Clown

The analogy of stickmen living on 2D paper universes is also useful with the concept of curved spaces.

Spacetime does not owe it's existence to matter in the sense of particles, atoms, etc. GR can describe universes where no matter exists at all, but this still doesn't leave room for independent existence of space, because the gravitational field still exists. The motion of an object will at the very least, be in relation to the field. The empty space in between galaxies is expanding.

Wow, cranky.

I think there should be a theory development forum here.

It has wonderful pictures. Some of these artists working on images for physics books (and CNN's space website) can make a mediocre book worthwhile. But maybe I just like that kind of art more than most folks.

Does what because everything moves?

Yes, Homer Simpson's hyperdounut. It's one possible multiply connected shape for the universe, while a hypersphere would be simply connected.

The universe is not expanding into a vacuum or any medium. The flat space in between galaxies itself expands, and there is no need for an embedding space.

Sounds like the block universe.

He's right, don't. Anyway, I think it's easier to just say the universe is infinite and that's why there's no edge or center. It's much easier than trying to explain Non-Euclidean geometry with clumsy balloon analogies.

In the normal rules of euclidean geometry, it would seem you would find a center of a finite universe. But remember that we aren't dealing with Euclidean geometry - we are dealing with the curved spacetime of general relativity. Without learning Non Euclidean geometry, you can use the analogy of any curved 2D surface to see how the universe could be without a center. Often, surfaces such as that of tables are flat, but they can also be curved as in the case of a balloon. In that case, at no X,Y point will you be able to find a center. The analogy has its shortcomings, mainly because the universe is not the curved area of a surface, it's a curved volume. Furthermore, the term "fabric of spacetime" is somewhat of a popular physics word that isn't meant to be taken too literally. Spacetime isn't considered to be an aether like medium, and there is no actual substance that empty space is consisted of. Following from that, the universe should be in no danger of tearing from the expansion.

I'll second that. WC2 has a good thing going, and I would have liked to see a WC3 built on that with additional units, updated graphics and new features. This new style with the RPG element ruined it for me.

Very true. Whenever a sucessful theory is replaced by something better, the new theory is usually an improvement on the old. It's never a case of throwing away the old completely.

In the case of the expanding universe, we'd need a new model that can explain the evidence as well as GR can.

There's also really no quantum theory of spacetime, so the classic model of the expanding universe is all we have to explain the data. As of now, the big bang is purely a classic theory.