Expert: Courtney Seligman - 2/16/2011

Question
In your 10/27/2009 answer about expansion of the universe you used terms like 'the center' and 'the edge' of the universe.  Is the concept that the universe is closed inconsistent with the current concept of expansion? It was my understanding that if the universe is a closed four dimensional manifold then it would have no center or edge. However it would have an antipodal distance (which might correspond to a radius in a three dimensional universe) and a volume. Does recent evidence indicate that the universe is not closed?

Answer
Reviewing the answer I think you are referring to, I don't see any reference to a center or an edge, so I may be looking at the wrong question; but I'm sure that if I referred to the center of the Universe I meant the "apparent" center, which is wherever you happen to be, and the "edge" would refer to the most distant regions we can see, not an actual physical edge, because as far as we know there isn't any such place. (I did notice an error in the expansion rate, which was thought to be about 75 km/sec/Mpc at that time, but is now generally agreed upon as more like 70 km/sec/Mpc).

In any event, those errors or confusing statements have little to do with your present question, so I'll consign them to my dustbin of lost memories, which sometimes seems as vast as the Universe itself.

It is true that for several decades there was considerable debate about whether the Universe might be closed, "flat", or open. In the late 60's, it appeared that the closed Universe concept had been thoroughly defeated, as there just isn't enough mass to give space the required curvature; but the apparent success of Inflation Theory, which suggested that the Universe was so nearly flat that it would be impossible to tell whether it was flat, closed or open, gave renewed hope to those cosmologists who (for whatever reason) preferred the idea of a closed Universe.

Recent measurements, however, particularly by the GALEX spacecraft, and of the distances of extremely distant galaxies determined from Type Ia supernovae, have proven beyond any reasonable doubt (to better than 99.9% certainty) that the Universe is "open", and neither flat nor closed. That hasn't kept those who are used to talking about flat or closed Universe models from continuing to do so; barring spectacular improvements in measurement technology, they will continue to cling to their outmoded theories until death removes them from the discussion (this is in fact often the way that old ideas disappear; acceptance of new ideas is rarely as universal or popular as history books imply).

So to summarize, the Universe is not closed or flat, but open; and it has no center or edge in any normal sense of either word. But wherever you are will appear to be the center of a sphere whose apparent edge is moving away from you at the speed of light, and whose radius in light years is the same as the current age of the Universe in years.

And of course, having wandered around a great deal of territory, not all of which directly applies to your question, I may not have answered it in a completely satisfactory way; so if you have any remaining questions, please feel free to ask for a clarification.