Black Hole Shadow Drift and Photon Ring Frequency Drift

Understand

The apparent angular size of the shadow of a black hole in an expanding Universe is redshift-dependent.

• Since cosmological redshifts change with time - known as the redshift drift - all redshift-dependent quantities acquire a time-dependence, and a fortiori so do black hole shadows.
• We find a mathematical description of the black hole shadow drift and show that the amplitude of this effect is of order $10^{-16}$ per day for M87$^{\star}$.
• While this effect is small, we argue that its non-detection can be used to constrain the accretion rate around supermassive black holes, as well as a novel probe of the equivalence principle.