There are two features of the universe that are difficult to understand, if the early universe had expanded at a constant rate. (1) Flatness/Critical Density, and (2) Isotropy/Homogeneity. Each part of the universe very precisely has escape speed with respect to any other part. And parts of the universe which could never have been in causal contact appear essentially identical. In the period 1979 - 1990, an idea which solved these and other problems was fleshed out. The idea was that very early in its history, the entire universe underwent a phase transition which caused a huge expansion in size during a very, very brief time interval.

To understand how the current universe looks, we need the very early universe to undergo a first-order phase transition at the age of about 10−36 to 10−33 seconds.  In this interval the radius of the universe would have increased from sub-quantum size to roughly 1 cm.  This inflation would be due to an inflation field (quanta: inflatons) with a huge, constant vacuum energy... a split-second de Sitter universe.  At the end of the transition, that vacuum energy would have to be zero.  The huge expansion would drastically cool the universe, but the required decay of the inflatons into particles with enormous kinetic energy would reheat things. The scale factor during expansion would be like eHt, and the key fact is that the quantum fluctuations existing during the inflationary era would be magnified to macroscopic size at the end of the era, and should still be seen in the Big Flash temperature and density variations.  In fact the level of variation seen by WMAP and Planck on the last scattering surface is around 10−4 to 10−5, precisely as expected from the inflationary scenario. These density variations were the seeds of all the structure currently seen in our universe, augmented by the driven standing waves existing in the later soup of fermions and photons.  This era of intense particle creation in a compact space-time would also create distinctive gravitational radiation, and would produce a unique quadrupole signature in the Big Flash, which it is very, very important to be able to observe.

Inflation could have occurred by a scalar field rolling down a potential energy hill. When the field rolls very slowly compared to the expansion of the Universe, inflation occurs. However, when the hill becomes steeper, inflation ends and reheating can occur.