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The standard model of cosmology (Λ CDM) is facing a serious crisis caused by the inconsistencies in the measurements of some fundamental cosmological parameters (Hubble constant H 0 and cosmic curvature parameter Ω k for example). On the other hand, a strictly linear evolution of the cosmological scale factor is found to be an excellent fit to a host of observations. Any model that can support such a coasting presents itself as a falsifiable model as far as the cosmological tests are concerned. In this article, the observational data of strong gravitational lensing (SGL) systems from SLACS, BELLS, LSD and SL2S surveys has been used to test the viability of linearly coasting cosmology. Assuming the spherically symmetric mass distribution in lensing galaxies, the ratio of angular diameter distance from lens to source and angular diameter distance of the source is evaluated and is used to constrain the power law cosmology. Further, updated type-Ia supernovae data set (Pantheon + ) with covariance matrix incorporating all statistical and systematic uncertainties is used to constrain the power law cosmology. It is found that the linear coasting is consistent with the SGL data within 1- σ uncertainties but Pantheon + sample does not support linear coasting. [ABSTRACT FROM AUTHOR]

Despite having a reasonably successful account of accelerated cosmology, understanding of the early evolution of Universe has always been difficult for mankind. Our promising strategy is based on a novel class of symmetric teleparallel theories of gravity called f(Q), in which the gravitational interaction is caused by the non-metricity scalar Q, which may help to solve some problems. We consider the locally rotationally symmetric (LRS) Bianchi type-I spacetime cosmological models and derive the motion of equations to study the early evolution of the cosmos. By assuming the hybrid expansion law (HEL) for the average scale factor, we are able to determine the solutions to the field equations of Bianchi type-I spacetime. We discuss the energy density profile, the equation of state, and the skewness parameter and conclude that our models preserve anisotropic spatial geometry during the early stages of the Universe with the possibility of an anisotropic fluid present. However, as time goes on, even in the presence of an anisotropic fluid, the Universe may move towards isotropy due to inflation while the anisotropy of the fluid dims away at the same time. It is seen from the squared speed of sound that Universe shows phantom nature at the beginning then approaches to dark energy at present epoch. We analyze both geometrical and physical behaviours of the derived model. [ABSTRACT FROM AUTHOR]

In this article, we have studied the solutions of Einstein–Maxwell field equations for compact objects in the presence of net electric charge. Interior physical 3-space is defined by Vaidya–Tikekar metric in spheroidal geometry. The metric is characterised by two parameters, namely, spheroidal parameter K and curvature parameter R. The nature of the interior fluid is considered to be anisotropic. Assuming strange matter equation of state (EOS) in the MIT Bag model for the interior matter content, namely, p = 1 3 (ρ - 4 B) , where B is the Bag constant, we determine various physical properties of the charged compact star. We have taken the value of surface density ρ s (= 4 B) as a probe to evaluate the mass–radius relation for the compact star in the presence of net electric charge and using the range of B necessary for possible stable strange matter. It is interesting to note that in this model there exist a maximum radius of a star which depends on B. We further note that compactness of the star corresponding to the maximum radius always lies below the Buchdahl limit (< 4 9) for the maximum allowed value of the pressure anisotropy and electromagnetic field. Energy and causality conditions hold good throughout the star in the presence of charge also. Prediction of mass of the strange stars is possible in the present model. We have determined mass, radius, surface red-shift and other relevant physical parameters of the compact objects. [ABSTRACT FROM AUTHOR]

In this manuscript, we have presented the compatible solutions under the framework of dynamical and non-dynamical Chern–Simons modified gravity. By taking an open choice of external field as a function of angular parameter θ , we obtained Gödel-type solutions of non-dynamical Chern–Simons modified gravity. It is mentioned that a vacuum Gödel-type solution exists in this theory. Non-static spherical symmetric solutions are also described for dynamical Chern–Simons modified gravity by considering external field as a function of radial parameter r. Later, results are similar to those of well-known Tolman–Bondi solutions found in the context of general relativity. It is observed that in case of Λ → κ p 0 both parameters A(t, r) and B(t, r) turned to be undefined. [ABSTRACT FROM AUTHOR]

We explore the possibility of using a gigaparsec-scale local void to reconcile the Hubble tension. Such a gigaparsec-scale void can be produced by multi-stream inflation where different parts of the observable universe follow different inflationary trajectories. These trajectories become different parts of the observable universe after inflation, when these scales return to the horizon. If these trajectories have different e-folding numbers, these parts of the universe have different energy densities, possibly creating a local large void. The impacts of such a void for cosmological observations are studied, especially those involving supernovae, Baryon Acoustic Oscillations (BAO) and the kinetic Sunyaev-Zel'dovich (kSZ) effect. We show that with the presence of the void, supernovae observations may be more consistent with the CMB. We also estimate the impacts of a local large void on BAO observations. In addition, we show that a local large void and hence its capabilities to ease the Hubble tension is limited by the kSZ effect. As a benchmark model, a 1.7 Gpc scale void with boundary width 0.7 Gpc and density contrast −0.14 may ease the Hubble tension, evading the kSZ limit. [ABSTRACT FROM AUTHOR]

In the framework of f(R) modified theory of gravity, the dynamical equations of motion of a cylindrical thin shell wormholes supported by a modified generalized Chaplygin gas are constructed, using the cut and paste scheme (Darmois Israel formalism). The mechanical stability analysis of a cylindrical thin shell wormhole is discussed using a linearized radial perturbation around static solutions at the wormhole throat. The presence of stable static solutions depends on the suitable values of some parameters of dynamical shell. [ABSTRACT FROM AUTHOR]

In the framework of Darmois-Israel formalism, the dynamics of motion equations of spherically symmetric thin shell wormholes that are supported by a modified Chaplygin gas in Einstein-Hoffman-Born-Infeld theory are constructed. The stability analysis of a thin shell wormhole is also discussed using a linearized radial perturbation around static solutions at the wormhole throat. The existence of stable static solutions depends on the value of some parameters of dynamical shell. [ABSTRACT FROM AUTHOR]

In a cosmological perspective, gravitational induction is explored as a source to mechanical inertia in line with Mach’s principle. Within the standard model of cosmos, considering the expansion of the universe and the necessity of retarded interactions, it is found that the assumed dynamics may account for a significant part of an object’s inertia.

We herein show that the classical Pound-Rebka (P-R) result is furnished within the framework of Yarman-Arik-Kholmetskii gravitation theory, which we will call YARK in short, in terms of the energy shift of resonant γ-radiation under Earth's gravitational field. Previous successes of YARK in satisfactorily dealing with the precession of the perihelion of Mercury, gravitational redshift, Shapiro delay, lensing effect, anomalous light-bending of gamma quanta, dark energy, accelerated cosmic expansion, analytical derivation of the Hubble constant, and additional time dilation in Mössbauer rotor setups makes it further applicable to past experiments like P-R that has so far been thought to validate the general theory of relativity (GTR). We discuss the P-R outcome of YARK and compare it with what is furnished by GTR. [ABSTRACT FROM AUTHOR]

The spatially homogeneous and anisotropic LRS Bianchi type-V Universe filled with variable modified Chaplygin gas in the framework of the Brans-Dicke (Brans and Dicke. Phys. Rev. 124, 925 (1961). scalar-tensor theory of gravitation has been studied. To obtain a determinate solution of the field equations we have used the condition that scalar field () is a function of average scale factor. Some physical and kinematical properties of the model are discussed. We have also analyzed the stability of the solution. [ABSTRACT FROM AUTHOR]

Analytical models of axially symmetric thin disks of finite extension in the presence of magnetic field are presented based on the well-known Morgan-Morgan solutions. The source of the magnetic field is constructed separating the equation corresponding to the Ampere's law of electrodynamics in spheroidal oblate coordinates. This produces two associated Legendre equations of first order for the magnetic potential and hence that can be expressed as a series of associated Legendre functions of the same order. The discontinuity of its normal derivate across the disk allows us to interpret the source of the magnetic field as a ringlike current distribution extended on all the plane of the disk. We also study the circular speed curves or rotation curve for equatorial circular orbits of charged test particles both inside and outside the disk. The stability of the orbits is analyzed for radial perturbation using an extension of the Rayleigh criterion. [ABSTRACT FROM AUTHOR]

The motion equations of matter in a gravitational field, acceleration field, pressure field, and other fields are considered based on the field theory. This enables us to derive simple formulas in the framework of the gravitational equilibrium model, which allow us to estimate the physical parameters of cosmic bodies. The acceleration field coefficient, η, and the pressure field coefficient, σ, are a function of the state of matter, and their sum is close in magnitude to the gravitational constant, G. In the presented model the dependence is found of the internal temperature and pressure on the current radius. The central temperatures and pressures are calculated for the Earth and the Sun, for a typical neutron star and a white dwarf. The heat flux and the thermal conductivity coefficient of these objects' matter are found, and the formula for estimating the entropy is provided. All the quantities are compared with the calculation results in different models of cosmic bodies. The discovered good agreement with these data proves the effectiveness and universality of the proposed model for estimating the parameters of planets and stars and for more precise calculation of physical quantities. [ABSTRACT FROM AUTHOR]

We continue to analyze the implications of the gravitational framework of our theoretical approach, christened YARK (abbreviated from Yarman-Arik-Kholmetskii), with respect to super-massive celestial bodies. We emphasize in particular that a gravitating test particle in the presence of a ponderable mass must adhere to the law of energy conservation, which remarkably does not yield any singularity according to YARK. Even so, for a given spherically shaped extremely compact super-massive body, one can achieve a theoretical radius below which 'light' of, say, the visible frequency range can indeed be trapped. Yet, such a radius comes out to be tens of times shorter than the threshold radius for black hole formation as established by the general theory of relativity (GTR). In accordance with our derivations, the minimal mass for a celestial object capable of recapturing emitted light in its environs - similar to textbook 'intermediate class black holes' - is found to be about 103 MS, where MS stands for the mass of the Sun. For less massive celestial objects, the crucial radius that produces a 'YARK black hole' (i.e., without singularity) corresponds to a higher density than the density of a baryon; and hence, such entities cannot apparently exist in nature. Black holes allowed therefore in our approach are not related, in any case, to the singularity conceptualization of GTR. As a consequence, we are able to present a resolution to the 'black hole information paradox'. The findings of YARK will be discussed hereinafter with regards to the foundations of observational cosmology. [ABSTRACT FROM AUTHOR]

In this paper we use a dynamic form of modified Newtonian gravity to reformulate the equations for gravitational time dilation. Here we introduce the generic equations for gravitational time dilation. It is shown that these equations agree exactly with gravitational time dilation in satellite navigation systems. The equations are also in agreement with a reanalysis of observations of gravitational red shifts in black hole accretion discs. Using these equations, we translate the time dilation into a finite value at the black hole event horizon. Thus this reformulation resolves the difficulties of the existence of black hole singularities. Importantly these dynamic gravitational equations provide testable predictions in the vicinity of black holes. [ABSTRACT FROM AUTHOR]

We study under what conditions the thermal peeling is present for dissipative local and quasi-local anisotropic spherical matter configurations. Thermal peeling occurs when different signs in the velocity of fluid elements appear, giving rise to the splitting of the matter configuration. The evolution is considered in the quasi-static approximation and the matter contents are radiant, anisotropic (unequal stresses) spherical local, and quasi-local fluids. The heat flux and the associated temperature profiles are described by causal thermodynamics consistent with this approximation. It is found that both types of configurations can exhibit thermal peeling when most of the radiated energy emerges from the first half of the distribution, and thermal peeling appears to be associated with extreme astrophysical scenarios (highly relativistic and very energetic gravitational system). [ABSTRACT FROM AUTHOR]

We investigate wave attributes of hot plasma around a Reissner-Nördstrom-de Sitter (RN-dS) metric in a Veselago medium. A perturbation scheme is implemented on general relativistic magnetohydrodynamical (GRMHD) equations that are further used for Fourier analysis. The linearly perturbed Fourier-analyzed GRMHD equations depict the dispersion of hot plasma waves. It is found that inclusion of charge in de Sitter space greatly affects the wave dispersion. A comparison of wave properties is presented, and results reassert the presence of the Veselago medium. [ABSTRACT FROM AUTHOR]

Modified Newtonian dynamics (MOND) provides a successful description of stellar and galactic dynamics on almost all astronomical scales. A key feature of MOND is the transition function from Newtonian to modified dynamics, which corresponds to the empirical mass discrepancy-acceleration (MDA) relation. However, the functional form of the MDA relation does not follow from theory in a straightforward manner; in general, empirical MDA relations are inserted ad hoc into analyses of stellar dynamics. I revisit the possibility of gravity being mediated by massive virtual particles, gravitons. Under certain reasonable assumptions, the resulting 'graviton picture' implies a MDA relation that is equivalent to the - empirical - 'simple μ' function of MOND, which is in very good agreement with observations. I conclude that the 'graviton picture' offers a simple description of gravitation on galactic scales, potentially playing a role for gravitation analogous to the role played by Bohr's model for atomic physics. [ABSTRACT FROM AUTHOR]

This review describes why the geometric nature of space-time, the Einstein equivalence principle, and the geodesic motion of particles show the possibility of building an extended relativistic theory of gravity on regions where the Tully-Fisher law is valid. It is also shown how a metric construction of gravity can have a modified Newtonian dynamics behaviour compatible with the Tully-Fisher law and the bending of light observed in individual, groups, and clusters of galaxies. It is also reviewed how this metric theory of gravity fits reasonably well on cosmological scales explaining the current acceleration of the universe. [ABSTRACT FROM AUTHOR]

Tests of MOND in elliptical galaxies are relatively rare because they often lack kinematic tracers in the regions where MOND effects are significant. Stellar shells observed in many elliptical galaxies offer a promising way to constrain their gravitational fields. Shells appear as glowing arcs around the host galaxy, with radii observed up to ∼100 kpc. The stars in axially symmetric shell systems move in nearly radial orbits. The radial distributions of shell locations and the spectra of stars in shells can be used to constrain the gravitational potential of their host galaxy. The symmetrical shell systems, being especially suitable for these studies, occur in approximately 3% of all early-type galaxies. Hence, if we overcome several problems (e.g., multiple shell generations present in the system, shells missed by observations, blurry shell edges, dynamical friction during the merger), the shells substantially increase the number of ellipticals in which MOND can be tested up to large radii. In this paper, we review our work on shell galaxies in MOND. We summarize Bílek et al. ( Astron. Astrophys. 559, A110 (2013)), where we demonstrated the consistency of shell radii in an elliptical NGC 3923 with MOND, and Bílek et al. (arXiv:1404.1109. 2014), in which we predicted a giant (∼200 kpc), yet undiscovered shell of NGC 3923. We explain the shell identification method that was used in these two papers. We further describe the expected shape of line profiles in shell spectra in MOND, which is very special because of the direct relation of the gravitational field and baryonic matter distribution (Bílek et al. 2014, in preparation). [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Physics is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Physics is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Physics is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper is devoted to study the acceleration and collision of charged particles in a general regular space-time. Using angular momentum, energy, and components of four-velocity, we explore the effect of charged particles on the center of mass energy. It is found that the collision energy of charged particles (independent of both singularity as well as horizon) is greater than that of uncharged particles. This depends not only on the mass to charge ratio of the black hole but also on the charge of the particle. Finally, we evaluate the collision energy of charged particles for a regular black hole, a particular example. [ABSTRACT FROM AUTHOR]

We present the plausibility of some models emerging from an algorithm devised to generate a one-parameter family of interior solutions for the Einstein equations. We explore how their physical variables change as the family parameter varies. The models studied correspond to anisotropic spherical matter configurations having a nonlocal equation of state. This particular type of equation of state, with no causality problems, provides at a given point the radial pressure not only as a function of the density but as a functional of the enclosed matter distribution. We have found that there are several model-independent tendencies as the parameter increases: the equation of state tends to be stiffer and the total mass becomes half of its external radius. Profiting from the concept of cracking of materials in general relativity, we obtain that these models become more potentially stable as the family parameter increases. [ABSTRACT FROM AUTHOR]

The equilibrium and stability properties of a planar gravitating plasma in the theory of ideal magnetohydrodynamic flows are investigated. The flow is taken to be barotropic. Variational principles for the steady state equations of aligned magnetic field and plasma flow are formulated. A simple and efficient way is proposed for investigating the stability properties of this state. Sufficient conditions for linear stability of the steady-state flow are obtained. [ABSTRACT FROM AUTHOR]

Herein, we analyze the dispersion modes of hot plasma around the Schwarzschild - de Sitter event horizon in the presence of a Veselago medium (left-handed). For this purpose, we apply Arnowitt, Deser, and Misner 3+1 formalism to develop general relativistic magnetohydrodynamics (GRMHD) equations for the Schwarzschild - de Sitter space-time. Implementation of linear perturbation yields perturbed GRMHD equations that are used for the Fourier analysis of rotating (nonmagnetized and magnetized) plasma. Wave analysis is described by the graphical representation of the wave vector, refractive index, change in refractive index, and phase and group velocities. The outcome of this work supports the validity of a Veselago medium. [ABSTRACT FROM AUTHOR]