Shadows in conformally related gravity theories

Abstract

Null geodesics are invariant under a conformal transformation, and thus it might seem natural to assume that the observables corresponding to the shadow of a space-time are also conformally invariant. Here, we argue instead, that since the Arnowitt-Deser-Misner mass and the active gravitational mass of an asymptotically flat space-time are not, in general, invariant under such conformal transformations, the shadow radius for photon motion in a space-time would be quantitatively different, when viewed from two different conformally related frames, although the expression for the shadow radius is similar. We then use this fact to propose a novel method to constrain the relevant parameters in a gravity theory conformally related to general relativity. As examples of our method, we constrain the parameter space in Brans-Dicke theory, and a class of brane-world gravity models, by using the recent observational data of M$87^{\ast }$ by the Event Horizon Telescope.

12 Pages, 3 Figures

References in corpus (8)

• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
• First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
• A Metric for Rapidly Spinning Black Holes Suitable for Strong-Field Tests of the No-Hair Theorem
• Black hole shadow in a general rotating spacetime obtained through Newman-Janis algorithm
• Selected topics in scalar-tensor theories and beyond
• Einstein-Æther Gravity in the light of Event Horizon Telescope Observations of M87*
• Constraining alternatives to the Kerr black hole
• Bumblebee gravity with a Kerr-Sen-like solution and its Shadow

Cited by in corpus (4)

• Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A${}^{\ast }$
• Image of Kerr-Melvin black hole with thin accretion disk
• Superradiant evolution of the shadow and photon ring of Sgr A${}^{\star }$
• Deciphering signatures of Kerr-Sen black holes in presence of plasma from the Event Horizon Telescope data