If the the universe is expanding or inflating and everything is moving away from everything else at an increasing rate, why do galaxies collide? At what scale are all things moving away from each other? How far out do we have to look before we can’t see anything that’s getting closer? What am I missing here?
Galaxies collide for the same reason that things on earth don’t fly apart. The local pull of gravity is greater than the force of the overall expansion of the universe.
Hubble’s Law puts a number to the overall expansion rate. It’s in kilometers/second per megaparsec. A megaparsec is 3,260,000 light-years. So the expansion only gets noticeable when applied to clusters of galaxies, not anything local.
Also, keep in mind that it’s not that the galaxies themselves moving away from each other, per se, but the actual fabric of spacetime that’s expanding.
Think of boats on a lake, but a large, continuous deluge of water is pouring into the lake, making the area of the lake grow and pushing the boats apart. This doesn’t mean a couple boats on a collision course wouldn’t still hit each other so long as they’re outpacing the expansion of the lake, itself.
Don’t have the foggiest idea what this means but I’d sure like to know the answer, SD style of course.
Well, the Andromeda galaxy is getting closer to the Milky Way galaxy, for one. This is often reporting as an impending collision, though a collision is not assured. (Actually, double-checking Wikipedia, it appears that the official word is that we will collide. That was announced back in May this year). It’s getting closer because of gravity and motions within our local cluster, and it’s close enough that the expansion of space-time isn’t a very big factor.
Get out far enough, though, and the expansion becomes more relevant.
Doing a little Googling, I find a link to another forum: Why are blue-shifted galaxies concentrated in specific sky directions? that discusses a list of blue-shifted galaxies one user claims to have put together (but doesn’t provide directly). It looks like you can follow his list to put the list together for yourself, but I’m not seeing distance in the output I get.
Maybe someone else can pick up the torch… I have to stop procrastinating now.
Or our money back?
Only if 30 million years late or more.