Expert: Courtney Seligman - 4/1/2010

Question
We see distant galaxies receeding away from us, doesn't this indicate that the universe is spherical, exploding from a central point? If so, has anyone used this data to triangulate the location of the Big Bang and come to the total radius of the sphere?

Answer
You have to make a distinction between what we can see, which is called the Observable Universe, and the "real" Universe, which extends far beyond what we can see (how far is anybody's guess, but most estimates range from a few hundred to a few million times the size of the Observable Universe).

The Observable Universe is a sphere, centered on the observer, which is expanding away from him, in all directions. It has a radius of about 14 billion light years, because if something is that far away, the expansion of space between it and the observer is carrying them away from each other at the speed of light, so that no light emitted by more distant objects can overcome the expansion of space between them and the observer, and is forever unobservable.

Now, it does not make any difference where the observer is, in this picture. No matter what direction he looks, he can see out to about 14 billion light years distance, because things closer than that are being carried away from him at less than the speed of light, and their light can reach him. But he cannot see anything further away.

If you pick a different observer, say halfway between the first one and the boundary of "his" Observable Universe, that observer would be moving away from the first one at half the speed of light. Looking further out, he would be able to see the 7 billion light years beyond him, AND another 7 billion light years beyond that, because his motion away from us means that things beyond him are not moving away from him as fast as they are moving away from the first observer.

Now if he turns around and looks back at the first observer, he would see them moving away from him at half the speed of light. He would also be able to see 7 billion light years beyond them, which is half the distance the first observer can see in that direction; but he would not be able to see the 7 billion light years beyond that, which the first observer can see, because being more than 14 billion light years from him, it would be carried away from him by its motion away from us, and by his motion away from the first observer, at more than the speed of light.

(You might want to draw a diagram to help with the above.)

Applying this to "us":

Someone 7 billion years off to our side can see another 7 billion light years beyond what we can see in his direction, because he is that much closer to things over there. But he sees 7 billion light years less than what we can see in what he sees as our direction, because he is 7 billion light years further away. But in one way, he still sees exactly the same thing -- a sphere, centered on him, 14 billion light years in radius. His sphere is not the same as ours, in that part of what he sees, we can't, and part of what we see, he can't. So his Observable Universe, although partially overlapping ours, is not exactly the same. But for both of us, and for any other observer, no matter where they are, our or their Observable Universes would have exactly the same size and appearance -- a sphere, centered on each observer, with a radius in light-years equal to the age of the Universe in years.

In other words, what we see is only a small fraction of a much larger structure, which is The Universe. The part we see, the Observable Universe, is a sphere centered on us. But you could move that sphere anywhere you want, and center it on any other observer, and they would see exactly the same thing -- an Observable Universe centered on them, expanding at the speed of light. Their Observable Universe would overlap ours, if they are within 14 billion light years of us. If they were further away, their Observable Universe and ours would contain completely different galaxies. But so far as we can tell, although the details might be different, the overall picture would remain the same.

So the spherical shape of the Observable Universe is an illusion caused by our point of view, and there is no center in the ordinary sense of the word; and there is no way to do any measurements which would tell you where such a center would be.