What caused inflation to stop?

[iamaelephant]

So I'm reading about the inflationary period and something occurred to me that never seems to be addressed in the popular physics books - what caused inflation to stop? So I buy the inflation argument, the evidence seems sound, but why would the universe expand so extraordinarily quickly for a tiny fraction of a second and then drop down to a more "reasonable" speed so quickly?

 

[Chalnoth]

So I'm reading about the inflationary period and something occurred to me that never seems to be addressed in the popular physics books - what caused inflation to stop? So I buy the inflation argument, the evidence seems sound, but why would the universe expand so extraordinarily quickly for a tiny fraction of a second and then drop down to a more "reasonable" speed so quickly?

The simplest models of inflation involve a field that has some potential energy. During inflation, the field is slowly moving towards the minimum of that potential (it moves slowly because the rapid expansion during inflation acts as a sort of friction). Once it does reach the minimum, however, the field starts to oscillate about said minimum. These oscillations cause the field to decay into other particles, so that there is a transition from this large potential energy density and very low temperature to a radiation-dominated, high-temperature state. This process is known as reheating.

The subsequent radiation-dominated state tends to slow its expansion rather rapidly with time.

 

[bapowell]

It's important to point out that inflation actually stops before reheating. Inflation only occurs for scalar fields that are potential energy dominated. As Chalnoth says, initially the field is at the top of a local maximum of its potential and is rolling slowly (potential energy dominated). It is perfectly OK to analogize with a ball rolling off a hill -- initially at rest and gravitational potential energy dominated. In such a case, the scalar field behaves like a cosmological constant and leads to accelerated expansion. However, as the field rolls of the maximum and picks up kinetic energy. Once this kinetic energy dominates over the potential, the field no longer gravitates like a cosmological constant and inflation ceases. Just before it begins its coherent oscillations, the scalar field gravitates more like a cosmic string network and is non-inflationary. As the oscillations set-in, it constitutes a Bose-Einstein condensate. Upon decay, the universe is reheated into hot radiation.