5.4. The General Idea of Inflation
The horizon problem especially is an extremely serious problem for the standard cosmology because at its heart is simply causality. Any solution to this problem is therefore almost certain to require an important modification to how information can propagate in the early universe. Cosmological inflation is such a mechanism.
Before getting into the details of inflation we will just sketch the general idea here. The fundamental idea is that the universe undergoes a period of accelerated expansion, defined as a period when > 0, at early times. The effect of this acceleration is to quickly expand a small region of space to a huge size, diminishing spatial curvature in the process, making the universe extremely close to flat. In addition, the horizon size is greatly increased, so that distant points on the CMB actually are in causal contact and unwanted relics are tremendously diluted, solving the monopole problem. As an unexpected bonus, quantum fluctuations make it impossible for inflation to smooth out the universe with perfect precision, so there is a spectrum of remnant density perturbations; this spectrum turns out to be approximately scale-free, in good agreement with observations of our current universe.