Re: Implications of a flat universe?

New evidence from observations of supernovae and the cosmic microwave background indicate that the geometry of the universe is flat. A flat universe is infinite in extent, so there is no 'edge' to the universe. You're right that the balloon analogy is only strictly accurate for closed geometries, but it is still a good way of visualizing an expanding universe. A more appropriate analogy for a flat universe would be an infinite rubber sheet which is being stretched in all directions simultaneously.

There's a subtle point here that is rarely appreciated. When we talk about expanding balloons or rubber sheets, we think of them as expanding into something. When referring to the geometry of space-time, however, space itself is expanding; it is not expanding into anything. In this respect, the above analogies are misleading, but they're the best ones I know for getting a handle on a difficult concept.

In one sense, there is a center to the universe, or rather, there are infinitely many centers of the universe. Because the speed of light is finite, we can only see things that are closer to us than the speed of light times the age of the universe. Thus, the most distant things we can see are contained in a sphere with this radius. This sphere is often referred to as 'the observable universe'. The cosmic microwave background is very exciting because it is the most distant thing we can see in the observable universe. Anything more distant is 'hidden' behind this opaque radiation, whereas anything outside the observable universe will always remain unknown to us unless the universe gets enough older that light from these regions has time to reach us.

An observer in an extremely distant galaxy might see a different pattern on the microwave background, but the statistical properties of the pattern should be the same as ours. Anything else would violate the cosmological principle that the universe should look the same, in a statistical sense, regardless of the location of the observer. To us, it looks like we're the center of the universe, but observers in distant galaxies believe that they are at the center of the expansion as well. Since the universe (if flat) is infinite in spatial extent, there is no center to the entire universe, although every observer is at the center of his or her observable universe.

Great questions! Here's some ideas for further reading:
Finkbeiner, A. Sky and Telescope September 1998, p.38
Adams, F.C. & Laughlin, G. Sky and Telescope August 1998, p.32
Kaufmann, W. Universe, Chapter 28
Silk, J. A Short History of the Universe, Chapter 6

P.S. According to current observations, the universe can only be flat if the cosmological constant is not equal to zero. Since the physical meaning of the cosmological constant is difficult to understand and because it makes things more complicated, many popular treatments of cosmology ignore it or assume that it is zero. The most recent edition of Kaufmann does include a special box describing some of the possible effects of a cosmological constant. The current favorite is a positive cosmological constant slightly smaller than the critical value.