Cosmology query about 'expanding space'.

If space is supposed to be expanding, can someone tell me how 2 galaxies can manage to collide, as ours and the Andromeda galaxy are reckoned to do at some point?

How can galaxies cross each others paths if they are all moving away from a theoretical centre?

Also, if gravity is such a weak force, how can it have an effect on objects moving hundreds of thousands of miles per hour, when it can’t even keep a rocket travelling at 17,000 mph from escaping its pull?

Gravity is proportional to the mass involved. The mass of a galaxy is in the range of 10 million to 1 trillion times that of the sun. You can’t compare that to the trivial mass of the earth. That same rocket wouldn’t achieve escape velocity off the sun. Speed in and of itself is only relevant to the amount of mass it’s trying to escape. A black hole can capture massed particles traveling near the speed the light. That’s nothing but the effect of a large amount of gravity.

Space as a whole is mostly empty. Gravitational attraction exists between nearly galaxies or nearly groups of galaxies. But the entire universe has such a small density that gravity is presently thought to be insufficient to stop its expansion.

OK, first of all, the cosmological expansion only becomes relevant on scales larger than galaxy clusters. On a cosmological scale, the Andromeda galaxy is extremely close to us.

Second, there is no center to the expansion of space. It’s occurring equally everywhere.

Third, the fact that something’s moving has no relevance whatsoever to what gravitational force it feels. Anything that has a mass will feel a force from everything else that has a mass, and will therefore accelerate. The reason there’s such a thing as escape speed is that gravity gets weaker with distance, so for something traveling fast enough, gravity gets weaker quickly enough that it never has a chance to completely stop it, but it’ll still always be slowing down.

The Hubble constant, which determines the rate of expansion, is about 70 km/s per megaparsec. That means that if you have two objects one megaparsec apart, the expansion will be pushing them apart at 70 km/s. Now at the scale of human beings, 70 km/s is pretty fast, but on the scale of galaxies that’s negligible, galaxies are trillions of kilometers long and moving at much faster speeds due to gravity. It’s only when you look at objects many many megaparsecs apart that the expansion rate gets high enough to overcome gravity.

You can also use that to see why the expanding universe doesn’t pull apart individual galaxies or solar systems. On that scale, the expansion rate is so tiny that we wouldn’t even able to measure it in a laboratory.

This seems to rule out any travelling to other galaxies then, unless we can attain speeds faster than the expansion, eh? Or we wait for one like Andromeda to pass us by or collide with us.

You’re about 1000 steps ahead of the state of the art. We still haven’t figured out how to get a human being to another star in our own galaxy within his or her own lifetime yet. Other galaxies may as well be other universes as far as that kind of thing goes.

You’re not getting it. The expansion of space will pull everything along with it. It would be like riding the current in a river, except that it is equal in every direction. A spaceship would not have to account for it.

We’re not even really sure what we’re expanding into. It’s like a 4th spacial dimension.

You may be thinking that “expanding space” means that there are things flying away from each other. It’s not necessarily like that. Things are getting further apart because the space in between them is getting stretched. Over the course of time, I guess you could say, 1 meter ain’t what it used to be.

It’s like we’re all ants on the surface of a balloon that’s being blown up. It’s not just the different ants on the balloon that are moving apart. it’s that there’s “more balloon” between them. That’s an entirely different thing from peculiar motion, where an ant would start walking toward another one.

So your question can be restated as “If the balloon is being blown up, why can the ants hit each other?” and the answer is obvious…they’re walking toward one another.

I know I skipped a lot of technical stuff there but it’s hard to explain and I’m not even sure I understand it myself. I gave the meat of it…can someone else make the potatoes?

The current rate of expansion is not enough to overcome the gravity of galactic clusters. So the 30-some-odd galaxies that make up the Local Group are with us for the long haul.

However, observers in the far distant future may have no way of discovering the true magnitude of the universe. All they’ll be able to see is a small collection of nearby galaxies and beyond a great void stretching out to infinity. All the other galaxies will be receding faster than the speed of light.