2022-0822

• [2208.09024] Cylindrically symmetric radial accretion onto Levi-Civita string-like source
Abstract: Radial steady-state accretion of polytropic matter is investigated under cylindrical symmetry in the Levi-Civita background metric. The model can be considered as a cylindrical analog of Bondi accretion in strong gravitational field. As a byproduct of this study, the issue of defining the line mass density is addressed and the role of the metric free parameters is discussed on the example of physical observables. The form of radial accretion equations is insensitive to the structure of the interior solution. Accordingly, the accretion solution analysis can be limited to a special Wilson form of Levi-Civita metric describing a structureless homogeneous string.

Two tips: one is accretion solutions of black holes, it is a way to study the environmental effects to the EMRIs; another is the analog of environment of black holes like acoustic black holes in curved spacetime, as the discussion of accretion gas around black holes like this.

• [2208.09048] Construction of a traversable wormhole from a suitable embedding function.
Abstract: In this work, we construct a traversable wormhole by providing a suitable embedding function ensuring the fulfilling of the flaring–out condition. The solution contains free parameters that are reduced through the study of the acceptable conditions of a traversable wormhole. We compute both the quantifier of exotic matter and the quasi–normal modes through the 13th order WKB as a function of the remaining free parameters. We obtain that the wormhole geometry can be sustained by a finite amount of exotic matter and seems to be stable under scalar perturbations.

• [2208.09362] The gravitational field outside a spatially compact stationary source in the most general class of fourth-order theories of gravity
Abstract: In the most general class of fourth-order theories of gravity, namely gravity, the metric for the external gravitational field of a spatially compact stationary source is provided, where is Ricci scalar, is Ricci square, and is Riemann square. A new type of gauge condition is proposed so that the linearized gravitational field equations of gravity are greatly simplified, and then, the stationary metric in the region exterior to the source is derived. The metric consists of General Relativity-like part and part, where the latter is the correction to the former. As with the previous one for a point-like source in references, the metric is characterized by two characteristic lengths, but since it can indicate the influence of the size and shape of a realistic source on the external gravitational field, it will have a wider range of applications. With this result, the corresponding metrics in various sub-models of gravity can also be deduced, and it is proved that in the linearized gravity, the Gauss-Bonnet curvature scalar does not contribute to the gravitational field outside the spatially compact stationary source.
 

2022-0823

• [2208.09737] Dyonic Matter Equations, Exact Point-Source Solutions, and Charged Black Holes in Generalized Born–Infeld Theory
Abstract: We derive the equations of motion governing static dyonic matters, described in terms of two real scalar fields, in nonlinear electrodynamics of the Born–Infeld theory type. We then obtain some exact finite-energy solutions of these equations in a few interesting special situations subject to dyonic point-charge sources and construct dyonically charged black holes with relegated curvature singularities. In the case of quadratic nonlinearity, in particular, we show that dyonic solutions enable us to restore electromagnetic symmetry, which is known to be broken in non-dyonic situations. We also show that in the context of k-essence cosmology the nonlinear electrodynamics models possess their own distinctive signatures in light of the underlying equations of state of the cosmic fluids they represent. In particular, the generalized models here are shown to resolve a density-pressure discrepancy issue exhibited by the original Born–Infeld model k-essence action function.

• [2208.09739] What do gravitational wave detectors say about polymer quantum effects?
Abstract: We compute the expected response of detector arms of gravitational wave observatories to polymerized gravitational waves. The mathematical and theoretical features of these waves were discussed in our previous work. In the present manuscript, we find both perturbative analytical, and full nonperturbative numerical solutions to the equations of motion of the detector arms using the method of geodesic deviations. These results show the modifications to both frequency and amplitude of the signal measured by the detector. Furthermore, we study the detectability of these signals in LISA by analyzing the modes in the frequency space.

Comment: I really want to know how to detect the quantum effects or quantum black holes by gravitational waves.

• [2208.10235] New entropies, black holes, and holographic dark energy
Abstract: The Bekenstein-Hawking entropy is a cornerstone of horizon thermodynamics but quantum effects correct it, while inequivalent entropies arise also in non-extensive thermodynamics. Reviewing our previous work, we advocate for a new entropy construct that comprises recent and older proposals and satisfies four minimal key properties. The new proposal is then applied to black holes and to holographic dark energy and shown to have the potential to cause early universe inflation or to alleviate the current Hubble tension. We then analyze black hole temperatures and masses consistent with alternative entropies.
 

2022-0824

• [2208.10961] The extended uncertainty principle effects on the phase transitions of Reissner-Nordstrom and Schwarzschild black holes
Abstract: In this paper, we investigate the phase transitions of Reissner-Nordstr¨om (RN) and Schwarzschild black holes for the extended uncertainty principle (EUP) framework. Considering temperature T, charge and electric potential as the state parameters, we show the van der Waals (vdW) like phase transition of RN black hole in diagrams and find the critical points depending on EUP parameter . Furthermore, we find Hawking-Page like phase transition for Schwarzschild black hole. The result imply that the black holes in asymptotically flat space have the similar phase structure with the black holes in anti-de Sitter (AdS) space.

• [2208.11095] Quantum gravitational signatures in next-generation gravitational wave detectors
Abstract: A recent study established a correspondence between the Generalized Uncertainty Principle (GUP) and Modified theories of gravity, particularly Stelle gravity. We investigate the consequences of this correspondence for inflation and cosmological observables by evaluating the power spectrum of the scalar and tensor perturbations using two distinct methods. First, we employ PLANCK observations to determine the GUP parameter γ0. Then, we use the value of γ0 to investigate the implications of quantum gravity on the power spectrum of primordial gravitational waves and their possible detectability in the next-generation detectors, like Einstein Telescope and Cosmic explorer.

Comment: key word is “Uncertainty principle”

• [2208.11028] Scattering Angles in Kerr Metrics
Abstract: Scattering angles for probes in Kerr metrics are derived for scattering in the equatorial plane of the black hole. We use a method that naturally resums all orders in the spin of the Kerr black hole, thus facilitating comparisons with scattering-angle computations based on the Post-Minkowskian expansion from scattering amplitudes or worldline calculations. We extend these results to spinning black-hole probes up to and including second order in the probe spin and any order in the PostMinkowskian expansion, for probe spins aligned with the Kerr spin. When truncating to third Post-Minkowskian order, our results agree with those obtained by amplitude and worldline methods.
 

2022-0825

• [2208.11127] Electrically charged spherical matter shells in higher dimensions: Entropy, thermodynamic stability, and the black hole limit
Abstract: We study the thermodynamic properties of a static electrically charged spherical thin shell in  dimensions by imposing the first law of thermodynamics on the shell. The shell is at radius , inside it the spacetime is Minkowski, and outside it the spacetime is Reissner-Nordström. We obtain that the shell thermodynamics is fully described by giving two additional reduced equations of state, one for the temperature and another for the electrostatic potential. We choose the equation of state for the temperature as a power law in the gravitational radius  with exponent , such that the  case gives the temperature of a shell with black hole thermodynamic properties, and for the electrostatic potential we choose an equation of state characteristic of a Reissner Nordström black hole spacetime. The entropy of the shell is found to be proportional to , where is the gravitational area corresponding to, with . We are then able to perform the black hole limit , find the Smarr formula, and recover the thermodynamics of a d-dimensional Reissner-Nordström black hole. We study the intrinsic thermodynamic stability of the shell with the chosen equations of state. We obtain that for  all the configurations of the shell are thermodynamically stable, for  stability depends on the mass and electric charge, and for  all the configurations are unstable, except for the shell at its own gravitational radius, which is marginally stable. We rewrite the stability conditions in terms of laboratory variables. We find that the sufficient condition for the stability of these shells is when the isothermal electric susceptibility  is positive, marginal stability happening when this quantity is infinite, and instability arising for configurations with a negative electric susceptibility.
 

2022-0829

• [2208.12713] Regular Rotating Black Holes: A Review
Abstract: This is the sketch of an invited chapter on regular rotating black holes for the edited book "Regular Black Holes: Towards a New Paradigm of the Gravitational Collapse" (Ed. C. Bambi, Springer Singapore, expected in 2023). The chapter intends to be a pedagogical review of the main characteristics of singularity-free rotating black holes. The interest in the subject has grown significantly in recent years, as shown by the increase in the number of published papers devoted to it. There have been advances in the modelling of both regular and rotating black holes coming from many different theoretical approaches. Undoubtedly, the recent observational developments (LIGO, the Event Horizon Telescope or, in the near future, the LISA project) and the possibility to probe our theoretical predictions has greatly contributed to awaken the interest. Please take into account that it is not the intention of this sketch to cite every article on the subject. Suggestions and corrections are welcome.

Comment: we can study the acoustic horizon of the regular black holes like this.

2022-0831

• [2208.13956] Circular orbits of Charged Particles around a Weakly Charged and Magnetized Schwarzschild Black Hole
Abstract: We study the circular orbits of charged particles around a weakly charged Schwarzschild black hole immersed in a weak, axisymmetric magnetic field. We start by reviewing the circular orbits of neutral particles and charged particles around only weakly charged and only weakly magnetized black holes. The case of a weakly magnetized and charged black hole is investigated then. In particular, we study the effect of the electromagnetic forces on the charged particles innermost stable circular orbits. We show that negative energy stable circular orbits are possible and that two bands of charged particles circular orbits, separated by a gap of no stable circular orbits can exist. The astrophysical aspects of our findings are discussed too.