Gravitation and Cosmology最新文献

In physics and cosmology, scalar fields are considered basic. In this study, we are interested to inspect the conduct of massless scalar field (SF) and massive scalar field (MSF) models in (f(R,T)) theory for Bianchi-I universe models. We discuss two cosmological models with respect to late cosmic acceleration, using constant scalar potential and exponential scalar potential models. Also, we study the behavior of a massive scalar field. Finally, we obtain our results in (f(R,T)) and general relativity (GR). In addition, we obtained an LRS Bianchi-I metric as a result of the solutions we made and selection of special constants.

We present the images of binary black holes (BHs) using the Majumdar–Papapetrou multi-BH solution, depending on the parameters of the problem: the BH masses, the distance between them, and the inclination of the observer. The images consist of shadows and photon rings. We find that a photon ring structure appears between the BHs. The trajectories of photons are calculated.

The influence of the gravitational fields of pulsars and magnetars on the emission of arions (strictly massless pseudoscalar Goldstone particles) during propagation of magnetodipole waves in a constant magnetic field has been evaluated. The solution of the equation was obtained, and the flux of arions emitted by magnetodipole waves during their propagation in a constant magnetic field was found. It has been shown that the amplitude of a created arion wave at a distance from the source of magnetodipole radiation of a pulsar or magnetar, ((rtoinfty)), in the considered case tends to a constant value. The intensity of arion emission in a solid angle element and the amount of arion energy (overline{I}), emitted in all directions per unit time grow quadratically with increasing distance traveled by the magnetodipole radiation of a pulsar or magnetar in a constant magnetic field. Such growth of the energy is due to the fact that the constant magnetic field is defined in the whole space. In reality, the galactic and intergalactic magnetic fields can exist in this form only in finite regions of space, outside which the force lines of their induction vector are curved. Therefore, it is possible to apply these results only in a region of space for which (rleq L_{textrm{coh}}<infty), where (L_{textrm{coh}}) is the coherence length, the distance at which the force lines of the induction vector can be considered to be straight. An estimate for the value of the coupling constant of photons with arions is obtained.

Based on the previously formulated theory of spherical perturbations in the cosmological medium of self-gravitating scalarly charged fermions with the Higgs scalar interaction and the similarity properties of such models, the formation of supermassive black hole (SMBH) seeds in the early Universe is studied. Using numerical simulation of the process, it is shown that the mass of SMBH seeds during the evolution process reaches a limiting value, after which it begins to slowly fall. The possible influence of nonlinearity on this process is discussed.

If we look from a quantum perspective, the most natural way in which the universe can be created is in entangled pairs whose time flow is oppositely related. This suggests the idea of the creation of a universe-antiuniverse pair. Assuming the validity of this hypothesis, in this paper, we show that the universe expands in an accelerated manner. The same reasoning holds for the anti-universe as well. This idea does not require any form of dark energy as used in the standard cosmological model (Lambda)CDM or in modified theories of gravity.

We propose noncanonical domain walls as a new dark energy model inspired by Grand Unified theories (GUTs). We investigate the cosmic dynamics and discover that the domain walls act as either dark energy or dark matter at different times, depending on the velocity (v) in the observer’s comoving frame. We find a single stable solution to the dynamics, i.e., only freezing ((v=0)) noncanonical domain walls can enter the phantom zone without having to experience ghost field instability. This means that the solution has the equation of state (EoS) (w_{textrm{dw}}<-1) without having to possess negative kinetic energy. These domain walls give rise to a late-time cosmic acceleration starting from (zapprox 0.24), resulting in (w_{textrm{dw}}=-1.5) and (w_{textrm{eff}}=-1.03) today. We learn that the EoS of the noncanonical domain walls is independent from the potential form. We also investigate the perturbation dynamics following the model. Our simulations show that, compared to (Lambda)CDM, the amplitude of the dark matter power spectrum in the noncanonical domain wall model is lower, while the CMB power spectrum is shifted slightly to lower (l) multipoles. The proposed model gives a smaller (sigma_{8}) as compared to that of (Lambda)CDM.

We investigate the optical behavior of a quantum Schwarzschild black hole with a space-time solution including a parameter (lambda) that encodes its discretization. Specifically, we derive the effective potential of this solution. In particular, we study circular orbits around the quantum black hole. Indeed, we find that the effective potential is characterized by a minimum and a maximum yielding double photon spheres denoted by (r_{p_{1}}) and (r_{p_{2}}). Then, we analyze the double shadow behavior as a function of the parameter (lambda), where we show that it controls the shadow circular size. An inspection of the Innermost Stable Circular Orbits (ISCO) shows that the radius (r_{textrm{ISCO}}) increases as a function of (lambda). Besides, we find that this radius is equal to (6M) for an angular momentum (L=2sqrt{3}) independently of (lambda). A numerical analysis shows that the photon sphere of radius (r_{p_{1}}) generates a shadow with a radius larger than (r_{textrm{ISCO}}). Thus, a truncation of the effective potential is imposed to exclude such behavior. Finally, the (lambda)-effect is inspected depending on the deflection angle of such a black hole, showing that it increases when higher values of the parameter (lambda) are considered. However, such an increase is limited by an upper bound given by ({6M}/{b}).

The process of particle collision in the vicinity of black holes is known to generate unbounded energies in the center-of-mass frame (the Bañados–Silk–West (BSW) effect) under specific conditions. We consider this process in the charged black hole metrics, namely, the Reissner–Nordström (RN) and Majumdar–Papapetrou (MP) metrics. We consider energy extraction from a Bardeen regular black hole due to the BSW effect. As in the RN case, we show that there is no restriction on energy extraction, but for real charged particles this effect is negligible. We derive necessary and sufficient conditions for this process. The conditions for the BSW effect in RN and MP metrics are shown to be identical, which is explained by the asymptotic equivalence of the two metrics near the horizons. Energy extraction in the RN metric is discussed. It is shown that if two real particles collide while falling onto a black hole, they are extremely unlikely to generate an ultra-massive particle. For the case of head-on collisions, we derive an upper bound on the extracted mass, which depends on the lapse function of the metric at the point of collision.

Currently, in order to explain the accelerated expansion phase of the Universe, several alternative approaches have been proposed, among which the most common are dark energy models and alternative theories of gravity. Although these approaches rest on very different physical aspects, it has been shown that both can be in agreement with the data in the current status of cosmological observations, thus leading to an enormous degeneration among these models. Therefore, until evidence of higher experimental accuracy is available, more conservative model-independent approaches are a useful tool for breaking this degenerated cosmological models picture. Cosmography as a kinematic study of the Universe is the most popular candidate in this regard. In this paper, we show how to construct the cosmographic equations for the (f(R,T)) theory of gravity within a conservative scenario of this theory, where (R) is the Ricci curvature scalar, and (T) is the trace of the energy-moment tensor. Such equations relate the (f(R,T)) function and its derivatives at current time (t_{0}) to the cosmographic parameters (q_{0}), (j_{0}), and (s_{0}). In addition, we show how these equations can be written within different dark energy scenarios, thus helping to discriminate among them. We also show how different (f(R,T)) gravity models can be constrained using these cosmographic equations.

In the framework of the Einstein–Maxwell-aether-axion theory, we consider a self-consistent model based on the concept of two-level control, which is carried out by the dynamic aether over the behavior of an axionically active electrodynamic system. The Lagrangian of this model contains two guiding functions, which depend on four differential invariants of the aether velocity: the scalar of expansion of the aether flow, the square of the acceleration four-vector, the squares of the shear and vorticity tensors. The guiding function of the first type is an element of the effective aetheric metric; this effective metric is involved in the formulation of kinetic terms for the vector, pseudoscalar and electromagnetic fields and predetermines features of their evolution. The guiding function of the second type is associated with the distribution of axions and describes its vacuum average value; basically, this function appears in the potential of the axion field and predetermines the position and depth of its minima. The self-consistent set of coupled master equations of the model is derived. An example of a static spherically symmetric system is considered as an application.