What are rarefractions?

Rarefraction is the reduction of an item's density, the opposite of compression. Like compression, which can travel in waves(sound waves, for instance), rarefraction waves also exist in nature. A common rarefraction wave is the area of low relative pressure following a shock wave.

Diminution in the density of something, especially air or a gas

If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.