Is just that a theory. From the little I read it appears that his theory is problem driven. It seems that his inflation theory answers questions that seem to be troubling cosmologist such as "flatness" etc. Which probably is why he recieved so many awards.

So he draws upon theory which has not been tested or verifiable for a prediction that the universe should expand exponentially.

The truth is no one knows what the physics of the universe is within minutes of the big bang, both temperature and density are too high too measure how matter behaves. As the saying goes this requires a great deal of faith to believe in this as well.