The cosmological evolution of primordial black holes ( PBHs ) is considered .
A comprehensive view of the accretion and evaporation histories of PBHs across the entire cosmic history is presented , with focus on the critical mass holes .
The critical mass of a PBH for current era evaporation is M _ { cr } \sim 5.1 \times 10 ^ { 14 } g. Across cosmic time such a black hole will not accrete radiation or matter in sufficient quantity to hasten the inevitable evaporation , if the black hole remains within an average volume of the universe .
The accretion rate onto PBHs is most sensitive to the mass of the hole , the sound speed in the cosmological fluid , and the energy density of the accreted components .
It is not easy for a PBH to accrete the average cosmological fluid to reach 30 M _ { \odot } by z \sim 0.1 , the approximate mass and redshift of the merging BHs that were the sources of the gravitational wave events GW150914 and GW151226 .
A PBH located in an overdense region can undergo enhanced accretion leading to the possibility of growing by many orders of magnitude across cosmic history .
Thus , two merging PBHs are a plausible source for the observed gravitational wave events .
However , it is difficult for isolated PBHs to grow to supermassive black holes ( SMBHs ) at high redshift with masses large enough to fit observational constraints .