It drove the hyper inflation when the universe was less than one-trillionth of a trillionth of a trillionth of a second old. Then it stopped when the universe was one/one-hundred billionth second old. Well it didn't exactly stop but it decayed and the universe split and broke away into perhaps an unlimited-universe type of multiverse as the decaying particle kept going on and splitting up the (then small amount of)space. Our universe was one that split off at that time but it kept going/decaying/splitting in the other remaining parts (a single space part before all the splitting).
The math works if the initial universe is much smaller than previously thought. If you have the universe infinitesimally small, then there is enough time for the heat to equalize, throughout the universe, equally, while it is less than one-trillionth of a trillionth of a trillionth of a second old, then the rapid expansion, from the (early hyper)inflation, caused by the inflaton particle, will drive the universe from much smaller than the size of an atom to the size of a grapefruit by the time it is one/one-hundred billionth of a second old.
The CMB picture around 2001-2003 (?) backed up the theory. The prediction wasn't falsified but strongly backed up.
A "big crunch" (universe previous to our presently existing 13.7 billion year old)universe is considered to be contra-indicated by the evidence that supports the Guth inflation theory. The respective views seem to be highly antagonistic toward each other. Once rises as the other falls.
It seems.
I have no idea though.
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