What I'm not seeing as clearly though is why its written off having a singularity is simpler on the pretense of its begging the question of where the volume came from initially.
(1) That's not a pretense! - it really does beg the question in the same way as the statement "God created the Universe" begs the question "who created God?"
(2) The macroscopic initial state requires more to be accounted for than the singularity one ... that's why the "question-begging" is important. For instance:
Wouldn't asking where the infinitely dense singularity came from be just as much of a legitimate question as to where the matter/energy came from?
Yep - perfectly legit. However, a macroscopic initial state also needs to explain the matter/energy
as well as where all the space-time came from. "Where the singularity came from?" is a much easier problem. (note: I didn't mean to say it's simpler because the other begs the question, I meant it's simpler,
in addition to the other begging the question. But since you got me thinking about it...)
There are a lot of cosmological models - we pick the one that makes life easiest for cosmologists. If you really want to understand why a particular model is favored you need to have studied general relativity, and topology, and such things. Of course, if you have, then the discussion can get a lot more precise.
You do realize that not all inflationary (big bang) models have a singularity as an initial condition - though they do have to account for why it looks like that from here.
In the end this is what is being said: it looks an awful lot like the Universes initial condition was some kind of a singularity ... the obvious model is one that says "well is
was a singularity" and see what you have to do to it to make it work for what we see ... maybe it's wrong? We'll know when the tweaks get too much and one of the other models manages to work out simpler.
Right now there is no way to give you a clear picture because I'm stuck with plain English. If you want to know why one model is simpler than another: you have to do the math.
BTW:
I kinda assumed that you meant that the state accounted for in the BB model by the rapid expansion phase was the macroscopic start point. Reasoning something like: if the really small scale universe cannot create the flatness/horizon thing, then perhaps that's where the Universe began? You don't have a "before" that stage to explain?
... otherwise you get Chronos' point that the state also fails to account for the flatness/horizon problem. (It'd have to be bigger than the scale you suggest I suspect...)
This sounds conceptually simpler and is the id of reasoning very useful for solving conjuring tricks. Unfortunately it makes life quite hard for the theorist.
I also assumed that you understood that the big-bang is not an explosion of anything
into space.
