The idea is there was no primevel atom but instead a portion of a quantum scalar field whose vacuum energy was not at a minmum. This is termed a false vacuum. The energy density was slightly less than critical and the total mass was ~ that of a garden pea. For this universe to exist, as we know it, its energy density had to be less than critical or it would have collapsed to a black hole instead of inflating. The classical big bang theory describes the evolution of the universe after the inflationary period. Here are 2 short articles on inflationary cosmology by the two guys who 'thunk' it up. Alan Guth and Andrei Linde. There are several versions Guth's, Linde's, Hawking's and Turok's, string cosmology, etc but they all have a quantum origin.
http://nedwww.ipac.caltech.edu/level5/Guth/Guth_contents.html
http://www.sciam.com/specialissues/0398cosmos/0398linde.html
Some interesting papers on string cosmolgy
http://www.to.infn.it/~gasperin/