As I understand the big bang theory, from a singularity the universe expanded at or above the speed of light for some period time, then slowed and is now again speeding up the expansion. That said as with a normal explosion - all thing moving outward somewhat evenly in all direction. If you turn the arrow of time then backward all things move back to the center of the explosion. So in other words even if all objects in space are moving away from each other as space expands, somewhere logically it all started as a singularity and all things move out in all directions somewhat evenly so again why no center of the universe? Is it that the universe is just to expansive for us to get a handle on and move the arrow of time backwards to the center? Or is it more like the string theory and we had two bains slamming into each other which created all the space at the same time and not from a singularity?

Thanks

Webo

I think that a lot of the confusion is due to only thinking of the spatial aspects of the Big Bang. While it may seem reasonable to ask where the center of a three dimensional volume is located, this ignores the temporal dimension; you should also be asking 'where is the center of time'.

Now from our point of view, the universe is about 14 billions years old, so if we define the center of time as being the middle point then we'd say that the center of time is 7 billion years ago, which is not only pretty meaningless but is also only applicable to now; when the universe was younger, let's say 10 billion years old, the 'center' would have been five billion years before then, or five billion years after the BB, and we can extrapolate this into the future to say that when the universe is forty billion years old its center will be in our future i.e. 20 billion years after the BB. A pretty meaningless deduction, I think you'll agree.

It's necessary then, to think in four-dimensional space-time, at the very least.

There there's the issue of how can something that has infinite size have a center?

When you look into how much space there is in an n-dimensional environment you find that there is infinite n-1 dimensional space within an n-dimensional environment e.g. there is room for an infinite two-dimensional area within a finite three-dimensional volume, just as there is room for an infinite length one-dimensional line within a finite two dimensional area (as the line is one-dimensional it has no width, so you could imagine drawing it in tightly packed folds, with each 'fold' being infinitely close to, but not touching the preceding fold; the same applies for the two-dimensional area within a finite three-dimensional volume).

In our four-dimensional space-time then, which seems to be finite i.e. it's only 14 billion years old, there would appear to be room for infinite three-dimensional space, which being infinite, cannot have a center.

About the best we can do then, is to say that the universe doesn't have a center, but had an origin, and that origin is located about 14 billion years ago.

In the Randall-Sundrum model, the

**branes** you refer to are analogous to four-dimensional areas slicing through through a five-dimensional volume.