International Journal of Modern Physics D

We give a brief review of the definition of the Wang–Yau quasi-local mass and discuss the evaluation of this quantity on surfaces of unit size at null infinity of an axisymmetric spacetime in Bondi–van der Burg–Metzner coordinates.

Large dimensionless numbers, arising out of ratios of various physical constants, intrigued many scientists, especially Dirac. Relying on the coincidence of large numbers, Dirac arrived at the revolutionary hypothesis that the gravitational constant $G$ should vary inversely at the cosmic time $t$. This hypothesis of Dirac, known as the Large Number Hypothesis (LNH), sparked off many speculations, arguments and new ideas in terms of applications. Works done by several authors with LNH as their basic platform are extensively reviewed in this work. Relationship between some of those works are pointed out here elaborately. Possibility of time variations of physical constants other than $G$ as well as large numbers in various realm of physical and biological sciences are also discussed.

The Cosmic Fabric model of gravity that was previously introduced by Tenev and Horstemeyer [T. G. Tenev and M. F. Horstemeyer, Int. J. Mod. Phys. D 27 (2018) 1850083; Rep. Adv. Phys. Sci. 2 (2018) 1850011] is further developed to provide a method for deriving the spacetime metric of the fabric's deformation due to a spherically symmetric gravitating body. We show that the derived metric is equivalent to the Schwarzschild metric up to a rescaling factor. From analyzing the membrane portion of the elastic energy due to an inclusion, we conclude that matter cannot be continuous at arbitrary sizes but must comprise discrete particles. Therefore, we show that the Cosmic Fabric model leads to results consistent with accepted theories at both continuum as well as sub-continuum length scales.

In order to examine a muon excess observed by the Pierre Auger Observatory, detailed Monte Carlo simulations were carried out for primary protons, iron nuclei and strangelets (hypothetical stable lumps of strange quark matter). We obtained a rough agreement between the simulations and the data for ordinary nuclei without any contribution of strangelets in primary flux of cosmic rays. Our simulations suggest that the shower observables are dominated by details of hadronic interaction models.

Wormholes (WHs) are a solution for General Relativity field equations which characterize a passage or tunnel that connects two different regions of spacetime and is filled by some sort of exotic matter that does not satisfy the energy conditions. On the other hand, it is known that in extended theories of gravity, the extra degrees of freedom once provided may allow the energy conditions to be obeyed and, consequently, the matter content of the WH to be nonexotic. In this work, we obtain, as a novelty in the literature, solutions for charged WHs in the $f(R,T)$-extended theory of gravity. We show that the presence of charge in these objects may be a possibility to respect some stability conditions for their metric. Also, remarkably, the energy conditions are respected in the present approach. In addition, we argue that our framework can be very useful to study the possibility of evolving $(2+1)$ and $(3+1)$-dimensional WH spacetime within the context of nonlinear electrodynamics, which open a new window to probe the physical quantities in a WH-type solution.

Gravity-induced condensation takes the form of momentum alignment in an ensemble of identical particles. Use is made of a one-dimensional Ising model to calculate the alignment per particle and the correlation length as a function of the temperature. These parameters indicate that momentum alignment is possible in the proximity of some astrophysical objects and in Earth, or near Earth laboratories. Momenta oscillations behave as known spin oscillations and obey identical dispersion relations.

In nonsupersymmetric-covariant quantum gravity theory, for each system of gravity coupled with single field is one-loop divergent. There were two kinds of methods to reach this result: diagrammatic calculation and heat kernel expansion. On the other hand, more symmetry requirements make the theory finite and it was analyzed how it is possible. In this paper, based on diagrammatic calculation, we consider Einstein–Scalar electrodynamics (Einstein–SQED) system which is the simplest model among the systems of gravity coupled with multiple fields having their own interaction. Although it is known that fermionic loop can cancel the divergent terms as in the case of supersymmetric theory, there might be accidental cancellation by the equation of motion at a low probability. It is worth to study because there is no such attempt for this model yet and it can be checked easily using the existing result. We introduce how to calculate the possible one-loop diagrams in Einstein–SQED system and show how divergent terms are not vanished with the results calculated earlier.

Under Newtonian dynamics, the relative motion of the components of a binary star should follow a Keplerian scaling with separation. Once orientation effects and a distribution of ellipticities are accounted for, dynamical evolution can be modeled to include the effects of Galactic tides and stellar mass perturbers, over the lifetime of the solar neighborhood. This furnishes a prediction for the relative velocity between the components of a binary and their projected separation. Taking a carefully selected small sample of 81 solar neighborhood wide binaries from the Hipparcos catalog, we identify these same stars in the recent Gaia DR2, to test the prediction mentioned using the latest and most accurate astrometry available. The results are consistent with the Newtonian prediction for projected separations below 7000 AU, but inconsistent with it at larger separations, where accelerations are expected to be lower than the critical $a_0=1.2\times 10^{-10}$m.s${}^{-2}$ value of MONDian gravity. This result challenges Newtonian gravity at low accelerations and shows clearly the appearance of gravitational anomalies of the type usually attributed to dark matter at galactic scales, now at much smaller stellar scales.

By considering the quantum gravity effects based on generalized uncertainty principle, we give a correction to Hawking radiation of charged fermions from accelerating and rotating black holes. Using Hamilton–Jacobi approach, we calculate the corrected tunneling probability and the Hawking temperature. The quantum corrected Hawking temperature depends on the black hole parameters as well as quantum number of emitted particles. It is also seen that a remnant is formed during the black hole evaporation. In addition, the corrected temperature is independent of an angle $𝜃$ which contradicts the claim made in the literature.

In this paper, we investigate the influence of nonisothermal processes on the evolution of the cloud's envelope around a newborn protostar. For this purpose, we study the evolution of a spherical cloud harboring a central hydrostatic newborn protostar. This model includes thermal effects due to heating of the cosmic rays and cooling of the gas and gas–dust energy transfer. We have ignored the effects of the magnetic field and rotation. The semianalytical Adomian decomposition method (ADM) is used to solve the system of nonlinear dynamical equations for different initial conditions. In this paper, the ADM allows us to follow the time evolution of the cloud's envelope and its mass accretion rate onto the newborn protostar. We find that the mass accretion rates of the envelope are increasing functions of time and highly depend on the choice of initial conditions. Moreover, we find that the nonisothermal processes affect the evolution of the mass accretion rates compared with the isothermal processes for different initial conditions.

The spin state of a wave packet of a spin-1/2 particle in a gravitational field is a mixed state, obtained by integrating out the momentum degree of freedom. The entropy of this spin mixed state depends on the deviation of spacetime from the flatness. In terms of a succession of local inertial frames, it is shown that spin decoherence occurs even if the particle is uniformly moving or static in a gravitational field.

We probe the influence of Lorentz-violating mechanism, treated exactly, on the radiative quantum corrections to critical exponents for massive $q$-deformed O($N$) $\lambda {\varphi }^4$ scalar field theories. We attain that task by employing three distinct and independent field-theoretic renormalization group methods. Firstly, we compute the critical exponents up to the finite next-to-leading order for then generalizing the results for any loop level. We show that the $q$-deformed critical exponents are insensible to the Lorentz symmetry breaking mechanism thus agreeing with the universality hypothesis.

With the recent progresses on the Type II supernovae, we attempt to investigate whether there does exist new physics beyond the standard cosmological paradigm, i.e. the cosmological constant $\mathrm{\Lambda }$ plus cold dark matter ($\mathrm{\Lambda }$CDM). Constraining four alternative cosmological models with a data combination of currently available Type II supernovae calibrated by the standard color method, Type Ia supernovae, baryon acoustic oscillations (BAO), cosmic microwave background (CMB) and cosmic chronometers, at the $1\sigma$ confidence level, we find that (i) a spatially flat universe is supported for the nonflat $\mathrm{\Lambda }$CDM model; (ii) the constrained equation-of-state of dark energy $\omega$ is consistent with the $\mathrm{\Lambda }$CDM hypothesis for the $\omega$CDM model, where $\omega$ is a free parameter; (iii) for the decaying vacuum model, there is no evidence of interaction between dark matter and dark energy in the dark sector of the universe; (iv) there is also no hint of dynamical dark energy for the dark energy density-parametrization scenario. It is very obvious that a larger Type II supernovae sample is required if we expect to draw definitive conclusions about the formation and evolution of the universe.

A space with deformed Poisson brackets for coordinates and momenta leading to the minimal length is considered. Features of description of motion of a body in the space are examined. We propose conditions on the parameters of deformation on which Poisson brackets for coordinates and momenta of the center-of-mass reproduce relations of deformed algebra, kinetic energy of a body is independent of its composition, and the weak equivalence principle is preserved in the deformed space. Influence of minimal length on the motion of the Sun–Earth–Moon system is studied. We find that deformation of the Poisson brackets leads to corrections to the accelerations of the Earth and the Moon toward the Sun, as a result the Eotvos-parameter does not vanish even if we consider equality of gravitational and inertial masses. The upper bound for the minimal length is estimated using results of the Lunar laser ranging experiment.

Gravastar models have recently been proposed as an alternative to black holes, mainly to avoid the problematic issues associated with event horizons and singularities. In this work, a regular variety of gravastar models within the context of Einstein–Born–Infeld (EBI) nonlinear electrodynamics are builded. These models presented here are truly regular in the sense that both the metric and its derivatives are continuous throughout spacetime, contrary to other cases in the literature where matching conditions are necessary in the interior and exterior regions of the event horizon. We investigated the accretion process for spherically symmetric spacetime geometries generated for a nonlinear electromagnetic field where the energy–momentum tensor has the same form that an anisotropic fluid that is the general EBI case. We analyze this procedure using the most general static spherically symmetric metric ansatz. In this theoretical context, we examined the accretion process for specific spherically symmetric compact configuration obtaining the accretion rates and the accretion velocities during the process and the flow of the fluid around the black hole. In addition, we study the behavior of the rate of change of the mass for each chosen metric.

In this paper, we present the entropy relations and bounds of Banados–Teitelboim–Zanelli (BTZ) black hole in two models of gravity's rainbow. Because of the effect of gravity's rainbow, one can find that the entropy product and sum both lost their universality and become mass-dependent. On the other hand, comparing the entropy bound of event horizon to the BTZ case, it is shown that the angular momentum $J$ enlarges the entropy bound while the gravity's rainbow parameter $\gamma$ diminishes it. For the entropy bound of Cauchy horizon, the gravity's rainbow parameter $\gamma$ enlarges it at the large $\gamma$ limit, while $\gamma$ diminishes it at the small $\gamma$ limit. These suggest some clues on the geometrical origin of black hole entropy bounds.