"Now, my main point: If one were to watch the history of the universe going backwards in time, one would see the energies increasing. Let me make the same point that [William Lane] Craig made about the physics getting speculative, but put in terms of energy. As the energy increases to 100 GeV, the physics becomes speculative – we're not really sure what happens at that point. As the energy increases to 1014 GeV (assuming it does increase to that point) the physics becomes extremely speculative, even unknown. In other words, we just don't know what happens once the energies get that high. The way Craig puts the point, it sounds like we know that there's a big bang, and we know what happens in the history of the universe once 10-12 seconds have passed, but we don't know what happens between the big bang and 10–12 seconds after the big bang. But in fact our lack of knowledge is much more fundamental. Because the physics doesn't tell us what happens once we trace the history of the universe backwards in time to these high energies, we don't even know if there's a big bang at all."
-Bradley Monton, "Physics-Based Intelligent Design Arguments are Based on False Physics"
(note: "1014" should read "10 to the 14th power", and "10-12" should read "10 to the negative twelfth power".)