The Universe is expanding at an increasing rate, and from what I've read there is still much debate around its fate. Will it continue to expand forever, slow to a constant rate of expansion, or slow and fall back in on itself? Why can we not yet prove that the Universe will eventually fall back in on itself? Gravity is a function of mass so if at some point in the future more mass exists than space in the universe it will begin to decelerate its expansion and eventually fall back in on itself. So we just need to be able to answer the following question; Is the formation of matter occurring faster than the formation of empty space? Or, Is the universe becoming more dense? And of course, is it becoming denser at a constant rate, increasing rate, or decreasing rate. The conundrum eventually becomes; how many integrals of this thought process are required to reach a constant? If everything started as a huge cloud of gas following the big bang it stands to reason the universe is getting more dense, because it certainly isn't gas now (of coarse there's still the issue of how fast is empty space being created). I'm just surprised we haven't thought of a way to prove a constant exists that supports the theory that the universe is becoming more dense and will eventually collapse back in on itself.