Is the expansion of the universe constant?
Constant over space or over time? As far as we know, the expansion of the Universe is effectively constant over space. That is to say, all parts of the Universe are expanding at the same rate. However, this is NOT the case for time; the expansion of the Universe is actually changing in speed over time. However, it has nothing to do with there being ‘no force in a vacuum’. Actually, there ARE forces in a vacuum, in particular gravity. Gravity actually serves to pull the Universe closer together and slow down the expansion, and for many years there were debates over whether the Universe would converge towards a constant expansion forever (an ‘open universe’), or converge towards zero expansion (a ‘flat universe’), or eventually pull itself back together into a Big Crunch and kill us all (a ‘closed universe’). However, in recent times we have discovered that there is in fact another force, which we call ‘dark energy’, and which opposes the pull of gravity more strongly than gravity itself
