, 19 September 2009 (UTC) 

 :To address the question: We're assuming that the universe is a place that we can comprehend, that behaves in a self-consistent manner. It turns out that this assumption works quite well, and we have mathematical descriptions of most ways in which the universe behaves that seem to match reality for the vast majority of the situations we've looked at. The most relevant mathematical description of reality for this case is general relativity, which describes how space, time, and matter interact. While the very early universe is not described very well (we'd need a good description of quantum gravity for that), the present and future states do seem to be described quite accurately. The most important conclusion drawn from looking at the equations is that an unchanging universe isn't consistent with the way we've observed the universe working. Parts would collapse, any sufficiently large volume would be within an event horizon, light from distant stars would be visible no matter which direction you looked (making the sky white), and so forth. The best match to observations is a description that involves an expanding universe. What happens later in the expansion depends on what additional assumptions are made. This article lists the possible assumptiosn and describes the consequences for each of them.   20:09