[Further to a comment and question see LETTERS]
It’s been a while since Cosmology graced these pages. So perhaps it’s time. Thanks Arthur – I only wish I was well-enough informed to answer you completely satisfactorily, Arthur.
Your first point is really not so much ‘why’ as ‘how’, it seems to me … but the pre-Bang singularity is not supposed to be inside any other space … the theory is that there is no ‘outside’ at this point, so the concept of ‘density’ is hard to apply, here.
Which brings us to the second point – which I think is very interesting. Why do they say that all matter was inside the Big Bang in its first moments? Again, my knowledge of the maths which really defines how the Big Bang model works, is not good enough (though it IS only a model, and one which is being questioned every day). But the theory is that the supposedly tiny ‘Bang’ in its first fractions of a second is not expanding into space. It IS space. and that space became the space we live in now. So unless everything is in there from the start, where would it come from ?
By the way, the question of how big the infant Big Bang space really was, is much more complicated than is normally discussed in layman’s terms. We know how big the observable Universe is now, and we THINK we know how fast the Universe is expanding. So we can extrapolate back to how big it was at any time. But that’s only the observable part of the Universe. I think it’s safe to say we have no idea how much more there is, out there beyond what we can see. And the other giant complication is that an assumption that the Universe was always expanding at the same rate that it is now leads to big problems (which we discussed in Bang !) – so the simple Big Bang theory is no longer simple. It includes a period of vastly accelerated growth called ‘Inflation’. There is no known reason for this to have happened … it is just a ‘fudge’ to fit the observable facts we know so far.
Finally, your analogy of shockwaves in a pond is attractive ! But not quite applicable. Again, there is no pond in the case of the Big Bang. It’s as if the stone and the initial disruption in the water ARE all there is in the beginning … and this ‘event’ expands to become the pond – which was not there before .. So maybe, if you look it at in this way, with a stone creating a pond … it’s not a bad analogy after all !
OK … I’m going to ask my friend Marcus Chown to comment … he will much more able than I to put this matter to rest ! But he may be busy!