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According to the current Big Bang with inflation cosmological model?

I was under the mistaken impression that there was very low volume, very high temperature/pressure, very low entropy and the Big Bang (perhaps prompted by the high pressure) resulted in an expansion of volume where energy didn't change much so temperature went down and entropy went up.

Now I've learned about inflation and how there is no conservation of energy at cosmological scales and that the universe gained an exponential amount of energy during the inflationary period. How did this happen with a decrease in temperature? Inflation was a period of supercooled expansion, was it not?

Why was positive pressure not enough to cause inflation such that we need a negative pressure in the form of the inflaton field (or later dark energy) to explain the expansion of the universe?

Thanks for your time

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you can formulate inflation either in a non-causal-patch way which refers to stuff outside the cosmological horizon, or in a causal patch way which doesn't. The statements you make are not absolute--- they are true in one picture and false in the other. You need to consider the stuff that is absolute between philosophical pictures. –  Ron Maimon Apr 9 '12 at 0:06
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The original theory of inflation proposed by Guth was that it was due to supercooling during a phase transition (possibly a symmetry breaking) but it was soon realised this didn't account for the universe we see. The main problem was ending the inflationary period. Current theories postulate an inflaton field without specifying what caused it. The physical origin of the inflaton field is one of the great unknowns at the moment.

Anyhow, the problem with regular pressure is that expansion rapidly dilutes it and the pressure falls. By contrast the inflaton field is a property of space so it does not get diluted by inflation and continues to drive it - forever, if you believe eternal inflation! That's why you need an inflaton field or, as you say, dark energy to drive the expansion.

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