The ants are thrashing around the glue spot and therfore have a random component to their motion. Hence the total motion of the ants is due both to this random component and the expansion component. For short separations, the random component could dominate over the expansion component. This is our causal horizon - beyond this distance something would have to travel faster than the speed of light in order to communicate with us. All observers are surrounded by such a horizon.
Horizons are okay.
Our assumption about homogeneity means that the stuff beyond the horizon is the same stuff we already know about. This assumption must be correct due to horizon overlaps and causality. This is the expansion rate that I measure. I measure the balloon to have a radius of 8 feet. This means the expansion age of the balloon is 8 days.