How did inflation add energy to the universe? What mechanism did this occur by? In other words, where did that energy come from? Was it due to the quantum fluctuation (or that scalar field rolling down a potential energy hill) to a lower, more stable energy state? This energy then manifested as exponential expansion, and then switched from expansion into heating (by filling up the universe with hot quark-gluon plasma)? Do we know the mechanism for this expansion->reheating switch?
The usual argument is that the net energy of the universe is zero because the positive energy of the matter and photons is balanced out by the negative gravitational potential energy. This would apply to the inflaton field as well. This has always felt vaguely unsatisfactory to me, but I don't know of a better explanation.
The "expansion->reheating switch" is dependant on the model of inflation used. In the original theory proposed by Guth inflation ended by quantum tunneling into the broken symmetry phase, so there was a nice clear physical process. Unfortunately this gave predictions that didn't match the observed universe, so it was abandoned. As I understand it it, current theories just tweak the potential to make inflation end without giving any nice physical reason for the exit.
You're obviously interested in the subject as you've asked a couple of questions on it. I would start with Guth's book as it's well written and an interesting read. The book is a bit out of date now, but it will give you a good starting point for further reading.
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