Everything is expanding – and so this is a natural question to ask. How can everything be expanding away from every other thing, and yet still collide?
Part of the blame for this confusion lies with the sorts of diagrams and language we use to demonstrate the expansion of the universe. If I say “the space between every galaxy is expanding, so that each galaxy appears to drift away from every other galaxy”, that’s a good way to get you to imagine an expansion of space. It also means that I’m ignoring everything else that’s going on that might be complicating the situation, to make the expansion of space idea as clear as possible.
In this case, what’s complicating the situation is our old friend gravity. If each galaxy in the universe were evenly spaced out – for instance, if they were all laid out as though they were points on a grid – then the simple description is also an accurate one. There wouldn’t be anything else going on. Each galaxy would continue to evolve in total isolation, slowly drifting farther away from anything else.
This isn’t what our universe looks like. Our universe looks much more cobwebby than gridlike, with big knots of galaxies, and little filaments of galaxies stretching away from each knot. The big knots are galaxy clusters, and can hold thousands of galaxies. Their smaller counterparts, galaxy groups, have a few galaxies in them. Our own galaxy is in a small group, with Andromeda, and a bunch of very small dwarf galaxies.
These clusters and groups are what happens when galaxies form close enough to each other that gravity can pull them together. If a galaxy is close enough to another galaxy, and not moving too fast, gravity will prevent them from ever truly separating again. These galaxies may spend many billions of years falling towards each other, and will generally miss each other on the first attempted collision, so will spend many more billions of years falling back together for a second, and then perhaps a third attempt. Our galaxy and Andromeda are in the first fall together stage, which will probably take about 3 billion more years before it’s hard to disentangle our two galaxies.
Fundamentally, the fact that we see galaxy collisions comes down to two things; galaxies didn’t form on a grid, and the force of expansion of our universe is less strong than the force of gravity for galaxies which are near each other. If the force of expansion were much, much stronger than it is, then even gravity might not be able to pull galaxies together, and each galaxy really would be an island universe, isolated for all time. Fortunately for us, gravity still reigns supreme as long as the conditions are right.
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