Electroweak Epoch - 10 -36 to 10 -12 Seconds
Cosmic Inflation - 10-36 to 10-32 Seconds
Astronomy & Cosmology -
The Early Universe
(or "What Banged in the Big Bang?")
John Gribbin has a nice history of Inflation Theory, Inflation for Beginners, as well as a couple of other articles. Universe Review has an excellent article with some basics, then a more technical analysis.
Following the breakdown of Grand Unification, the Universe enters a very brief period of inflation. The concept of Inflation was first mooted by Alan Guth in the early 1980s, as an attempt to explain the uniformity of the Cosmic Microwave Background (CMB). It proposed that the Universe, or a small fraction of a universe, expanded at an enormous rate when it was approximately 10-36 seconds old. It relies on the idea that extreme densities can cause gravity to become a repulsive force known as a false vacuum. It is the energy of this false vacuum that drives Inflation. During the inflationary period, which was incredibly short, about 10-32 seconds, it grew in volume by a factor of at least 1026, resulting in a Universe that grew from about the size of a single proton to around 10 cm (4 inches) across, about the size of a grapefruit. Note that this refers to "our little patch" of the Universe. Some cosmologists theorize that, if the Universe in its entirety was subject to inflation, it would be millions of light-years across at the end of inflation, but would be forever outside of our ability to see it due to the finite speed of light. Other "little patches" could have experienced inflation differently, resulting in, for example, a different temperature for their cosmic microwave background. Some other "little patches" may have expanded faster or slower, or for a longer or shorter time, or may not have experienced inflation at all so remain very small.
Prior to inflation, the region would have been more or less in a state of thermal equilibrium, and inflation would tend to smooth out any small irregularities, larger than the Planck length, that were there leading to the homogenous CMB we find today. In the same way that inflating a balloon tends to make a fixed sized area on its' surface flatter, inflation also flattened the curvature of space-time. At the same time, it provides a mechanism to explain how galaxies and stars formed due to tiny quantum fluctuations in the primordial matter that inflated.
Collapse of the Inflaton Field - 10-32 to 10-12 seconds
The release of the potential energy from the collapse of the inflaton field, that drove inflation, results in a hot, dense quark/gluon plasma. Highly energetic particle interactions produced many exotic particles including W± and Z bosons and, possibly, the Higgs boson. At the end of this period, the W± and Z bosons decayed, and the weak nuclear force became a short-range force.