3.4. The start of inflation
In the original basic pictures of inflation, where a period of inflation was inserted into a SBB model, various arguments were made about why the Universe had to enter an inflationary phase based on symmetry restoration at high temperatures [1, 6, 7]. However, starting with a SBB before inflation assumes a great deal about initial conditions (which after all we are trying to explain).
Currently cosmologists that think about these things take one of two perspectives. One group tries to treat ``the most random possible'' initial conditions , and discuss how inflation can emerge from such a state of matter. The other group tries to construct the ``wavefunction of the Universe'' based on deep principles . In some cases the wavefunction of the Universe indicates that inflation is the most likely starting point for regions that take on SBB-like behavior. I have a personal prejudice against ``principles'' of initial conditions, because I do not see how that gets your further than simply postulating the initial conditions of the SBB.
Coming from the random initial conditions point of view, a popular argument is that even if the probability of starting inflation is extremely small in the primordial chaos, once inflation starts it takes over the Universe. When measured in terms of volume, the exponential expansion really does appear to take over. In fact, even though there is a classical instability, the small quantum probability of remaining on the flat inflationary part of the potential is amplified by the exponential inflationary growth, and in many models most of the Universe inflates forever (so-called ``eternal inflation''). So perhaps as long as inflation is possible, one does not even have to think too hard about what went before.
All of these issues are plagued by key unresolved questions (for example to do with putting a measure on the large spaces one is contemplating). I will discuss some of the open questions associated with these issues in Section 6.