Maximum PBH Mass and Primordiality

In order to avoid unacceptable $\mu$-distortions inconsistent with observational data on the Cosmic Microwave Background, Primordial Black Holes (PBHs) must be less massive than ${10}^{12}{M}_{\odot }$, quite closely above the highest black hole mass yet observed. This comparableness leads us to posit that all supermassive black holes originate as PBHs.