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I am an ignoramus on this subject, and I assume that these statements quoted from wikipedia are more or less accurate:(http://en.wikipedia.org/wiki/Inflaton)
1) Prior to the expansion period, the inflaton field was at a higher-energy state.
2) Random quantum fluctuations triggered a phase transition whereby the inflaton field released its potential energy as matter and radiation as it settled to its lowest-energy state.
So, in the "classical/standard" theory (that means, no string theories, or newer ones), what was assumed to prevent the highly dense universe to become a black hole before inflation started?