Out of curiousity, what do you personally think will finally finish off the human race, j.d? Also, how come the expanding universe won't just push galaxies further apart?
As for what I personally think will do us in -- bluntly (albeit vaguely) our own stupidity... or, rather, our lack of motivation to overcome such. That's the damnable thing, to me... so many of the things we face that could mean our extinction we could overcome if we'd just learn to stop thinking in the same fashion we have for the past 10,000 or so years... While surface thoughts are somewhat altered (though the change is more apparent than substantive), our basic ways of looking at things (especially our interactions with each other) really haven't changed since the earliest civilizations arose. Unless we do, we are likely to not resolve the problems of: pollution; global warming; overpopulation (and scarcity of resources); collisions with roaming celestial bodies; or the supervolcanoes, for a start.
And as for why the expanding universe doesn't push them further apart: because they're not all moving in the same direction. Whether or not the universe began with a single big bang (and it may have been from several), other factors have altered the orientation of the matter moving within it, from supermassive black holes, to gravitational attraction between galaxies that were close enough for that to overcome the initial impetus of the Big Bang, to dark matter (or dark energy), etc. If it was more than one, of course, it could be because the initial matter was already being propelled toward a collision. Those are still questions where we're gathering lots of information -- and more is coming in all the time -- to refine our understanding of the origins of the universe, as well as being able to predict more accurately its future development.
One of the most exciting aspects of this is the recent idea of multi-dimensional space (for example, the universe may indeed be an eleven- (or even higher) dimensional one, but our part of it may be a three-dimensional pocket within that higher-dimensional frame... therefore, there is much we would not be able to observe directly, because we simply never developed the sensory equipment to do so; we can only observe indirectly, by the effects on that which we can observe. Some of this may be resolved within the next few years through the new supercollider that should be operational soon. (And as for the above concept... Shades of Lovecraft!
)
I highly recommend Lisa Randall's
Warped Passages for a good introduction to some of these concepts, their bases, and the possibility that we may soon be actually able to verify (or falsify) at least some of them empirically.
