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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?

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The inflaton loses energy, and this gets transferred to the universe. Inflation is actually less weird, at least from a conservation of energy standpoint, than dark energy is. –  Jerry Schirmer Apr 9 '12 at 15:03
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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 The book is a bit out of date now, but it will give you a good starting point for further reading.

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I'm not sure I understand the first paragraph of your reply. So the energy of the inflaton field was always there and still is, but now it's turned into the positive energy of matter/photons/etc? I thought the inflaton field had negative energy though? What was the positive energy in the universe during the inflationary epoch? So if the current universal energy checkbook balance is positive energy of matter/photons/etc + negative gravitational potential = 0, then how was the checkbook "balanced" during the inflationary epoch? –  Ocsis2 Apr 9 '12 at 14:15
It also exponentially increased the (negative) gravitational energy, and this balanced out the exponential increase in the inflaton field energy. The inflaton field had (has?) a negative pressure, but it's not negative energy. –  John Rennie Apr 9 '12 at 14:17
This makes more sense but now it sets me back to my first question. -_- Even if the energy is still being cancelled out to 0, how did both positive and negative forms of energy increase in the first place? Was it through quantum tunneling between different energy states? –  Ocsis2 Apr 9 '12 at 14:19
I'm not sure I understand the question. If you assume an inflaton field exists, without worrying about it's physical origin, then the equations of GR predict exponential expansion. This is purely classical - there's nothing quantum about it. I'd guess you're really asking about the physics behind the inflaton field i.e. what causes it, and the simple answer is that no-one knows. –  John Rennie Apr 9 '12 at 14:29
I guess what I'm asking is where did the inflaton field energy come from? It didn't come all at once at the start of inflation, right? Because it occurred for a period of time and throughout this period of time the universe expanded and the energy increased. So the total energy increase couldn't have all been attributed to its causal mechanism then? So what was the mechanism for this increase of energy throughout the process of inflation? (Taking for granted now that both positive/negative energy increased equivalently preserving the zero sum energy balance). –  Ocsis2 Apr 9 '12 at 14:52
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