Gravitation and Cosmology最新文献

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.

Using integration of the wave equation in time domain, we show that scalar field perturbations around the ((2+1))-dimensional asymptotically flat black hole with Gauss–Bonnet corrections is dynamically stable even for the near-extreme values of the coupling constant.

This paper purports to develop a consistent microscopic theory of deformed Lorentz symmetry and the corresponding deformed geometry. Among the key geometric predictions of this approach, one lies in both the deformed line element (DLE) and the deformed maximum attainable velocity (DMAV) of a particle leading to potentially observable signatures in ultra-high energy astrophysics. In particular, the DMAV has in the past often been tested in a phenomenological approach to cosmic-ray and astrophysical-photon physics in order to extract constraints on those velocities. To this aim, we develop the theory of, so-called, master space (MS({}_{p})) induced supersymmetry, subject to certain rules. We derive the Standard Lorentz Code (SLC) in a new perspective of global double MS({}_{p})-SUSY transformations in terms of Lorentz spinors ((underline{theta},underline{bar{theta}})) referred to MS({}_{p}). The MS({}_{p}), embedded in the background 4D-space, is an unmanifested indispensable individual companion to the particle of interest as the intrinsic property devoid of any external influence. While all particles are living on (M_{4}), their superpartners can be viewed as living on MS({}_{p}). In the sequel, we turn to the deformation of these spinors: (underline{theta}tounderline{tilde{theta}}=lambda^{1/2},underline{theta}), etc., where (lambda) appears as a deformation scalar function of the Lorentz invariance (LIDF). This yields both the DLE and DMAV, respectively, in the form (tilde{ds}=lambda ds) and (tilde{c}=lambda c), provided the invariance of DLE, and the same value of DMAV in free space holds for all inertial systems. Thus the LID (Lorentz invariance deformation) generalization of global MS({}_{p})-SUSY theory formulates the generalized relativity postulates in a way that preserve the relativity of inertial frames, in spite of the appearance of modified terms in the LID dispersion relations. We complement this conceptual investigation with testing of various LIDFs in the UHECR- and TeV-(gamma) threshold anomalies by implications for several scenarios: the Coleman and Glashow-type perturbative extension of SLC, the LID extension of standard model, the LID in quantum gravity motivated space-time models, the LID in loop quantum gravity models, and the LID for the models preserving the relativity of inertial frames.

We give a concise introduction to biquaternionic analysis and the so-called algebrodynamical approach to field theory and highlight some of its connections to twistors, shear-free null congruences and classical field/particle dynamics. We also attempt to extend the analysis to another (“cyclic”) class of solutions to the equations of biquaternionic differentiability and explore some of the properties of the associated congruences and static singularities which allow for the construction of classical models of particles.

The cosmic expansion phenomenon is being studied through the interaction of newly proposed dark energy model (the Modified Rényi holographic dark energy (MRHDE) model) with cold dark matter (CDM) in the framework of Sáez–Ballester (SB) theory of gravitation. To determine the solution of the field equations, the concept of a time-dependent deceleration parameter (DP) is used. The Universe begins with an initial singular state and changes with time from an early deceleration phase to a late acceleration phase. In this paper, it is shown that this expanding solution is stable against perturbations with respect to anisotropic spatial directions. Some important features of the models thus obtained are discussed.

We present the manifest proof of the validity of the local weak and strong energy conditions in all Einstein–Maxwell–Yang–Mills space-times where nonnull electromagnetic and Yang–Mills fields are present. To this end, we make use of the new tetrads introduced previously. These new tetrads have remarkable properties in curved four-dimensional Lorentzian space-times. For example, they diagonalize locally and covariantly any stress-energy tensor in Einstein–Maxwell space-times and also in Einstein–Maxwell–Yang–Mills space-times for nonnull electromagnetic and Yang–Mills fields. We use these properties in order to prove the energy conditions for any space-time with these characteristics.