Is a black hole shadow a reliable test of the no-hair theorem?

Abstract

Capturing the image of the shadow cast by the event horizon of an illuminated black hole is, at the most basic level, an experiment of extreme light deflection in a strongly curved spacetime. As such, the properties of an imaged shadow can be used to probe the general relativistic Kerr nature of astrophysical black holes. As an example of this prospect, it is commonly asserted that a shadow can test the validity of the theory's famous `no hair theorem' for the black hole's mass and spin multipole moments. In this paper, we assess this statement by calculating the shadow's equatorial radius in spacetimes with an arbitrary multipolar structure and within a slow rotation approximation. We find that when moments higher than the quadrupole are taken into account, the shadow acquires a high degree of degeneracy as a function of the deviation from the Kerr multipole moments. The results of our analysis suggest that dark objects with strongly non-Kerr multipolar structure could nevertheless produce a Kerr-like shadow with its characteristic quasi-circular shape.

Accepted for publication as a Regular Article in Physical Review D

References in corpus (22)

• First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
• First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
• Black Hole Shadows, Photon Rings, and Lensing Rings
• Light ring stability in ultra-compact objects
• Gravitational Test Beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole
• Photon Rings around Kerr and Kerr-like Black Holes
• Lensing by Kerr Black Holes
• Chaotic lensing around boson stars and Kerr black holes with scalar hair
• Fundamental photon orbits: black hole shadows and spacetime instabilities
• Can different black holes cast the same shadow?
• Photon rings of spherically symmetric black holes and robust tests of non-Kerr metrics
• Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry
• EHT tests of the strong-field regime of General Relativity
• Photon ring test of the Kerr hypothesis: Variation in the ring shape
• Can supermassive black hole shadows test the Kerr metric?
• Asymptotically flat, parameterized black hole metric preserving Kerr symmetries
• Multipole moments in scalar-tensor theory of gravity
• Black Hole Images as Tests of General Relativity: Effects of Plasma Physics
• Measuring the shape of a black hole photon ring
• Visibility of Black Hole Shadows in Low-luminosity AGN
• Chaotic photon orbits and shadows of a non-Kerr object described by the Hartle-Thorne spacetime
• Recent LIGO-Virgo discoveries

Cited by in corpus (7)

• Shadows and parameter estimation of rotating quantum corrected black holes and constraints from EHT observation of M87* and Sgr A*
• Shadows aspect of rotating black holes in the Einstein-AdS SU(N)-nonlinear sigma model
• Motion of particles around a magnetically charged Euler-Heisenberg black hole with scalar hair and the Event Horizon Telescope
• Spacetime Measurements with the Photon Ring
• Fractal signatures of non-Kerr spacetimes in the shadow of light-ring bifurcations
• Vacuum Spacetime With Multipole Moments: The Minimal Size Conjecture, Black Hole Shadow, and Gravitational Wave Observables
• Shadow constraints of charged black hole with scalar hair and gravitational waves from extreme mass ratio inspirals