Expert: Steve Nelson - 2/9/2008

Question
just a silly question from a man whose interested..

if the universe is expanding and the galaxies moving away from eachother.....how come sometimes they collide?

also, if the expansion will go on "forever".  as the galaxies repulse over long distances or something (please explain if you can)  hare does the energy for this reaction come from?  The one thing i do know is that movement needs energy.  but nobody has ever explained what gravitational energy is, hare it comes from, i know magnetic energy comes from the Earth's molten spinning core (for instance) but all i hear about gravitational energy is that it has something to do with mass, they used to describe it (when i was a school) that mass bends space, and like to balls in a hummock, things roll together.  but that seems to be out of the window these days with gravitational repulsion.

please feel free to edit the above question.  i do ramble.

thanks

Constantine

Answer
Galaxies are still subject to local gravitation within their cluster and do move in different directions.  But on average they're all moving apart because space itself is expanding.  It's not repulsion, really, it's that space itself is increasing in size.  That's where the increasing expansion of the universe is coming from.  

For more technical details, consult hyperphysics:  http://hyperphysics.phy-astr.gsu.edu/Hbase/grav.html