International Journal of Modern Physics D

The important role of magnetic fields in the phenomena in and evolution of the Universe is well appreciated. A salient example of this is to make (often episodic) large magnetic fields in AGN accretion disks and their emanation of well-collimated and longitudinally extended astrophysical jets. Such typical cases or related astrophysical processes, we find, provide a fertile ground for exciting large-amplitude oscillations in the magnetic fields that constitute the spine of the jets. The energy sources of these oscillations can be traced originally to the gravitational energy of the central object. During their long propagation along the jet, because of the gradual changes of the density and magnetic fields, these large magnetic pulsations turn into relativistic amplitude electromagnetic (EM) pulses, which in turn induce intense wakefields that are capable of acceleration of electrons, positrons, and ions to high energies. In this review, we survey a variety of astrophysical objects ranging from as large as the cosmic AGN accretion disks and their jets to as small as microquasars, to find or predict that there exist common astrophysical processes of emission of high-energy particles and gamma (and other EM) emissions. A variety of these objects will be ideally observed and studied in the multimessenger astrophysical observations. One example that already stuck out was the case of the simultaneous observations of gravitational wave emission and gamma-ray pulse from the collision of the two neutron stars and their subsequent structure formation (such as a disk) around them.

In the teleparallel $f(T)$ gravity scenario, we consider a five-dimensional thick brane. This scenario is interesting because this theory can provide explanations for inflation, radiation and dark matter under certain conditions. It is convenient to assume, for our study, a polynomial profile of the function $f(T)$. Indeed, some polynomial profiles can produce internal structures for which a brane splitting occurs. For functions $f(T)$ with this capability, geometrically contracted matter field configurations are obtained. These contractions of the matter field for the profiles of $f(T)$ reproduce compact-like settings. To complement the study, we analyze the stability of the brane using the concept of Configurational Entropy (CE). The CE arguments are interesting because they tell us the most stable and likely configurations from the brane in this gravitational background. Therefore, we can indicate the best profile of the function $f(T)$.

In this work, we reach the equations of motion of Morris–Thorne wormhole geometry by means of the Einstein Field Equations and Klein–Gordon Equation of Scalar–Tensor Theory. We discuss the anisotropic matter energy distribution. We determine a relation between the radial and the transverse pressures. Hence, we express the anisotropic energy–momentum tensor in terms of one pressure class, by means of that relation. Besides that, we check the isotropic case and show that there is no traversable wormhole (WH), in the zero redshift function situation, if the energy–momentum distribution of the universe is isotropic. In addition, we represent the conditions in order that the Null Energy Condition (NEC) is satisfied in the zero redshift function case, for anisotropic distribution. We also propose a special class of traversable WH shape functions. We will be calling the WHs corresponding to that class of functions as the Yukawa Type WHs. We expressed the NEC for those WHs particularly. Furthermore, we determine the radial and the transverse pressures in zero redshift function situation.

Effective one body numerical relativity waveform models for spin-aligned binary black holes (SEOBNR) are based on the effective one body theoretical framework and numerical relativity simulation results. SEOBNR models have evolved through version 1 to version 4. We recently extended SEOBNRv1 model to SEOBNRE (Effective One Body Numerical Relativity waveform models for Spin-aligned binary black holes along Eccentric orbit) model which is also valid for spin-aligned binary black hole coalescence along eccentric orbit. In this paper, we update our previous SEOBNRE model to make it consistent to SEOBNRv4 which is the most widely used SEOBNR waveform model. This upgraded SEOBNRE model improves accuracy compared to previous SEOBNRE model, especially for highly spinning black holes. For spin-aligned binary black holes with mass ratio $1\le q\lesssim 10$, dimensionless spin $-0.9\lesssim \chi \lesssim 0.995$ and orbital eccentricity $0\le e_0\lesssim 0.6$ at reference frequency $M{f}_0=0.002$ ($M$ is the total mass of the binary black hole, $f_0\approx 40\frac{10{\mathrm{\text{M}}}_{\odot }}{M}$Hz), the upgraded SEOBNRE model can always fit numerical relativity waveform better than 98.2%. For most cases, the fitting factor can even be better than 99%.

By applying the Newman–Janis Algorithm, we investigate optical behaviors of rotating Ayón–Beato–García black holes in four-dimensional Einstein Gauss–Bonnet Gravity. Exploiting the Hamilton–Jacobi mechanism, we first obtain the needed null geodesic equations of motion. We then provide analytic and numerical methods to study the associated optical aspect for certain regions of the involved moduli space. Concretely, we analyze and examine the shadow behaviors in terms of one-dimensional real closed curves. Particularly, we find various shapes including the D-ones. After that, we study graphically the corresponding astronomical observables. We compute and discuss the energy emission rate. We observe that the rotating parameter and the Gauss–Bonnet coupling have a relevant influence on the shadow geometric configurations and the energy emission rate. Moreover, we find that the nonlinear electrodynamic charge does not affect such optical behaviors. Finally, we provide a possible link with the event horizon telescope activities by imposing certain conditions on the involved black hole parameters in the $M87^{*}$ imaging aspect.

In this paper, we study black hole entropy universality within the conformal Weyl gravity paradigm. We do this by first computing the entropy of specific vacuum and non-vacuum solutions, previously unexplored in conformal Weyl gravity via both the Nöther current method and Wald’s entropy formula. For the vacuum case, we explore the near horizon near extremal Kerr metric, which is also a vacuum solution to conformal Weyl gravity and not previously studied in this setting. For the non-vacuum case, we couple the conformal Weyl gravity field equations to a near horizon (linear) $U(1)$ gauge potential and analyze the respective found solutions. We highlight the non-universality of black hole entropy between our studied black hole solutions of varying symmetries. However, despite non-universality, the respective black hole entropies are in congruence with Wald’s entropy formula for the specific gravity theory. Finally and despite non-universality, we comment on the construction of a near horizon CFT dual to one of our unique non-vacuum solutions. Due to the non-universality, we must introduce a parameter (similarly to entropy calculations in LQG) which we also call $\gamma$ and relating to the Weyl anomaly coefficient. The construction follows an ${AdS}_2/{CFT}_1$ correspondence in the near horizon, which enables the computation of the full asymptotic symmetry group of the chosen non-vacuum conformal Weyl black hole and its near horizon quantum CFT dual. We conclude with a discussion and outlook for future work.

In this paper, we investigate spherically symmetric gravitational collapse of thick matter shell without radiation in the Einstein gravity with cosmological constant. The orbit of the infalling thick matter is determined by imposing an equation of state for the matter near interface, where pressure constituted of the transverse component and the longitudinal one is proportional to energy density. We present analytic solutions for the equation of state and discuss parameter region to satisfy physical conditions such as the absence of the shell crossing singularity, the monotonic increase of the emergent infinite redshift surface and energy conditions. We finally show that adopting the definition presented in arXiv:2005.13233 the total energy in this time-dependent system is invariant under the given time evolution.

In this paper, we have studied inflationary paradigm through an inflationary equation-of-state. With a single parameter equation-of-state as a function of the scalar field responsible for accelerated expansion, we find an observationally viable model satisfying all the constraints as laid down by the recent observations. The resulting model can efficiently cover a wide range of tensor-to-scalar ratio ranging from $r\sim {𝒪}(10^{-1})$ to ${𝒪}(10^{-6})$, other inflationary observables being consistent with the latest data. Nowadays, ultimate eliminator between inflationary models is the tensor-to-scalar ratio, the model presented here is capable of keeping up with the future probes of tensor-to-scalar ratio at the same time having good agreement with other inflationary observables.

Motivated by the work of P. H. Frampton et al. [Phys. Lett. B 708 (2012) 204; I. Brevik et al., Phys. Rev. D 84 (2011) 103508, arXiv:1107.4642 [hep-th]; I. Brevik et al., Phys. Rev. D 86 (2012) 063007, arXiv:1208.4770 [gr-qc]] this study reports on the reconstruction schemes for the inhomogeneous equation of state (EoS) first introduced by S. Nojiri et al., [Phys. Rev. D 72 (2005) 023003, arXiv:hep-th/0505215 [hep-th]] and the cosmology in the scenario of coupled fluid. We have considered the viscosity through the Eckart approach in an interacting system. The EoS parameter for the Little Rip, the $f(\rho )$ in terms of cosmic time for Pseudo Rip, is reconstructed for the EoS for Bounce cosmology inhomogeneous viscous fluid. The EoS parameter is rebuilt for the coupled dark fluid in the nonviscous system for the Little Rip, Pseudo Rip and Bounce cosmology. The reconstructed EoS parameter for inhomogeneous viscous fluid was phantom for the Little Rip cosmology. The reconstructed EoS parameters of holographic Ricci dark energy (HRDE) of the coupled fluid in nonviscous systems are phantom for both the Little Rip and Pseudo Rip cosmology. The model HRDE of the coupled fluid in the nonviscous scenario for Pseudo-Rip cosmology indicates the possibility of singularity. The same model for the bounce cosmology violates the null energy condition and has the EoS parameter almost symmetric around the bouncing point.

General Relativity (GR) has been tested extensively in the solar system and is being tested in the new environment of the Galactic Center (GC) black hole where the dimensionless gravitational potential ($GM/c^{2}r$) is 100 times stronger than the one encountered in solar system. Therefore, the neighborhood of the GC black hole is a naive opportunity to test modified theories of gravity. In this work, effect of $f(R)$ gravity near the black hole is studied. The difference of pericenter shift between GR and $f(R)$ gravity is studied for compact orbits having semi-major axis equal to and below $a=1000$ au (S0-2 like orbits). In a model-dependent approach, we choose $f(R)\propto R^2$ (power law gravity) model which is cosmologically motivated and study the deviation in orbital pericenter shift for both zero spin and non-zero spin of the black hole. It is found that effect of $f(R)$ gravity becomes prominent for compact orbits. In model-independent approach to $f(R)$ gravity with the generic scalaron fields ($\psi =f^{\prime }(R)$), we extract the parameters of $f(R)$ gravity from the current bounds on Parametrized Post-Newtonian (PPN) parameters ($\gamma ,\beta$) near the GC black hole. The screening of $f(R)$ gravity is also investigated for these bounds on PPN parameters. It has been found that sufficiently massive scalarons ($10^{-16}$eV) are completely screened but light and intermediate mass scalarons ($10^{-22}$eV and $10^{-19}$eV) are unscreened towards S0-2 like orbits as well as in the orbit of the newly discovered short period star S4716 ($a=407$au). The possibility of detection of the $f(R)$ gravity effects due to these unscreened scalarons is forecasted with existing and upcoming astrometric capabilities of Extremely Large Telescopes (ELTs).

The dimensional quadrupole mass moment $Q_2$ of a black hole is negative; thus, Eq. (98) of Ref. 1 must be corrected to $Q_2=-S^2/c^{2}M$.