Black Hole Shadow Drift and Photon Ring Frequency Drift

The apparent angular size of the shadow a black hole in an expanding Universe is redshift-dependent. Since cosmological redshifts change with time - known as redshift drift all redshift-dependent quantities acquire time-dependence, and fortiori so do shadows. We find mathematical description show that amplitude this effect order $10^{-16}$ per day for M87$^{\star}$. While small, we argue its non-detection can be used to constrain accretion rate around supermassive holes, well novel probe equivalence principle. If general relativity assumed, infer from data obtained by Event Horizon Telescope M87$^{\star}$ maximum $|\dot{M}/{M}| \leq 10^5 M_{\odot}$ year. On other hand, case effective gravitation coupling, derive constraint $|\dot{G}/G| 10^{-3}-10^{-4}$ on visibility frequency universal interferometric signatures photon rings also discussed, which very similar drift. This particular interest future experiments involving spectroscopic techniques, could make observations their drifts viable.