Ébano L. Martins | Searcharxiv

arXiv2024

New sources of ghost fields in $k$-essence theories for black-bounce solutions

In the present study, we generalize the possible ghost field configurations within the framework of $k$-essence theory to the Simpson-Visser metric area function $Σ^2=x^2+a^2$. Our analysis encompasses field configurations for the region-defined metric function $dA_\pm$ as well as the general solution that asymptotically behaves as Schwarzschild-de Sitter for $x\to-\infty$. Specifically, we investigate two scalar field configurations and define the associated potential for each one. Through rigorous calculations, we verify that all equations of motion are satisfied. Notably, our findings indicate that even when proposing new configurations of ghost scalar fields, the energy conditions remain unchanged. This result serves to validate the wormhole solutions obtained in previous studies.

gr-qc↗

arXiv2022

Scattering and absorption of a scalar field impinging on a charged black hole in the Einstein-Maxwell-dilaton theory

This study revisits the absorption and scattering process by which a massless scalar field impinges on a charged dilatonic black hole. First, we review the classical analysis to obtain the deflection angle and the differential scattering cross-section. Then, using the partial wave method, we determine the total absorption cross-section numerically in terms of the decoupling parameter called $Mω$, finding that the amplitude of the dilatonic black hole is lower than the Reissner-Nordström one for mild frequencies. In the high-frequency limit, the absorption cross-section exhibits two different complex behaviors; the fine structure and the hyperfine structure. For the differential scattering cross-section, smaller values of $Mω$ lead to more significant amplitudes; the opposite scenario is obtained by increasing the charge-to-mass ratio. To fully grasp the main properties of the charged dilatonic black hole, we consider a different framework where the compact object is impinged by a charged massive scalar field. The superradiant effect is lessened for intermediate frequency concerning the Reissner-Nordström case. However, this effect does not necessarily imply the existence of any dynamical instability. In order to trigger the superradiant instability, unstable modes must remain trapped outside the event horizon with a mechanism based on the reflecting-mirror boundary conditions. In this way, a charged scalar field plus a charged black hole configure a charged black hole bomb. We provide an analytic formula (lower bound) for the values of the charge field, which can trigger this superradiant instability. We extend this minimal setup by considering the dilaton perturbations while freezing the other degree of freedom. The new perturbation scheme enhances the superradiance scattering and reduces the lower bound of the charge-to-mass ratio to develop a superradiant instability.

gr-qc↗