Most astronomical objects become harder to detect at greater distances. Galaxies fade, stars dim, signals weaken with the square of the distance. The exceptions are objects detected not by what they emit but by what they block.
Lacki (2026) shows that a black hole with a mass exceeding a trillion suns — a “stupendously large black hole” or SLAB — would cast a shadow against the cosmic microwave background. The shadow would appear as a cold spot: a region where the oldest light in the universe is missing because the black hole absorbed it. The remarkable result is that this shadow becomes easier to detect at higher redshift — beyond about z = 1.6, the signal strength increases with distance. A SLAB at z = 10 is easier to see than one at z = 1.
The reason is geometric. The CMB comes from a fixed surface — the last scattering surface at z ~ 1100. A black hole between us and that surface blocks a solid angle of CMB photons proportional to its apparent size. At moderate redshift, the black hole subtends a large angle but only blocks a fraction of the CMB along that line of sight. At higher redshift, closer to the CMB surface, the shadow becomes more complete because fewer unblocked photon paths remain around the black hole. The contrast improves even as the angular size shrinks.
Current CMB data already excludes SLABs above 10^17 solar masses anywhere within the observable universe. The non-detection is itself a measurement — it constrains the maximum mass a black hole can reach over cosmic time.
The general principle: detection by absence inverts the normal distance-signal relationship. When you look for what is missing rather than what is present, the background matters more than the object. A bright, uniform background makes shadows sharper. The farther the object is from the observer — but closer to the background — the sharper the silhouette. Looking for what isn't there follows different scaling laws than looking for what is.