Shadows in conformally related gravity theories

TL;DR

This paper demonstrates that the shadow of a black hole is not conformally invariant and proposes a method to constrain parameters in conformally related gravity theories using observational data from M87*.

Contribution

It reveals the non-invariance of black hole shadows under conformal transformations and introduces a new approach to test gravity theories with observational data.

Findings

Constrain parameters in Brans-Dicke theory using EHT data.

Limit parameter space in brane-world gravity models.

Show that black hole shadows differ in conformally related frames.

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$87^{*}$ by the Event Horizon Telescope.