Expert: Courtney Seligman - 9/24/2009

Question
Where is cosmology now on the overall shape of the universe? Is it a big sphere, with the galaxies all moving away from each other? Then what is in the center, a big space? What about the old question, if one headed out from earth in a spaceship on a straight line trajectory, with the earth always in the center of the rear view mirror, where would you go? Would you wind up eventually heading back to earth? Is the universe analogous in three dimensions to the two dimensional surface of a sphere, finite yet unbounded?

Answer
It is probably a blob of no particular shape, which is growing in all directions at an effectively infinite rate, meaning a rate so large that most of it is moving away from most of the rest of it faster than the speed of light, which makes most of it unobservable, no matter where you are. Only a very tiny portion of it would be close enough to any given observer for them to be able to observe it. Given that, what if any shape it has would be unknowable, but locally, would be called 'hyperbolic', which is more or less the opposite of spherical. Whereas on a sphere the opposite side of the Universe would be in all directions from you, so that if you moved away from it in one direction you would get closer in the opposite direction, in a hyperbolic space there is only one direction to a given object, and as you move away from it you just get further and further from it, no matter how you measure the distance.

It should be noted that when I say it is growing in all directions, that does not mean that it is growing outward into some previously empty space. Its size could be thought of in the same way as a picture that you are editing with a photo-editing program. If you want to see a particular part of the picture in more detail, you can zoom in on that part, making the overall size of the picture appear to be larger; but the picture doesn't change, just the apparent distance between objects in the picture. Similarly, the Universe doesn't really change its size relative to any (nonexistent) surroundings. The distances between things just get bigger and bigger, because the empty space between them gets larger and larger.

If you headed away from the Earth forever, you would just keep getting further and further from it. There is no limit to how far you can go. At any given moment, the furthest distance is finite; but over time, the furthest distance will increase at an exponential rate (e.g., when it is twice as big it will be growing twice as fast), and in an infinite future, become infinite. So it is now finite and unbounded, but in the infinite future it will be infinite and unbounded.

As far as we know, there is no center as such. Every part of it is as close to the center as any other part. But we can only know about the observable part, which is probably so small compared to the whole that even if there were a center, we wouldn't have any way to tell where it is. In any event, what is in the center should be the same as everywhere else. Everywhere in the Universe there are galaxies, clusters of galaxies, super-clusters, web-like structures filled with galaxies and clusters, and relatively empty voids. Which galaxies you see in one place would be different from the ones you see in a different place, but the overall view would be about the same.