The theory of inflation (below) offers a very neat explanation for the homogeneity and flatness of our universe, i.e. offers a very neat solution to the horizon and flatness problems of the Big Bang model. But what physical mechanism could have caused inflation? What could cause the infant universe to undergo a psychotic, exponential expansion in the first fractions of an instant?
The basic idea is that the infant universe may have undergone a phase transition. In particle physics, it has long been predicted that the quarks in the quark-gluon plasma must have undergone a phase transition to become trapped in hadrons in the first billionth of a second. Cosmologists now believe that, even earlier in the history of the universe, the fundamental constituents of matter/energy may have undergone a phase transition at even higher energies. Under certain circumstances, such a phase transition would be accompanied by the appearance of energy in empty space (so-called vacuum energy). Calculations show that this vacuum energy could act as an enormous force of repulsion, causing a rapid, exponential expansion of spacetime.
Technically speaking, it is thought that the very early universe cooled into a metastable state of false vacuum - it was then nudged towards genuine equilibrium by a process of quantum tunnelling. However, it was soon shown that the latter process is too violent to result in the universe we observe today. Instead, Linde , Albrecht and Steinhardt calculated that a more realistic model is a universe that moves from false vacuum to equilibrium via a gradual process as shown below.
New inflation vs old inflation
Finally, it was assumed in all the early versions of inflation theory that the vacuum energy liberated by the phase transition disappeared immediately after the hyper-expansion. However, it has recently been discovered that the rate of expansion of the universe is currently accelerating! This observation was a great surprise as it is not predicted by any of the Friedmann models; it is now believed that the cause of the acceleration (known as dark energy) may be a small amount of vacuum energy left over from inflation….more on this next week.
Update: if you can’t wait, there is a very nice summary of dark energy here