The vacuum energy that drives the rapid expansion in an inflationary cosmology comes from a scalar field that is part of the spontaneous symmetry breaking dynamics of some unified theory particle theory, say, a Grand Unified Theory or string theory.

This field is sometimes called the inflaton. The average value of the inflaton at temperature T is the value at the minimum of its potential energy at that temperature. The location of this minimum changes with temperature, as is shown in the animation to the right.

For temperatures T above some critical temperature T_{crit}, the minimum of the potential is at zero. But as the temperature cools, the potential changes and a second minimum develops in the potential at a nonzero value. This signals something called a phase transition, like when steam cools and condenses into water. For water the critical temperature T_{crit} where this phase transition happens is 100°C, or 373°K.

The two minima in the potential represent the two possible phases of the inflaton field, and of the Universe, at the critical temperature. One phase has the minimum of the field f=0, and the other phase represents the vacuum energy if the ground state has f=f_{0}.

According to the inflationary model, at the critical temperature, spacetime starts to under go this phase transition from one minimum to the other. But it doesn’t do it smoothly, it stays in the old “false” vacuum too long. This is called supercooling. This region of false vacuum expands exponentially fast, and the vacuum energy of this false vacuum is the cosmological constant for the expansion. It is this process that is called Inflation and solves the flatness, horizon and monopole problems.

This region of false vacuum expands until bubbles of the new broken symmetry phase with f=f_{0 }form and collide, and eventually end the inflationary phase. The potential energy of the vacuum is converted through to kinetic energy of matter and radiation, and the Universe expands according to the Big Bang model already outlined.

# The Process Of Inflation

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