Jack's Notes

The Second Friedmann Equation describes the acceleration or deceleration of the expansion (or contraction). It describes the forces pushing or pulling on the expansion. 

is again the Scale Factor. 

is the rate at which the rate of expansion is changing.

The value of this ratio depends on , which is the density of matter and energy. The more density, the harder gravity pulls inwards to try to stop the expansion (or speed up the collapse). 

The negative sign means that density is pulling inwards. 

As for pressure , we see that for the acceleration to increase the velocity of expansion, the pressure must be negative, which creates a sort of repulsive gravitational force.

The source of this negative pressure remains unknown, although the author once again puts forth the Blast Wave as a worthwhile candidate for consideration.