No-Hair Theorem in the Wake of Event Horizon Telescope

Abstract

Thanks to the release of the extraordinary EHT image of shadow attributed to the M87* supermassive black hole (SMBH), we have a novel window to assess the validity of fundamental physics in the strong-field regime. Motivated by this, we consider Johannsen & Psaltis metric parameterized by mass, spin and an additional dimensionless hair parameter which in the high rotation regimes is able to provide a suitable framework for the test of the no-hair theorem (NHT) using the EHT data. Incorporating the into the standard Kerr spacetime enrich it in the sense that, depending on setting the positive and negative values for that, we deal with alternative compact objects: deformed Kerr naked singularity and Kerr BH solutions, respectively. Shadows associated with these two possible solutions indicate that the deformation parameter affects the geometry shape of standard shadow such that it become more oblate and prolate with < 0 and > 0, respectively. By scanning the window associated with three shadow observables: oblateness, deviation from circularity, and shadow diameter, we perform a numerical analysis within the range a * = 0.9 ∓ 0.1 of the dimensionless rotation parameter, to find the constraints on the hair parameter in both possible solutions. For both possible signs of , we extract a variety of upper bounds that are in interplay with a * . Our analysis suggests that as the rotation parameter approaches the extreme limit, although the allowable range of both hair parameters becomes narrower, the hairy Kerr BH solution is a more promising candidate to play a role of the alternative compact object instead of standard Kerr BH. The lack of tension between hairy Kerr BH with the current observation of the EHT shadow of the M87* SMBH carries this message that the possibility of NHT violation is not excluded.

Photon regions, shadow observables and constraints from M87* of a charged rotating black hole

“…These observations are consistent with the image of Kerr black hole predicted from GR, but they cannot rule out Kerr or non-Kerr black holes in MoG. Thus, the EHT observations of shadow are then applied as an important tool to test black hole in strong gravitational field regime, as the observational data could be used to constrain the black hole parameters in MoG, and even to distinguish different theories of gravity [46][47][48][49][50][51][52][58][59][60].…”

“…Thus, physicists could use shadow to unreveal the near horizon features of black hole by analytical investigations or numerical simulation of their shadows and therein. Moreover, the size and distortion of shadow [31,32], which could be calculated via the boundary of shadow, has been widely investigated to estimate the black hole parameters in both GR and MoG, with or without additional sources surrounding the black hole [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52]. This direction could be seen as one aspect of black hole shadows to distinguish GR and other theories of gravity, or to acquire the information of the surrounding matter, though it was found that those theoretical features of shadow are usually not sufficient to distinguish black holes in different theories or confirm the details of the surrounding matter.…”

Magnetic Reconnection and Energy Extraction from a Spinning Black Hole with Broken Lorentz Symmetry

“…For instance, the role of electric charge on the BZ mechanism has been investigated recently in [68,69]. Theoretically, considering the contribution of electric charge is well motivated from several aspects: first, according to the no-hair theorem, any astrophysical BH described by three observable parameters-mass, spin, and electric charge [70,71]. Second, unlike the argument that the presence of plasmas around astrophysical BHs leads to a prompt discharge, the relevant environment is prone to some processes at both the classical and relativistic levels that result in the production of a small nonzero electric charge [72].…”

Rotating black holes with an anisotropic matter field

“…Due to this property, the shape of the capturecross-section by light is transformed into an asymmetric one. An interesting astrophysical phenomenon is that the shadow of a rotating black hole has the asymmetric bright side due to this property [43,44,45,46,47,48,49,50,51,52,53,54].…”