Quote from DonAnd my take is that this was the primordial situation with all atoms falling in parallel "straight down." However, once a couple collisions happened, the order was interrupted by collisions and conglomerations in parts of the cosmos. In other parts, the parallel falling continued. And so on.
I've never been able to reconcile a 'primordial' downward movement with the concurrent claim that there was no beginning.
I have been considering this point as well.
Epicurus' third elementary principle proposes that "the universe is as it always was and always will be". If the distribution of matter in the universe has always been the same, it seems to follow that there would not have been a period where most particles were moving in a consistent, parallel stream in the same, perfectly "downward" movement. At all points in time, there was never an absolute "up" and "down", and at all points time, particles were falling against each other, so a deviation from perfectly straight paths would not be necessary, because linear motion can describe this.