master Space-Teleparallel Supergravity ((widetilde{MS}_{p})-TSG) [1], which reviews the acceleration and inertial effects, with two more consequences. We first address the “locality hypothesis” for extension of Lorentz invariance within Special Relativity to accelerated observers. This replaces the accelerated observer with a continuous infinity of hypothetical momentarily comoving inertial observers along its word line. This assumption is valid only if the curvature of the world line could be ignored. In the general case, this is actually untenable. In the framework of (widetilde{MS}_{p})-TSG theory, the locality hypothesis introduces strict restrictions, replacing the curved (widetilde{MS}_{p}) with the flat MS({}_{p}). Our strategy, therefore, goes beyond the locality hypothesis to recover (widetilde{MS}_{p}) by invoking a general deformation MS({}_{p}towidetilde{MS}_{p}), which, as a corollary, is solely responsible for acceleration and inertia effects. This significantly improves the standard metric and other relevant geometric structures referred to a noninertial frame in Minkowski space-time for relativistic velocities and arbitrary characteristic acceleration lengths. Second, we address the inertial effects in semi-Riemannian and more general post-Riemannian geometries. We derive the relativistic inertial force in semi-Riemannian space, and the inertial force acting on an extended rotating body moving in Riemann–Cartan space. The relativistic Weak Equivalence Principle is a consequence of the theory, at which inertial effects gradually decrease at large Lorentz factors and vanish in the photon limit.