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Originally Posted by Squink
If this big bang as exploding firework model is to have any validity, it needs to explain a couple of things.
•Why don't we see a central void? When something explodes, the debris cloud starts out spherical, but after a little time has passed, you always get an expanding shell of material around a central void, rather like a planetary nebula. If the universe contains such a void, it should be pretty easy to find.
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There are different kinds of explosions. An atomic bomb explosion tends to send out a “bubble” or a “thin shell” of a shock wave. But there is another kind that is like an explosion in a pile of dirt or sand. That kind sends out particles in all directions, with the central stuff not moving too much at all and the outer stuff moving the fastest. But this is only speculation, which originally was based on the idea that if the galaxies are moving apart now, they must have been closer together in the past and maybe very close together in the distant past.
But no one really knows what happened or why the galaxies are moving (or appear to be moving). If our universe is bigger than suspected, the “center” could be so far away from us we might not be able to see it.
If the “explosion” was really uniform and the universe bigger than we think, then we would see what we see, but we wouldn’t have to be near the center at all.
One reason astronomers were able to detect our position in our own galaxy was because of the “sideways” motion of some of the distant stars (I think that’s the way to explain it). But, the problem with detecting any “sideways” motion of any of the distant redshifted galaxies is because they are so far away, a slight “sideways” motion is just too small to be noticed.
Newton offered a third alternative to the 1) collapsing or 2) expanding list of possibilities. He said the entire universe might be revolving, something like the way our solar system works. Well, if it’s revolving, it would have to be somewhat overall “flat” shaped similar to a galaxy, but from where we are I don’t think we see anything but a visual “sphere”. So I suppose it would have to be much larger than we think it is, in order for it to be revolving without our noticing it yet. But, actually, that’s pretty much the way the 19th Century astronomers thought of our own galaxy, non-rotating and made up of “fixed” stars and “nebula”.
Personally, I don’t know what’s going on with the overall universe. I don't even know if it's really "expanding". I’m just basing some of my comments on what I’ve read about the different theories over the years.
What Eddington did in his book in 1933 was just avoid the “center” problem by saying that the universe probably existed as a small sphere (many light-years across) for a long time and then started to expand. He just avoided discussing the very beginning.