We solve dynamical equations of motion to determine the conditions under which an over-dense region in the early universe will lead to collapse to a black hole , starting from horizon crossing of the over-dense region to the point of gravitational instability .
Here we focus on the sensitivity to QCD and electroweak phase transitions .
We then solve rate equations to determine the mass distribution of black holes in the present universe .
A second order phase transition or rapid crossover would have significant consequences only if the index of primordial density fluctuations n > 1.25 .
However , a first order transition would lead to a black hole dominated universe for any realistic value of n including n = 1 .