Cosmological black hole production in grand unified theories

Cosmological black hole production is reviewed with due consideration to interactions assumed relevant in the very early Universe. In the framework of the standard hot big bang and grand unified theories it is shown that primordial black holes would form too prolifically to be consistent with observations, if they are present in an extended mass range. This poses some constraints on the form of fluctuations present in the very early Universe, be they primordial or induced spontaneously by phase transitions in the cooling Universe.