B Galaxy Collisions & Expansion: What's Going On?

[Star32503]

Hi, this is just a question I've had for years and have not been able to figure out because of conflicting information. I have read that space is expanding and that it's speeding up, but also have heard that in about 5 billion years our galaxy and Andromeda will collide. How is this possible?

Also, if a galaxy is moving away from another galaxy and each one is moving away from the other where does the extra space come from? At least this is what I got from watching a Stephen Hawking show some time ago.

Thank you,
Andre

 

[kimbyd]

Short but still detailed answer:
Typical galaxy motions with respect to the background are about 1000km/s.

The typical rate of expansion between any two galaxies reaches 1000km/s at roughly 45 million light years. Andromeda is about 2.5 million light years away.

So basically, galaxies do move around. And if they're close enough those motions may allow them to collide. But once they get far enough away, the expansion becomes far more important. Andromeda is so close to us that the expansion doesn't really matter at all.

 

[Vanadium 50]

Let's start with an easier problem - can two cars collide if the universe is expanding?

 

[Ibix]

Also, if a galaxy is moving away from another galaxy and each one is moving away from the other where does the extra space come from?

Space doesn't have to come from anywhere.

The solutions to Einstein's field equations are four dimensional manifolds. To see the one describing our universe on a cosmological scale as "space expanding" you have to imagine slicing it into a series of 3d manifolds, each one being "the universe at one instant", then compare each one to the next. But first, this is an artificial process - nothing is coming from anywhere. We just sliced up a solid. So in a sense, the only available answer to your question is "when we know where the universe comes from (and if asking that question even makes sense) we'll be able to answer". Another thing to think of is that our current models regard the universe as spatially infinite. So each of the "slices" is the same size (infinite), whatever you might naively think from the fact that everything is getting further apart.