Typically, primordial black hole formation requires density contrasts (regional variations in the universe's density) of around δ δ ρ ρ / ρ ρ ∼ ∼ 0.1 (10%), where ρ ρ is the average density of the universe. Several mechanisms could produce dense regions meeting this criterion during the early universe, including reheating, cosmological phase transitions and (in so-called "hybrid inflation models") axion inflation. Since primordial black holes didn't form from stellar gravitational collapse, their masses can be far below stellar mass (~2×10 g). Stephen Hawking calculated in 1971 that primordial black holes could have a mass as small as 10 g. But they can have any size, so they could also be large, and may have contributed to the formation of galaxies.