Thread: expansion and collisions

More or less, yeah. Just think of mutual gravitation effectively cancelling out the expansion.

The question was: How can galaxies collide if the universe is expanding?
This example might help:
If you put a small amount of air in a balloon, place the balloon in a bell jar and slowly remove the air around the outside of the balloon from the jar the balloon will expand. The molecules of O2, CO2, and Nitrogen in the air sample will continue to collide with each other as the balloon expands, but the collisions will become less frequent and the overall temperature will become lower. The universe acts similarly. The galaxies remain intact but collide less frequently as universe becomes less dense.

Unlike galaxies, O2, CO2, and N do not react with each other. If you were to introduce a molecule that oxidizes to the balloon, it would combine with the O2 at a rate that would decrease as the space between the molecules increases and the temperature decreases. If the reaction produces heat, that would add to the temperature and increase the rate of expansion. Again, the universe acts similarly. If, however, it absorbs heat, like a black hole, it would cause the expansion to slow. It is not always necessary to dabble in Relativity to get a feel for these things.