ok, the problem with all this theorizing you guys are doing is that your using the rules of classical physics to describe phenomenon of an "object", if you will, in which classical physics does not apply. For instance, superposition. If there is a universe with a certain placement of molecules and another universe with the same molecules in a different placement, a universe also exists with the molecules in both places at the same time. so, this means that there is also the possibility of one space being occupied by two or more molecules. SOOO, if someone were to trael back in time to the moments where the universe was but a singularity(but NOT a blackhole, something that can ONLY exist if time exists outside of the singularity), this person would be forced into the only place that time exists, the same space as the singularity, and in time the particles of the person would be forced to be of another, universe, of the start of many universes, and the negative direction of the density of the first universe would cause the particle to become unstable(something that increasing the density, strangely, does not do...) and you have a "big bang".
So, in the end, you'll really have changed nothing.
Of course there's more to superposition and quantum physics than that(which I severally simplified) but theres really no reason to get into wave functions, now is there?
Oh yeah, and you would not be broken down to radiation in this other universe, also thanks to superposition, because the universe would then simply have the same particles in two different places at the same time. And do keep in mind, this phenomenon has been observed many times in our own universe....
EDIT: Bump :p