Hunting for extra dimensions in the shadow of M87*

Abstract

The Event Horizon Telescope has recently provided the first image of the dark shadow around the supermassive black hole M87*. The observation of a highly circular shadow provides strong limits on deviations of M87*'s quadrupole moment from the Kerr value. We show that the absence of such a deviation can be used to constrain the physics of extra dimensions of spacetime. Focusing on the Randall-Sundrum ${\mathrm{AdS}}_{5}$ brane-world scenario, we show that the observation of M87*'s dark shadow sets the limit $\ensuremath{\ell}\ensuremath{\lesssim}170\text{ }\text{ }\mathrm{AU}$, where $\ensuremath{\ell}$ is the ${\mathrm{AdS}}_{5}$ curvature radius. This limit is among the first quantitative constraints on exotic physics obtained from the extraordinary first ever image of the dark shadow of a black hole.