Narrow Results

This research paper aims to redefine the complexity factor in $f(R,G)$ gravity and the appearance of electric charge, where $R$ is the Ricci scalar and $G$ is the Gauss–Bonnet term. In this context, we intend to analyze the splitting of the Riemann tensor after considering the anisotropic distribution of the charged fluid related to the spherically symmetric spacetime. We interpret $Y_{\mathrm{\text{TF}}}$ as the complexity factor among all the determined structural scalars that encompasses the characteristics of anisotropic pressure and the effective representation of the energy density. The correction terms associated with modified theory are considered to calculate some significant results related to the Weyl scalar, Tolman mass, and the complexity factor (CF). Moreover, the expression for CF is established by using the structure scalars determined in our paper, and the diminishing complexity restraint is utilized to determine the solutions for the various models. The celestial object having non-uniform energy density and anisotropic pressure asserts the maximum intricacy. But, if the effects of non-uniform energy density and anisotropic distribution of pressure are eradicated due to the presence of dark source terms associated with modified gravity then these fluids may not exhibit any complexity. Consequently, it is revealed that the constituents of effective and electromagnetic parts directly influence the structure scalars and CF.

A de-Sitter gauge theory of the gravitational field is developed using a spherical symmetric Minkowski space–time as base manifold. The gravitational field is described by gauge potentials and the mathematical structure of the underlying space–time is not affected by physical events. The field equations are written and their solutions without singularities are obtained by imposing some constraints on the invariants of the model. An example of such a solution is given and its dependence on the cosmological constant is studied. A comparison with results obtained in General Relativity theory is also presented.

We indicate a limit of the post-Newtonian gravity equations with incompressible-fluid matter source in which Newtonian gravity is approximately decoupled from relativistic gravitomagnetic effects. In this gravito-magneto-hydrodynamic limit, we perform fully developed turbulence calculations. We demonstrate that gravitomagnetic effects reduce the vortical complexity and nonlinearity of turbulence, even leading to its extinction within large volumes, and generate departures from Kolmogorov turbulence scalings, that are explained via a combination of dimensional and exact analysis arguments.

With the increasing popularity of online social networking platforms, the amount of social data has grown exponentially. Social data analysis is essential as spamming activities and spammers are escalating over online social networking platforms. This paper focuses on spammer detection on the Twitter social networking platform. Although existing researchers have developed numerous machine learning methods to detect spammers, these methods are inefficient for appropriately detecting spammers on Twitter due to the imbalance of spam and nonspam data distribution, the involvement of diverse features and the applicability of data mechanisms by spammers to avoid their detection. This research work proposes a novel hybrid approach of the gravitational search algorithm and the decision tree (HGSDT) for detecting Twitter spammers. The individual decision tree (DT) algorithm is not able to address the challenges as it is unstable and ineffective for the higher level of favorable data for a particular attribute. The gravitational search algorithm (GSA) constructs the DTs with improved performance as the gravitational forces act as the information-transferring agents through mass agents. Moreover, the GSA is efficient in handling the data of higher dimensional search space. In the HGSDT approach, the construction of the DT and splitting of nodes are performed with the heuristic function and Newton’s laws. The performance of the proposed HGSDT approach is determined for the Social Honeypot dataset and 1KS-10KN dataset by conducting three different experiments to analyze the impact of training data size, features and spammer ratio. The result of the first experiment shows the need of a higher proportion of training data size, the second experiment signifies the more importance of textual content-based features compared to the other feature categories and the third experiment indicates the requirement of balanced data to attain the effective performance of the proposed approach. The overall performance comparison indicates that the proposed HGSDT approach is superior to the incorporated machine learning methods of DT, support vector machine and back propagation neural network for detecting Twitter spammers.

The possibility of parity violation through space–time torsion has been explored in a scenario containing fields with different spins. Taking the Kalb–Ramond (KR) field as the source of torsion, an explicitly parity violating U(1) _EM gauge-invariant theory has been constructed by extending the KR field with a Chern–Simons term.

A covariant scheme for material coupling with GL(N,R) gauge formulation of gravity is studied. We revisit a known idea of a Yang–Mills type construction, where quadratical power of cosmological constant has to be considered in consistence with vacuum Einstein's gravity. Then, matter coupling with gravity is introduced and some constraints on fields and background appear. Finally, exploring the N=3 case we elucidate that introduction of auxiliary fields decreases the number of these constraints.

Milgrom has proposed that the appearance of discrepancies between the Newtonian dynamical mass and the directly observable mass in astronomical systems could be due to a breakdown of Newtonian dynamics in the limit of low accelerations rather than the presence of unseen matter. Milgrom's hypothesis, modified Newtonian dynamics or MOND, has been remarkably successful in explaining systematic properties of spiral and elliptical galaxies and predicting in detail the observed rotation curves of spiral galaxies with only one additional parameter — a critical acceleration which is on the order of the cosmologically interesting value of cH₀. Here we review the empirical successes of this idea and discuss its possible extension to cosmology and structure formation.

A mechanism of formation of a planet from a gaseous cloud and dust is proposed. The particles of the clouds are combined into clusters and the gravitational contraction of the system is modeled using central gravitational field. Initially the clusters are randomly distributed in the space, and during the contraction process they form hollow spherical layers. The thermodynamic approach considers each cluster as a volume comprising an initial number of the particles to give the temperature of the layers. This method has been used to model the formation of the Earth at early stages. Our results demonstrate that the gravitational contraction leads to quantization of the structure and formation of spherical shells. Particularly, the Earth consists of at least three spherical layers of high density.

Various TeVeS-inspired and f(R)-inspired theories of gravity have added an interesting twist to the search for dark matter and vacuum energy, modifying the landscape of astrophysics day by day. These theories can be together called a Non-uniform Dark Energy fluid (a Nu-Lambda fluid or a VΛ fluid); a common thread of these theories, according of an up-to-date summary by HZL¹, is a non-uniform vector field, describing an uneven vacuum energy fluid. The so-called "alternative" gravity theories are in fact in the standard GR gravity framework except that the cosmological "constant" is replaced by a nontrivial non-uniform vacuum energy, which couples the effects of Dark Matter and Dark Energy together by a single field. Built initially bottom-up rather than top-down as most gravity theories, TeVeS-inspired theories are healthily rooted on empirical facts. Here we attempt a review of some sanity checks of these fast-developing theories from galaxy rotation curves, solar system constraints, and gravitational lensing. We will also discuss some theoretical aspects of these theories related to the vacuum energy, and point out some analogies with electromagnetism and the Casimir effect.

We discuss the effects of non-inertial motion in reactions occurring in laboratory, stars, and elsewhere. It is demonstrated that non-inertial effects due to large accelerations during nuclear collisions might have appreciable effects nuclear and atomic transitions. We also explore the magnitude of the corrections induced by strong gravitational fields on nuclear reactions in massive, compact stars, and the neighborhood of black holes.

We derive the field equations and the equations of motion for scalar fields and massive test particles in modified theories of gravity with an arbitrary coupling between geometry and matter by using the Palatini formalism. We show that the independent connection can be expressed as the Levi–Cività connection of an auxiliary, matter Lagrangian dependent metric, which is related with the physical metric by means of a conformal transformation. Similarly to the metric case, the field equations impose the nonconservation of the energy–momentum tensor. We derive the explicit form of the equations of motion for massive test particles in the case of a perfect fluid, and the expression of the extra-force is obtained in terms of the matter-geometry coupling functions and of their derivatives. Generally, the motion is non-geodesic, and the extra force is orthogonal to the four-velocity. It is pointed out here that the force is of a different nature than in the metric formalism. We also consider the implications of a nonlinear dependence of the action upon the matter Lagrangian.

We consider Kerr–Newman–AdS–dS black holes near extremality and work out the near-horizon geometry of these near-extremal black holes. We identify the exact U(1)_L×U(1)_R isometries of the near-horizon geometry and provide boundary conditions enhancing them to a pair of commuting Virasoro algebras. The conserved charges of the corresponding asymptotic symmetries are found to be well-defined and nonvanishing and to yield central charges c_L≠0 and c_R = 0. The Cardy formula subsequently reproduces the Bekenstein–Hawking entropy of the black hole. This suggests that the near-extremal Kerr–Newman–AdS–dS black hole is holographically dual to a non-chiral two-dimensional conformal field theory.

The prevailing view in modern cosmology is that the universe is comprised of immense quantities of exotic materials (i.e. Dark Matter and Dark Energy) that have yet to be positively identified. However, there is also a small group of scientists who believe that the answer to this dilemma is to be found in the modification of gravity (i.e. General Relativity). This short paper states that if we make the bold assumption that all objects/observers are comprised of sets of spacetime coordinates that change (albeit slowly) as the universe ages, then three puzzles that currently confront cosmologists, astronomers and astrophysicists can easily be answered using relatively simple calculations. The condition necessary to explore this possibility can be obtained if one postulates that relativistic gravitational potential lessens (in absolute magnitude) everywhere as the universe ages (n). That is, the spacetime metric g_μν(x)→g_μν(x, n). If gravity behaves in this manner, then it can be shown that it is the causitive agent of indeterminism in nature.

The paper studied classical oscillations of background metric in the multidimensional anisotropic model of Kazner in the de-Sitter stage and obtained dependence of fluctuations on dimension of spacetime with infinite expansion. Stability of the model could be achieved when the number of spacelike dimensions equals to or more than four. We calculated the contributions to the density of "vacuum energy", that are provided by proper oscillations of background metric and compared with contribution of cosmological arising from particles due to expansion. As it turned out, contribution of gravitational oscillation of metric into density of "vacuum energy" should play a significant role in the de-Sitter stage.

In the paper, multidimensional anisotropic metric and density of vacuum energy in the Kasner's model are investigated. It is shown that the presence of scalar field in model is equivalent to metric in the spacetime with additional dimensions and we propose the idea of generating additional dimensions by massless scalar field. We propose a method of renormalization of metric that describes conversion from spacetime with scalar field to higher-dimensional spacetime. We obtain the expression for cosmological constant which depends on the initial conditions for anisotropic metric coefficients. Using the method of Bogolubov, we investigate the influence of anisotropic metric onto the cosmological birth of particles and obtain the effective mass of scalar field depending on the cosmological constant.

We conjecture that weak interactions are peculiar manifestations of quantum gravity at the Fermi scale, and that the Fermi constant is related to the Newtonian constant of gravitation. In this framework one may understand the violations of fundamental symmetries by the weak interactions, in particular parity violations, as due to fluctuations of the spacetime geometry at a Planck scale coinciding with the Fermi scale. As a consequence, gravitational phenomena should play a more important role in the microworld, and experimental settings are suggested to test this hypothesis.

One way to make the variational principle based on the Einstein–Hilbert action well-defined (i.e. functionally differentiable) is to add a surface term involving the integral of the trace of the extrinsic curvature. I provide a simple derivation of this result which is constructive in the sense that it starts from the variation of Einstein–Hilbert action and obtains the correct boundary term. This is to be contrasted with the usual derivations in which one first adds this term and then shows that the unwanted parts cancel out in the variation of the total action. The approach described here also clearly identifies the variables that need to be fixed in the boundary as the three-metric, directly from the action principle.

In 1680, Cassini proposed oval curves as alternative trajectories for the visible planets around the sun. The Cassini ovals were of course overshadowed by the Kepler's first law (1609), namely the planets move around the sun describing conic orbits. Here, we describe the possibility that the Cassini's idea works at larger or smaller scales. Indeed, we consider the Spiric curves (which are a generalization of the Cassini oval) and present the first steps towards a Spiric gravitational theory. We show that from our formalism an ellipse associated with a planet can be obtained as a particular case.

An application of the quantum N-portrait to the Universe is discussed, wherein the spacetime geometry is understood as a Bose–Einstein condensate of N soft gravitons. If near or at the critical point of a quantum phase transition, indications are found that the vacuum energy is partly suppressed by 1/N, as being due to quanta not in the condensate state. Time evolution decreases this suppression, which might have implications for cosmic expansion.

We examine in detail the cosmology based on quantal (Bohmian) trajectories as suggested in a recent study [A. F. Ali and S. Das, Phys. Lett. B741, 276 (2014)]. We disagree with the conclusions regarding predicting the value of the cosmological constant $\mathrm{\Lambda }$ and evading the Big Bang singularity. Furthermore, we show that the approach of using a quantum corrected Raychaudhuri equation (QRE), as suggested in A. F. Ali and S. Das, Phys. Lett. B741, 276 (2014), is unsatisfactory, because, essentially, it uses the Raychaudhuri equation (RE), which is a kinematical equation, in order to predict dynamics. In addition, even within this inconsistent framework, the authors have adopted unjustified assumptions and carried out incorrect steps leading to doubtful conclusions.