Expert: Courtney Seligman - 10/4/2010

Question
I hope I'm right in thinking that when an explosion takes place, there is an inwards force as well as an outwards force. If this is true, then is it possible that such a force could stop the centre of an exploding object from annihilating itself? If that is also true, could there be an object in space that is the original point of the big bang?

Answer
With a "bomb" of finite size, the central portion could be compressed by the force of the outer explosion. In fact, that is how atomic and hydrogen bombs are triggered. An outer shell explodes, forcing the inner part inwards, increasing its density and temperature until it goes "critical".

However, in the case of the Big Bang, there is no "central" portion. At the time of the Big Bang, the entire Universe consisted of a single point of space-time of zero size (well, a size with a decimal point, then 40-some zeroes before you get to any non-zero value; but still, close enough to make no difference). As a result, even if it were possible for one part of the "explosion" to compress another, there would be no "central" region.

Aside from that, in that first instant the Universe expanded from essentially zero size to a virtually infinite size (most estimates of that size are hundreds of thousands of times larger than the 13+ billion light years we can actually observe), and its various parts became "disconnected" from each other, in that they were so far apart that they could never have any effect on each other. So the "outer" parts couldn't affect the "inner" parts, even at that time.

So although your analogy would be useful in many circumstances, I'm afraid it doesn't apply to the Big Bang. For that particular case the motion, whether thought of as an explosion or an expansion, only goes in one direction -- outward.