Expanding Universe Questions... eh, another one.

So the universe is expanding, and has expanded for ~14 billion years. As I understand it, the expansion of space has progressed at a speed that is greater than c. This is why when we point telescopes to some great distance we can see how the universe looked at an earlier stage of it’s development. Also, as I understand it, the reason earth isn’t flying away from the sun, and the sun flying away form the local group of stars is gravity. I picture it in my head like a bunch of people (stars) standing on a huge block of ice on the sea. As the ice breaks apart and begins to spread, people who were holding hands (gravity) will tend to stick together even as some other groups of people (galaxies) will drift away.

Assuming I’m right about all this, I have a question: Since space is expanding at a speed greater than c, shouldn’t we be able to see our OWN galaxy somewhere behind us as it was in the past? Couldn’t we (if we had a strong enough telescope) theoretically be able to see earth as it was in the past, just as we can see what early galaxies looked like?

No. If I interpret your idea correctly, you are thinking that maybe we have somehow ‘outran’ our own light emissions by riding a superluminal spatial expansion. That is not how it works. The basic misconception there is the idea that we are moving away from where we were as a direct result of the expansion. This is incorrect. It’s the rest of space that is ‘moving’ away from us (and every other galaxy as well). We are moving from where we used to be, but that is an entirely different process - the result of normal motion like rotating around the galaxy. Such motion cannot, of course, put us ahead of our own light.

You have a basic fact wrong. The apparent speed of recession varies with distance from us. This is called Hubble’s Law. So nearby galaxies are receding slowly (if at all) while it is only the farthest ones (that we can see) that are receding at near the speed of light. We can’t see ones farther than that since the photons will never reach us.

If the universe is closed (wraps around) and if the distance of wrap around is too large, then we cannot see our own galaxy.

This would be true even if the universe wasn’t expanding.

Imagine a universe that’s completely static. There was no Big Bang and all the galaxies are at rest relative to each other. An observer in the universe can still “look into the past” by pointing his telescope at a distant object simply because light travels at a finite speed. If he’s looking at a galaxy a million light years away, he’s looking a million years into the past. If he’s looking at a galaxy a billion light years away, he’s looking a billion years into the past. The “looking into the past” effect is a function of how far away things are, not how fast they’re moving relative to us.

Note also that there’s nothing special about our position: From our reference frame, we’re staying put and everything else is moving away from us at various speeds, but on the planet Zardox, 13 billion lightyears away, the Zardoxians see themselves at rest and everything moving away from them.

The ice floes analogy isn’t too bad, though, as analogies go. I’ll probably use that one in the future.

Are they expanding fast and we’re relatively stable in a fixed pattern or if you were standing in one of the expanding galaxies, would you perceive yourself as relatively stable and the Milky Way be expanding fast away from you?

Yes to both.

Everything is being pushed away from everything else by the expansion of space. (Because of gravity and other forces, not everything is actually moved, of course, but the pressure is there). The expansion is relative to distance, so the farther something is the faster they expand away from us (and the faster we expand away from them).

I also like the ice floe analogy.

I think what you’re missing is that photons of light are just as embedded in space as particles are. So as space expands, the photons are carried along as well. Think of photons as ants crawling away from the people: You’re never going to catch up to an ant that started from you, because the ants are being carried along with the ice, too.

Brilliant! The analogy is complete then. Ice floes + people + ants. Do we really need E = MC^2?

Thanks Dopers!