|MadSci Network: Astronomy|
Most cosmologists would say that the current evidence suggests that the Universe will expand forever, but models which include a reversal of the expansion and then a big crunch haven't yet been ruled out.
[In particular, I believe that the far future history of the Universe depends in part on the characteristics of "dark energy" that is causing the Universe's expansion to accelerate in the present epoch. It may be possible for dark energy to have characteristics, e.g., such as changes in time, that would permit the Universe to collapse in the far distant future. Moderator]
In the present day Universe, the expansion means that clusters of galaxies are getting further and further away from each other, carried along by the expansion. If the expansion reversed, then this would reverse.
[More explicitly, the expansion of the Universe does not mean that matter is expanding. Rather on large scales (on scales larger than roughly 120 million light years) the distances between objects increases with time. Presumably, during a Big Crunch, this process would reverse, but matter would not shrink. For more details on the Universe's expansion, consult the Cosmology Tutorial. Moderator]
On smaller scales, though, it's local gravity that matters; even though the Universe is currently expanding, our Galaxy is approaching its nearest large neighbour, the Andromeda Galaxy. (We'll actually collide in a few billion years). Even if the Universe's expansion does reverse and a "Big Crunch" ensues, it will take many billions (perhaps even trillions) of years before the Big Crunch will occur. Thus, the Andromeda Galaxy-Milky Way Galaxy collision will occur well before the Big Crunch.
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