Narrow Results

In the framework of massive gravity theory which is coupled to the Maxwell and Born–Infeld electrodynamics, we review derivation of the exact four-dimensional black hole solutions by use of a static and spherically symmetric geometry. The exact solutions are asymptotically AdS and show two-horizon, one-horizon, extreme and black holes without horizon. The Maxwell and Born–Infeld black hole solutions are compatible in the limit of large nonlinearity parameter and, both reduce to the Reissner–Nordström black hole in the absence of massive gravitons. By reviewing thermodynamic and conserved quantities, and making use of a Smarr-type mass formula, we check validity of the thermodynamical first law for both of Maxwell and Born–infeld black holes. We investigate thermal stability of the black holes by use of the canonical ensemble and geometrical approaches, separately. By calculating the specific heats and Ricci scalars of alternative thermodynamic metrics, we compare the results of geometrothermodynamics with those of canonical ensemble method. Also, we perform a global stability analysis by use of the grand canonical ensemble and noting the Gibbs free energy of the Maxwell and Born–Infeld black holes.

In this paper, we study the Dirac particle near the event horizon of the charged Bañados-Teitelboim-Zanelli (BTZ) black hole and Reissner–Nordström (R-N) black holes to obtain its energy spectrum for discussing the weak gravity conjecture (WGC). The corresponding energy has both real and imaginary parts. We encounter the quasi-normal mode. Because the considered black holes have mass $M$ and charge $Q$, this property allows us to examine the WGC using the energy spectrum with specific conditions. We attempt to investigate the WGC for these black holes by utilizing the Dirac particle energy spectrum obtained near charged black holes. Also, we impose conditions on the energy spectrum of particles for which the WGC holds for charged black holes, i.e. $Q∕M>1$. To approve the WGC near the event horizon of black holes, we determine that the Dirac particle has a specified charge viz $q=-\frac{1}{2\omega }$ and $q=-\frac{1}{ln(\frac{r_{+}}{\ell })}$ for R-N and charged BTZ black holes, respectively.

The Jordan frame (JF) field equations of scalar-tensor (ST) theory are strongly coupled and, the exact solutions cannot be obtained easily. By using the conformal transformation (CT), the ST action has been translated to the Einstein frame (EF) where the theory is known as the Einstein-dilaton (Ed) gravity. Also, an $(n+1)$-dimensional electromagnetic Lagrangian has been introduced which remains invariant under CT. The Ed-conformal-invariant field equations, which are confronted with the mathematical indeterminacy problem, have been solved by use of a power-law ansatz function. We have introduced two classes of black holes (BHs) which are asymptotically non-flat and non-AdS. The Ed exact solutions can produce BHs with three, two, one and without horizons. By calculating the thermodynamic quantities, and making use of the Smarr mass relation it has been shown that the thermodynamical first law is valid in the EF. Thermal stability of Ed BHs has been analyzed by considering specific heats, thermodynamic Ricci scalars and Gibbs free energies, separately. Then using the inverse CTs, the ST exact solutions have been obtained which show two classes of horizonless, one-horizon, two-horizon and three-horizon BHs. We found that CTs preserve thermodynamic quantities and, thermodynamic properties of the ST BHs are just like those of Ed ones.

Effect of maximum amount of charge a compact star can hold, is studied here. We analyze the different features in the renewed stellar structure and discuss the reasons why such huge charge is possible inside a compact star. We studied a particular case of a polytropic equation of state (EOS) assuming the charge density is proportional to the mass density. Although the global balance of force allows a huge charge, the electric repulsion faced by each charged particle forces it to leave the star, resulting in the secondary collapse of the system to form a charged black hole.

In this paper, we studied how nonlinear vacuum electrodynamics can affect charged collapsar spacetime structure and properties of particles movement in this spacetime. Analysis of uncharged particle orbits stability shows that the main features of this orbit in Reissner–Nordström spacetime remains actual for Einstein–Born–Infeld theory. At the same time, there is a significant quantitative difference between stable orbits parameters in these theories and this fact may influence on accretion rate predictions for charged collapsar.

Testing gravity theories and constraining their parameters using data from astrophysical observations of quasiperiodic oscillations (QPOs) is a powerful approach to study the features of black holes (BHs) and gravity theories in the strong field regime. To this end, we investigate the dynamics of the test particles in the vicinity of some charged BHs such as Reissner–Nordström (RN), regular Bardeen and Ayon–Beato–Garcia (ABG). In particular, we calculate harmonic oscillations and the innermost stable circular orbits (ISCOs) of test particles around these BHs. As an astrophysical application, we discuss the twin peak QPOs in relativistic precession (RP) model and give the possible values of the upper and lower frequencies of the QPOs at the distance from ISCO to infinity around the extremely charged BHs. Moreover, we also investigate the position of the orbit, where twin peak QPO may take place around central (charged) BH in the microquasar GRS 1915-105. We also study the dependence of the distance between the QPO position and ISCO from the BH charge. Using the upper and lower frequencies of the QPO, we obtain the relationship between the mass and charge of the central BH in the microquasar. Finally, we compare the effects of the spin of the Kerr BH and BH charges on the QPO in microquasar GRS 1915-105. We find that the BH charge can mimic the spin up to $a/M=0.2927$ in the Bardeen model and up to $a/M=0.7687$ in the case of special regular BH, admitting the same QPO frequencies.

Using the symmetry of the near-horizon geometry and applying quantum field theory of a complex scalar field, we study the spontaneous pair production of charged scalars from near-extremal rotating, electrically and/or magnetically charged black holes. Analytical expressions for pair production, vacuum persistence and absorption cross section are found, and the spectral distribution is given a thermal interpretation. The pair production in near-extremal black holes has a factorization into the Schwinger effect in AdS and Schwinger effect in Rindler space, measuring the deviational from extremality. The associated holographical correspondence is confirmed at the 2-point function level by comparing the absorption cross section ratio as well as the pair production rate both from the gravity and the conformal field theories. The production of monopoles is discussed.

According to the no-scalar-hair conjecture, black-hole solutions in scalar-tensor theories of gravity (STT) can be fully classified by their asymptotic preserved charges - mass, electric (magnetic) charge and angular momentum, exactly as in General Relativity (GR). This makes the black holes in STT undistinguishable from those in GR for a distant observer. There is a series of theorems that confirm the conjecture in the vacuum and electrovacuum cases. The present talk is dedicated to new non-unique, numerical solutions that serve as counter-examples of the no-scalar-hair conjecture. They describe charged black holes coupled to non-linear electrodynamics in special classes of STT that posses primary scalar hair.