Hunting for extra dimensions in the shadow of M87*
  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 AdS$_5$ brane-world scenario, we show that the observation of
M87*'s dark shadow sets the limit $\ell \lesssim 170\,{\rm AU}$, where $\ell$
is the 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.
